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class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Isometric_contraction"> <div class="vector-toc-text"> <span class="vector-toc-numb">1.1</span> <span>Isometric contraction</span> </div> </a> <ul id="toc-Isometric_contraction-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Isotonic_contraction" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Isotonic_contraction"> <div class="vector-toc-text"> <span class="vector-toc-numb">1.2</span> <span>Isotonic contraction</span> </div> </a> <ul id="toc-Isotonic_contraction-sublist" class="vector-toc-list"> <li id="toc-Concentric_contraction" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Concentric_contraction"> <div class="vector-toc-text"> <span class="vector-toc-numb">1.2.1</span> <span>Concentric contraction</span> </div> </a> <ul id="toc-Concentric_contraction-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Eccentric_contraction" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Eccentric_contraction"> <div class="vector-toc-text"> <span class="vector-toc-numb">1.2.2</span> <span>Eccentric contraction</span> </div> </a> <ul id="toc-Eccentric_contraction-sublist" class="vector-toc-list"> <li id="toc-Eccentric_contractions_in_movement" class="vector-toc-list-item vector-toc-level-4"> <a class="vector-toc-link" href="#Eccentric_contractions_in_movement"> <div class="vector-toc-text"> <span class="vector-toc-numb">1.2.2.1</span> <span>Eccentric contractions in movement</span> </div> </a> <ul id="toc-Eccentric_contractions_in_movement-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> </ul> </li> </ul> </li> <li id="toc-Vertebrate" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Vertebrate"> <div class="vector-toc-text"> <span class="vector-toc-numb">2</span> <span>Vertebrate</span> </div> </a> <button aria-controls="toc-Vertebrate-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Vertebrate subsection</span> </button> <ul id="toc-Vertebrate-sublist" class="vector-toc-list"> <li id="toc-Skeletal_muscle" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Skeletal_muscle"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.1</span> <span>Skeletal muscle</span> </div> </a> <ul id="toc-Skeletal_muscle-sublist" class="vector-toc-list"> <li id="toc-Neuromuscular_junction" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Neuromuscular_junction"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.1.1</span> <span>Neuromuscular junction</span> </div> </a> <ul id="toc-Neuromuscular_junction-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Excitation–contraction_coupling" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Excitation–contraction_coupling"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.1.2</span> <span>Excitation–contraction coupling</span> </div> </a> <ul id="toc-Excitation–contraction_coupling-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Sliding_filament_theory" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Sliding_filament_theory"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.1.3</span> <span>Sliding filament theory</span> </div> </a> <ul id="toc-Sliding_filament_theory-sublist" class="vector-toc-list"> <li id="toc-Cross-bridge_cycle" class="vector-toc-list-item vector-toc-level-4"> <a class="vector-toc-link" href="#Cross-bridge_cycle"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.1.3.1</span> <span>Cross-bridge cycle</span> </div> </a> <ul id="toc-Cross-bridge_cycle-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Gradation_of_skeletal_muscle_contractions" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Gradation_of_skeletal_muscle_contractions"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.1.4</span> <span>Gradation of skeletal muscle contractions</span> </div> </a> <ul id="toc-Gradation_of_skeletal_muscle_contractions-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Length-tension_relationship" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Length-tension_relationship"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.1.5</span> <span>Length-tension relationship</span> </div> </a> <ul id="toc-Length-tension_relationship-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Force-velocity_relationships" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Force-velocity_relationships"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.1.6</span> <span>Force-velocity relationships</span> </div> </a> <ul id="toc-Force-velocity_relationships-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Smooth_muscle" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Smooth_muscle"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.2</span> <span>Smooth muscle</span> </div> </a> <ul id="toc-Smooth_muscle-sublist" class="vector-toc-list"> <li id="toc-Mechanisms_of_smooth_muscle_contraction" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Mechanisms_of_smooth_muscle_contraction"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.2.1</span> <span>Mechanisms of smooth muscle contraction</span> </div> </a> <ul id="toc-Mechanisms_of_smooth_muscle_contraction-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Neuromodulation" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Neuromodulation"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.2.2</span> <span>Neuromodulation</span> </div> </a> <ul id="toc-Neuromodulation-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Cardiac_muscle" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Cardiac_muscle"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.3</span> <span>Cardiac muscle</span> </div> </a> <ul id="toc-Cardiac_muscle-sublist" class="vector-toc-list"> <li id="toc-Excitation-contraction_coupling" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Excitation-contraction_coupling"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.3.1</span> <span>Excitation-contraction coupling</span> </div> </a> <ul id="toc-Excitation-contraction_coupling-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> </ul> </li> <li id="toc-Invertebrate" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Invertebrate"> <div class="vector-toc-text"> <span class="vector-toc-numb">3</span> <span>Invertebrate</span> </div> </a> <button aria-controls="toc-Invertebrate-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Invertebrate subsection</span> </button> <ul id="toc-Invertebrate-sublist" class="vector-toc-list"> <li id="toc-Circular_and_longitudinal_muscles" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Circular_and_longitudinal_muscles"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.1</span> <span>Circular and longitudinal muscles</span> </div> </a> <ul id="toc-Circular_and_longitudinal_muscles-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Obliquely_striated_muscles" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Obliquely_striated_muscles"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.2</span> <span>Obliquely striated muscles</span> </div> </a> <ul id="toc-Obliquely_striated_muscles-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Asynchronous_muscles" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Asynchronous_muscles"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.3</span> <span>Asynchronous muscles</span> </div> </a> <ul id="toc-Asynchronous_muscles-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-History" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#History"> <div class="vector-toc-text"> <span class="vector-toc-numb">4</span> <span>History</span> </div> </a> <ul id="toc-History-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-See_also" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#See_also"> <div class="vector-toc-text"> <span class="vector-toc-numb">5</span> <span>See also</span> </div> </a> <ul id="toc-See_also-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-References" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#References"> <div class="vector-toc-text"> <span class="vector-toc-numb">6</span> <span>References</span> </div> </a> <ul id="toc-References-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Further_reading" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Further_reading"> <div class="vector-toc-text"> <span class="vector-toc-numb">7</span> <span>Further reading</span> </div> </a> <ul id="toc-Further_reading-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-External_links" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#External_links"> <div class="vector-toc-text"> <span class="vector-toc-numb">8</span> <span>External links</span> </div> </a> <ul id="toc-External_links-sublist" class="vector-toc-list"> </ul> </li> </ul> </div> </div> </nav> </div> </div> <div class="mw-content-container"> <main id="content" class="mw-body"> <header class="mw-body-header vector-page-titlebar"> <nav aria-label="Contents" class="vector-toc-landmark"> <div id="vector-page-titlebar-toc" class="vector-dropdown vector-page-titlebar-toc vector-button-flush-left" > <input type="checkbox" id="vector-page-titlebar-toc-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-vector-page-titlebar-toc" class="vector-dropdown-checkbox " aria-label="Toggle the table of contents" > <label id="vector-page-titlebar-toc-label" for="vector-page-titlebar-toc-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--icon-only " aria-hidden="true" ><span class="vector-icon mw-ui-icon-listBullet mw-ui-icon-wikimedia-listBullet"></span> <span class="vector-dropdown-label-text">Toggle the table of contents</span> </label> <div class="vector-dropdown-content"> <div id="vector-page-titlebar-toc-unpinned-container" class="vector-unpinned-container"> </div> </div> </div> </nav> <h1 id="firstHeading" class="firstHeading mw-first-heading"><span class="mw-page-title-main">Muscle contraction</span></h1> <div id="p-lang-btn" class="vector-dropdown mw-portlet mw-portlet-lang" > <input type="checkbox" id="p-lang-btn-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-p-lang-btn" class="vector-dropdown-checkbox mw-interlanguage-selector" aria-label="Go to an article in another language. Available in 33 languages" > <label id="p-lang-btn-label" for="p-lang-btn-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--action-progressive mw-portlet-lang-heading-33" aria-hidden="true" ><span class="vector-icon mw-ui-icon-language-progressive mw-ui-icon-wikimedia-language-progressive"></span> <span class="vector-dropdown-label-text">33 languages</span> </label> <div class="vector-dropdown-content"> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="interlanguage-link interwiki-ar mw-list-item"><a href="https://ar.wikipedia.org/wiki/%D8%A7%D9%86%D9%82%D8%A8%D8%A7%D8%B6_%D8%B9%D8%B6%D9%84%D9%8A" title="انقباض عضلي – Arabic" lang="ar" hreflang="ar" data-title="انقباض عضلي" data-language-autonym="العربية" data-language-local-name="Arabic" class="interlanguage-link-target"><span>العربية</span></a></li><li class="interlanguage-link interwiki-ast mw-list-item"><a href="https://ast.wikipedia.org/wiki/Contraici%C3%B3n_muscular" title="Contraición muscular – Asturian" lang="ast" hreflang="ast" data-title="Contraición muscular" data-language-autonym="Asturianu" data-language-local-name="Asturian" class="interlanguage-link-target"><span>Asturianu</span></a></li><li class="interlanguage-link interwiki-bs mw-list-item"><a href="https://bs.wikipedia.org/wiki/Mi%C5%A1i%C4%87na_kontrakcija" title="Mišićna kontrakcija – Bosnian" lang="bs" hreflang="bs" data-title="Mišićna kontrakcija" data-language-autonym="Bosanski" data-language-local-name="Bosnian" class="interlanguage-link-target"><span>Bosanski</span></a></li><li class="interlanguage-link interwiki-ca mw-list-item"><a href="https://ca.wikipedia.org/wiki/Contracci%C3%B3_muscular" title="Contracció muscular – Catalan" lang="ca" hreflang="ca" data-title="Contracció muscular" data-language-autonym="Català" data-language-local-name="Catalan" class="interlanguage-link-target"><span>Català</span></a></li><li class="interlanguage-link interwiki-cs mw-list-item"><a href="https://cs.wikipedia.org/wiki/Svalov%C3%BD_stah" title="Svalový stah – Czech" lang="cs" hreflang="cs" data-title="Svalový stah" data-language-autonym="Čeština" data-language-local-name="Czech" class="interlanguage-link-target"><span>Čeština</span></a></li><li class="interlanguage-link interwiki-de mw-list-item"><a href="https://de.wikipedia.org/wiki/Muskelkontraktion" title="Muskelkontraktion – German" lang="de" hreflang="de" data-title="Muskelkontraktion" data-language-autonym="Deutsch" data-language-local-name="German" class="interlanguage-link-target"><span>Deutsch</span></a></li><li class="interlanguage-link interwiki-el mw-list-item"><a href="https://el.wikipedia.org/wiki/%CE%9C%CF%85%CF%8A%CE%BA%CE%AE_%CF%83%CF%85%CF%83%CF%84%CE%BF%CE%BB%CE%AE" title="Μυϊκή συστολή – Greek" lang="el" hreflang="el" data-title="Μυϊκή συστολή" data-language-autonym="Ελληνικά" data-language-local-name="Greek" class="interlanguage-link-target"><span>Ελληνικά</span></a></li><li class="interlanguage-link interwiki-es mw-list-item"><a href="https://es.wikipedia.org/wiki/Contracci%C3%B3n_muscular" title="Contracción muscular – Spanish" lang="es" hreflang="es" data-title="Contracción muscular" data-language-autonym="Español" data-language-local-name="Spanish" class="interlanguage-link-target"><span>Español</span></a></li><li class="interlanguage-link interwiki-eo mw-list-item"><a href="https://eo.wikipedia.org/wiki/Muskola_kontrahi%C4%9Do" title="Muskola kontrahiĝo – Esperanto" lang="eo" hreflang="eo" data-title="Muskola kontrahiĝo" data-language-autonym="Esperanto" data-language-local-name="Esperanto" class="interlanguage-link-target"><span>Esperanto</span></a></li><li class="interlanguage-link interwiki-eu mw-list-item"><a href="https://eu.wikipedia.org/wiki/Muskulu-uzkurdura" title="Muskulu-uzkurdura – Basque" lang="eu" hreflang="eu" data-title="Muskulu-uzkurdura" data-language-autonym="Euskara" data-language-local-name="Basque" class="interlanguage-link-target"><span>Euskara</span></a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%DA%A9%D8%B4%D8%B4_%D9%85%D8%A7%D9%87%DB%8C%DA%86%D9%87" title="کشش ماهیچه – Persian" lang="fa" hreflang="fa" data-title="کشش ماهیچه" data-language-autonym="فارسی" data-language-local-name="Persian" class="interlanguage-link-target"><span>فارسی</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/Contraction_musculaire" title="Contraction musculaire – French" lang="fr" hreflang="fr" data-title="Contraction musculaire" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-gl mw-list-item"><a href="https://gl.wikipedia.org/wiki/Contracci%C3%B3n_muscular" title="Contracción muscular – Galician" lang="gl" hreflang="gl" data-title="Contracción muscular" data-language-autonym="Galego" data-language-local-name="Galician" class="interlanguage-link-target"><span>Galego</span></a></li><li class="interlanguage-link interwiki-ko mw-list-item"><a href="https://ko.wikipedia.org/wiki/%EA%B7%BC%EC%88%98%EC%B6%95" title="근수축 – Korean" lang="ko" hreflang="ko" data-title="근수축" data-language-autonym="한국어" data-language-local-name="Korean" class="interlanguage-link-target"><span>한국어</span></a></li><li class="interlanguage-link interwiki-hy mw-list-item"><a href="https://hy.wikipedia.org/wiki/%D5%84%D5%AF%D5%A1%D5%B6%D5%A1%D5%B5%D5%AB%D5%B6_%D5%AF%D5%AE%D5%AF%D5%B8%D6%82%D5%B4" title="Մկանային կծկում – Armenian" lang="hy" hreflang="hy" data-title="Մկանային կծկում" data-language-autonym="Հայերեն" data-language-local-name="Armenian" class="interlanguage-link-target"><span>Հայերեն</span></a></li><li class="interlanguage-link interwiki-hi mw-list-item"><a href="https://hi.wikipedia.org/wiki/%E0%A4%AA%E0%A5%87%E0%A4%B6%E0%A5%80_%E0%A4%B8%E0%A4%82%E0%A4%95%E0%A5%81%E0%A4%9A%E0%A4%A8" title="पेशी संकुचन – Hindi" lang="hi" hreflang="hi" data-title="पेशी संकुचन" data-language-autonym="हिन्दी" data-language-local-name="Hindi" class="interlanguage-link-target"><span>हिन्दी</span></a></li><li class="interlanguage-link interwiki-id mw-list-item"><a href="https://id.wikipedia.org/wiki/Kontraksi_otot" title="Kontraksi otot – Indonesian" lang="id" hreflang="id" data-title="Kontraksi otot" data-language-autonym="Bahasa Indonesia" data-language-local-name="Indonesian" class="interlanguage-link-target"><span>Bahasa Indonesia</span></a></li><li class="interlanguage-link interwiki-it mw-list-item"><a href="https://it.wikipedia.org/wiki/Contrazione_muscolare" title="Contrazione muscolare – Italian" lang="it" hreflang="it" data-title="Contrazione muscolare" data-language-autonym="Italiano" data-language-local-name="Italian" class="interlanguage-link-target"><span>Italiano</span></a></li><li class="interlanguage-link interwiki-mk mw-list-item"><a href="https://mk.wikipedia.org/wiki/%D0%9C%D1%83%D1%81%D0%BA%D1%83%D0%BB%D0%BD%D0%B0_%D0%BA%D0%BE%D0%BD%D1%82%D1%80%D0%B0%D0%BA%D1%86%D0%B8%D1%98%D0%B0" title="Мускулна контракција – Macedonian" lang="mk" hreflang="mk" data-title="Мускулна контракција" data-language-autonym="Македонски" data-language-local-name="Macedonian" class="interlanguage-link-target"><span>Македонски</span></a></li><li class="interlanguage-link interwiki-nl mw-list-item"><a href="https://nl.wikipedia.org/wiki/Spiercontractie" title="Spiercontractie – Dutch" lang="nl" hreflang="nl" data-title="Spiercontractie" data-language-autonym="Nederlands" data-language-local-name="Dutch" class="interlanguage-link-target"><span>Nederlands</span></a></li><li class="interlanguage-link interwiki-no mw-list-item"><a href="https://no.wikipedia.org/wiki/Muskelkontraksjon" title="Muskelkontraksjon – Norwegian Bokmål" lang="nb" hreflang="nb" data-title="Muskelkontraksjon" data-language-autonym="Norsk bokmål" data-language-local-name="Norwegian Bokmål" class="interlanguage-link-target"><span>Norsk bokmål</span></a></li><li class="interlanguage-link interwiki-uz mw-list-item"><a href="https://uz.wikipedia.org/wiki/Muskul_qisqarishi" title="Muskul qisqarishi – Uzbek" lang="uz" hreflang="uz" data-title="Muskul qisqarishi" data-language-autonym="Oʻzbekcha / ўзбекча" data-language-local-name="Uzbek" class="interlanguage-link-target"><span>Oʻzbekcha / ўзбекча</span></a></li><li class="interlanguage-link interwiki-pl mw-list-item"><a href="https://pl.wikipedia.org/wiki/Skurcz_mi%C4%99%C5%9Bnia" title="Skurcz mięśnia – Polish" lang="pl" hreflang="pl" data-title="Skurcz mięśnia" data-language-autonym="Polski" data-language-local-name="Polish" class="interlanguage-link-target"><span>Polski</span></a></li><li class="interlanguage-link interwiki-pt mw-list-item"><a href="https://pt.wikipedia.org/wiki/Contra%C3%A7%C3%A3o_muscular" title="Contração muscular – Portuguese" lang="pt" hreflang="pt" data-title="Contração muscular" data-language-autonym="Português" data-language-local-name="Portuguese" class="interlanguage-link-target"><span>Português</span></a></li><li class="interlanguage-link interwiki-ro mw-list-item"><a href="https://ro.wikipedia.org/wiki/Contrac%C8%9Bie_muscular%C4%83" title="Contracție musculară – Romanian" lang="ro" hreflang="ro" data-title="Contracție musculară" data-language-autonym="Română" data-language-local-name="Romanian" class="interlanguage-link-target"><span>Română</span></a></li><li class="interlanguage-link interwiki-ru mw-list-item"><a href="https://ru.wikipedia.org/wiki/%D0%9C%D1%8B%D1%88%D0%B5%D1%87%D0%BD%D0%BE%D0%B5_%D1%81%D0%BE%D0%BA%D1%80%D0%B0%D1%89%D0%B5%D0%BD%D0%B8%D0%B5" title="Мышечное сокращение – Russian" lang="ru" hreflang="ru" data-title="Мышечное сокращение" data-language-autonym="Русский" data-language-local-name="Russian" class="interlanguage-link-target"><span>Русский</span></a></li><li class="interlanguage-link interwiki-sr mw-list-item"><a href="https://sr.wikipedia.org/wiki/Kontrakcija_mi%C5%A1i%C4%87a" title="Kontrakcija mišića – Serbian" lang="sr" hreflang="sr" data-title="Kontrakcija mišića" data-language-autonym="Српски / srpski" data-language-local-name="Serbian" class="interlanguage-link-target"><span>Српски / srpski</span></a></li><li class="interlanguage-link interwiki-sv mw-list-item"><a href="https://sv.wikipedia.org/wiki/Muskelkontraktion" title="Muskelkontraktion – Swedish" lang="sv" hreflang="sv" data-title="Muskelkontraktion" data-language-autonym="Svenska" data-language-local-name="Swedish" class="interlanguage-link-target"><span>Svenska</span></a></li><li class="interlanguage-link interwiki-te mw-list-item"><a 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class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Activation of tension-generating sites in muscle</div> <p class="mw-empty-elt"> </p> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Smooth_muscle_cell_contraction.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/3/35/Smooth_muscle_cell_contraction.png/220px-Smooth_muscle_cell_contraction.png" decoding="async" width="220" height="204" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/3/35/Smooth_muscle_cell_contraction.png/330px-Smooth_muscle_cell_contraction.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/3/35/Smooth_muscle_cell_contraction.png/440px-Smooth_muscle_cell_contraction.png 2x" data-file-width="4843" data-file-height="4482" /></a><figcaption>Depiction of <a href="/wiki/Smooth_muscle" title="Smooth muscle">smooth muscle</a> contraction</figcaption></figure> <p><b>Muscle contraction</b> is the activation of <a href="/wiki/Tension_(physics)" title="Tension (physics)">tension</a>-generating sites within <a href="/wiki/Muscle_cell" title="Muscle cell">muscle cells</a>.<sup id="cite_ref-Widmaier_et_al_2008_1-0" class="reference"><a href="#cite_note-Widmaier_et_al_2008-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Silverthorn_2016_2-0" class="reference"><a href="#cite_note-Silverthorn_2016-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> In <a href="/wiki/Physiology" title="Physiology">physiology</a>, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as when holding something heavy in the same position.<sup id="cite_ref-Widmaier_et_al_2008_1-1" class="reference"><a href="#cite_note-Widmaier_et_al_2008-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> The termination of muscle contraction is followed by <b>muscle relaxation</b>, which is a return of the muscle fibers to their low tension-generating state.<sup id="cite_ref-Widmaier_et_al_2008_1-2" class="reference"><a href="#cite_note-Widmaier_et_al_2008-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p><p>For the contractions to happen, the muscle cells must rely on the change in action of two types of <a href="/wiki/Myofilament" title="Myofilament">filaments</a>: thin and thick filaments. </p><p>The major constituent of thin filaments is a chain formed by helical coiling of two strands of <a href="/wiki/Actin" title="Actin">actin</a>, and thick filaments dominantly consist of chains of the <a href="/wiki/Motor_protein" title="Motor protein">motor-protein</a> <a href="/wiki/Myosin" title="Myosin">myosin</a>. Together, these two filaments form myofibrils - the basic functional organelles in the skeletal muscle system. </p><p>In <a href="/wiki/Vertebrate" title="Vertebrate">vertebrates</a>, <a href="/wiki/Muscle_cell#Muscle_contraction_in_striated_muscle" title="Muscle cell">skeletal muscle contractions</a> are <a href="/wiki/Nervous_system" title="Nervous system">neurogenic</a> as they require <a href="/wiki/Synapse" title="Synapse">synaptic input</a> from <a href="/wiki/Motor_neurons" class="mw-redirect" title="Motor neurons">motor neurons</a>. A single motor neuron is able to innervate multiple muscle fibers, thereby causing the fibers to contract at the same time. Once innervated, the protein filaments within each skeletal muscle fiber slide past each other to produce a contraction, which is explained by the <a href="/wiki/Sliding_filament_theory" title="Sliding filament theory">sliding filament theory</a>. The contraction produced can be described as a <a href="#Gradation_of_skeletal_muscle_contractions">twitch</a>, summation, or tetanus, depending on the frequency of <a href="/wiki/Action_potentials" class="mw-redirect" title="Action potentials">action potentials</a>. In skeletal muscles, muscle tension is at its greatest when the muscle is stretched to an intermediate length as described by the length-tension relationship. </p><p>Unlike skeletal muscle, the contractions of <a href="/wiki/Smooth_muscle" title="Smooth muscle">smooth</a> and <a href="/wiki/Cardiac_muscle" title="Cardiac muscle">cardiac muscles</a> are <a href="/wiki/Myogenic_contraction" class="mw-redirect" title="Myogenic contraction">myogenic</a> (meaning that they are initiated by the smooth or heart muscle cells themselves instead of being stimulated by an outside event such as nerve stimulation), although they can be modulated by stimuli from the <a href="/wiki/Autonomic_nervous_system" title="Autonomic nervous system">autonomic nervous system</a>. The mechanisms of contraction in these <a href="/wiki/Muscle_tissue" class="mw-redirect" title="Muscle tissue">muscle tissues</a> are similar to those in skeletal muscle tissues. </p><p>Muscle contraction can also be described in terms of two variables: length and tension.<sup id="cite_ref-Widmaier_et_al_2008_1-3" class="reference"><a href="#cite_note-Widmaier_et_al_2008-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> In natural movements that underlie <a href="/wiki/Animal_locomotion" title="Animal locomotion">locomotor activity</a>, muscle contractions are multifaceted as they are able to produce changes in length and tension in a time-varying manner.<sup id="cite_ref-Biewener_1998_3-0" class="reference"><a href="#cite_note-Biewener_1998-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> Therefore, neither length nor tension is likely to remain the same in <a href="/wiki/Skeletal_muscle" title="Skeletal muscle">skeletal muscles</a> that contract during locomotion. Contractions can be described as <a href="/wiki/Isometric_exercise" title="Isometric exercise">isometric</a> if the muscle tension changes but the muscle length remains the same.<sup id="cite_ref-Widmaier_et_al_2008_1-4" class="reference"><a href="#cite_note-Widmaier_et_al_2008-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Aidley_1998_4-0" class="reference"><a href="#cite_note-Aidley_1998-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Sircar_2008_5-0" class="reference"><a href="#cite_note-Sircar_2008-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Bullock_et_al_2001_6-0" class="reference"><a href="#cite_note-Bullock_et_al_2001-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> In contrast, a muscle contraction is described as <a href="/wiki/Isotonic_contraction" title="Isotonic contraction">isotonic</a> if muscle tension remains the same throughout the contraction.<sup id="cite_ref-Widmaier_et_al_2008_1-5" class="reference"><a href="#cite_note-Widmaier_et_al_2008-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Aidley_1998_4-1" class="reference"><a href="#cite_note-Aidley_1998-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Sircar_2008_5-1" class="reference"><a href="#cite_note-Sircar_2008-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Bullock_et_al_2001_6-1" class="reference"><a href="#cite_note-Bullock_et_al_2001-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> If the muscle length shortens, the contraction is concentric;<sup id="cite_ref-Widmaier_et_al_2008_1-6" class="reference"><a href="#cite_note-Widmaier_et_al_2008-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Kumar_2004_7-0" class="reference"><a href="#cite_note-Kumar_2004-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> if the muscle length lengthens, the contraction is eccentric. </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Types">Types</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=1" title="Edit section: Types"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:1015_Types_of_Contraction_new.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/d/d4/1015_Types_of_Contraction_new.jpg/300px-1015_Types_of_Contraction_new.jpg" decoding="async" width="300" height="438" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/d/d4/1015_Types_of_Contraction_new.jpg/450px-1015_Types_of_Contraction_new.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/d/d4/1015_Types_of_Contraction_new.jpg/600px-1015_Types_of_Contraction_new.jpg 2x" data-file-width="741" data-file-height="1083" /></a><figcaption> Types of muscle contractions</figcaption></figure> <p>Muscle contractions can be described based on two variables: force and length. Force itself can be differentiated as either tension or load. Muscle tension is the force exerted by the muscle on an object whereas a load is the force exerted by an object on the muscle.<sup id="cite_ref-Widmaier_et_al_2008_1-7" class="reference"><a href="#cite_note-Widmaier_et_al_2008-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> When muscle tension changes without any corresponding changes in muscle length, the muscle contraction is described as isometric.<sup id="cite_ref-Widmaier_et_al_2008_1-8" class="reference"><a href="#cite_note-Widmaier_et_al_2008-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Aidley_1998_4-2" class="reference"><a href="#cite_note-Aidley_1998-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Sircar_2008_5-2" class="reference"><a href="#cite_note-Sircar_2008-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Bullock_et_al_2001_6-2" class="reference"><a href="#cite_note-Bullock_et_al_2001-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> If the muscle length changes while muscle tension remains the same, then the muscle contraction is isotonic.<sup id="cite_ref-Widmaier_et_al_2008_1-9" class="reference"><a href="#cite_note-Widmaier_et_al_2008-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Aidley_1998_4-3" class="reference"><a href="#cite_note-Aidley_1998-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Sircar_2008_5-3" class="reference"><a href="#cite_note-Sircar_2008-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Bullock_et_al_2001_6-3" class="reference"><a href="#cite_note-Bullock_et_al_2001-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> In an isotonic contraction, the muscle length can either shorten to produce a concentric contraction or lengthen to produce an eccentric contraction.<sup id="cite_ref-Widmaier_et_al_2008_1-10" class="reference"><a href="#cite_note-Widmaier_et_al_2008-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Kumar_2004_7-1" class="reference"><a href="#cite_note-Kumar_2004-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> In natural movements that underlie locomotor activity, muscle contractions are multifaceted as they are able to produce changes in length and tension in a time-varying manner.<sup id="cite_ref-Biewener_1998_3-1" class="reference"><a href="#cite_note-Biewener_1998-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> Therefore, neither length nor tension is likely to remain constant when the muscle is active during locomotor activity. </p> <div class="mw-heading mw-heading3"><h3 id="Isometric_contraction">Isometric contraction</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=2" title="Edit section: Isometric contraction"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1236090951">.mw-parser-output .hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Isometric_exercise" title="Isometric exercise">Isometric exercise</a></div> <p>An isometric contraction of a muscle generates tension without changing length.<sup id="cite_ref-Widmaier_et_al_2008_1-11" class="reference"><a href="#cite_note-Widmaier_et_al_2008-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Aidley_1998_4-4" class="reference"><a href="#cite_note-Aidley_1998-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Sircar_2008_5-4" class="reference"><a href="#cite_note-Sircar_2008-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Bullock_et_al_2001_6-4" class="reference"><a href="#cite_note-Bullock_et_al_2001-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> An example can be found when the muscles of the hand and <a href="/wiki/Forearm" title="Forearm">forearm</a> grip an object; the <a href="/wiki/Joint" title="Joint">joints</a> of the hand do not move, but muscles generate sufficient force to prevent the object from being dropped. </p> <div class="mw-heading mw-heading3"><h3 id="Isotonic_contraction">Isotonic contraction</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=3" title="Edit section: Isotonic contraction"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In <a href="/wiki/Isotonic_contraction" title="Isotonic contraction">isotonic contraction</a>, the tension in the muscle remains constant despite a change in muscle length.<sup id="cite_ref-Widmaier_et_al_2008_1-12" class="reference"><a href="#cite_note-Widmaier_et_al_2008-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Aidley_1998_4-5" class="reference"><a href="#cite_note-Aidley_1998-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Sircar_2008_5-5" class="reference"><a href="#cite_note-Sircar_2008-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Bullock_et_al_2001_6-5" class="reference"><a href="#cite_note-Bullock_et_al_2001-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> This occurs when a muscle's force of contraction matches the total load on the muscle. </p> <div class="mw-heading mw-heading4"><h4 id="Concentric_contraction">Concentric contraction</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=4" title="Edit section: Concentric contraction"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In concentric contraction, muscle tension is sufficient to overcome the load, and the muscle shortens as it contracts.<sup id="cite_ref-Faulkner2003_8-0" class="reference"><a href="#cite_note-Faulkner2003-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> This occurs when the force generated by the muscle exceeds the load opposing its contraction. </p><p>During a concentric contraction, a muscle is stimulated to contract according to the <a href="/wiki/Sliding_filament_theory" title="Sliding filament theory">sliding filament theory</a>. This occurs throughout the length of the muscle, generating a force at the origin and insertion, causing the muscle to shorten and changing the angle of the joint. In relation to the <a href="/wiki/Elbow-joint" class="mw-redirect" title="Elbow-joint">elbow</a>, a concentric contraction of the <a href="/wiki/Biceps" title="Biceps">biceps</a> would cause the arm to bend at the elbow as the hand moved from the leg to the shoulder (a <a href="/wiki/Biceps_curl" class="mw-redirect" title="Biceps curl">biceps curl</a>). A concentric contraction of the <a href="/wiki/Triceps_brachii_muscle" class="mw-redirect" title="Triceps brachii muscle">triceps</a> would change the angle of the joint in the opposite direction, straightening the arm and moving the hand towards the leg. </p> <div class="mw-heading mw-heading4"><h4 id="Eccentric_contraction">Eccentric contraction</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=5" title="Edit section: Eccentric contraction"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">See also: <a href="/wiki/Eccentric_training" title="Eccentric training">Eccentric training</a></div> <p>In eccentric contraction, the tension generated while isometric is insufficient to overcome the external load on the muscle and the muscle fibers lengthen as they contract.<sup id="cite_ref-Ucal_9-0" class="reference"><a href="#cite_note-Ucal-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> Rather than working to pull a joint in the direction of the muscle contraction, the muscle acts to <a href="/wiki/Acceleration" title="Acceleration">decelerate</a> the joint at the end of a movement or otherwise control the repositioning of a load. This can occur involuntarily (e.g., when attempting to move a weight too heavy for the muscle to lift) or voluntarily (e.g., when the muscle is 'smoothing out' a movement or resisting gravity such as during downhill walking). Over the short-term, <a href="/wiki/Strength_training" title="Strength training">strength training</a> involving both eccentric and concentric contractions appear to increase <a href="/wiki/Muscular_strength" class="mw-redirect" title="Muscular strength">muscular strength</a> more than training with concentric contractions alone.<sup id="cite_ref-Colliander_10-0" class="reference"><a href="#cite_note-Colliander-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> However, exercise-induced muscle damage is also greater during lengthening contractions.<sup id="cite_ref-pmid22539728_11-0" class="reference"><a href="#cite_note-pmid22539728-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> </p><p>During an eccentric contraction of the <a href="/wiki/Biceps_brachii_muscle" class="mw-redirect" title="Biceps brachii muscle">biceps muscle</a>, the elbow starts the movement while bent and then straightens as the hand moves away from the <a href="/wiki/Shoulder" title="Shoulder">shoulder</a>. During an eccentric contraction of the <a href="/wiki/Triceps_brachii_muscle" class="mw-redirect" title="Triceps brachii muscle">triceps muscle</a>, the elbow starts the movement straight and then bends as the hand moves towards the shoulder. <a href="/wiki/Desmin" title="Desmin">Desmin</a>, <a href="/wiki/Titin" title="Titin">titin</a>, and other z-line <a href="/wiki/Protein" title="Protein">proteins</a> are involved in eccentric contractions, but their mechanism is poorly understood in comparison to <a href="/wiki/Cross-bridge_cycling" class="mw-redirect" title="Cross-bridge cycling">cross-bridge cycling</a> in concentric contractions.<sup id="cite_ref-Ucal_9-1" class="reference"><a href="#cite_note-Ucal-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> </p><p>Though the muscle is doing a negative amount of <a href="/wiki/Mechanical_work" class="mw-redirect" title="Mechanical work">mechanical work</a>, (work is being done <i>on</i> the muscle), chemical energy (of fat or <a href="/wiki/Glucose" title="Glucose">glucose</a>, or temporarily stored in <a href="/wiki/Adenosine_triphosphate" title="Adenosine triphosphate">ATP</a>) is nevertheless consumed, although less than would be consumed during a concentric contraction of the same force. For example, one expends more energy going up a flight of stairs than going down the same flight. </p><p>Muscles undergoing heavy eccentric loading suffer greater damage when overloaded (such as during <a href="/wiki/Muscle_hypertrophy" title="Muscle hypertrophy">muscle building</a> or <a href="/wiki/Strength_training" title="Strength training">strength training</a> exercise) as compared to concentric loading. When eccentric contractions are used in weight training, they are normally called <i>negatives</i>. During a concentric contraction, contractile muscle <a href="/wiki/Myofilament" title="Myofilament">myofilaments</a> of <a href="/wiki/Myosin" title="Myosin">myosin</a> and <a href="/wiki/Actin" title="Actin">actin</a> slide past each other, pulling the Z-lines together. During an eccentric contraction, the myofilaments slide past each other the opposite way, though the actual movement of the myosin heads during an eccentric contraction is not known. Exercise featuring a heavy eccentric load can actually support a greater weight (muscles are approximately 40% stronger during eccentric contractions than during concentric contractions) and also results in greater muscular damage and <a href="/wiki/Delayed_onset_muscle_soreness" title="Delayed onset muscle soreness">delayed onset muscle soreness</a> one to two days after training. Exercise that incorporates both eccentric and concentric muscular contractions (i.e., involving a strong contraction and a controlled lowering of the weight) can produce greater gains in strength than concentric contractions alone.<sup id="cite_ref-Colliander_10-1" class="reference"><a href="#cite_note-Colliander-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-12" class="reference"><a href="#cite_note-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> While unaccustomed heavy eccentric contractions can easily lead to <a href="/wiki/Overtraining" title="Overtraining">overtraining</a>, moderate training may confer protection against injury.<sup id="cite_ref-Colliander_10-2" class="reference"><a href="#cite_note-Colliander-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading5"><h5 id="Eccentric_contractions_in_movement">Eccentric contractions in movement</h5><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=6" title="Edit section: Eccentric contractions in movement"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Eccentric contractions normally occur as a braking force in opposition to a concentric contraction to protect joints from damage. During virtually any routine movement, eccentric contractions assist in keeping motions smooth, but can also slow rapid movements such as a punch or throw. Part of training for rapid movements such as <a href="/wiki/Pitcher" title="Pitcher">pitching</a> during baseball involves reducing eccentric braking allowing a greater power to be developed throughout the movement. </p><p>Eccentric contractions are being researched for their ability to speed rehabilitation of weak or injured tendons. <a href="/wiki/Achilles_tendinitis" title="Achilles tendinitis">Achilles tendinitis</a><sup id="cite_ref-13" class="reference"><a href="#cite_note-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-14" class="reference"><a href="#cite_note-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup> and <a href="/wiki/Patellar_tendonitis" class="mw-redirect" title="Patellar tendonitis">patellar tendonitis</a><sup id="cite_ref-15" class="reference"><a href="#cite_note-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup> (also known as jumper's knee or patellar tendonosis) have been shown to benefit from high-load eccentric contractions. </p> <div class="mw-heading mw-heading2"><h2 id="Vertebrate">Vertebrate</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=7" title="Edit section: Vertebrate"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:Muscle_Tissue_(1).svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/f/f9/Muscle_Tissue_%281%29.svg/220px-Muscle_Tissue_%281%29.svg.png" decoding="async" width="220" height="165" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/f/f9/Muscle_Tissue_%281%29.svg/330px-Muscle_Tissue_%281%29.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/f/f9/Muscle_Tissue_%281%29.svg/440px-Muscle_Tissue_%281%29.svg.png 2x" data-file-width="512" data-file-height="384" /></a><figcaption>In vertebrate animals, there are three types of muscle tissues: 1) skeletal, 2) smooth, and 3) cardiac</figcaption></figure> <p>In <a href="/wiki/Vertebrate" title="Vertebrate">vertebrate animals</a>, there are three types of <a href="/wiki/Muscle_tissue" class="mw-redirect" title="Muscle tissue">muscle tissues</a>: skeletal, smooth, and cardiac. <a href="/wiki/Skeletal_muscle" title="Skeletal muscle">Skeletal muscle</a> constitutes the majority of muscle mass in the body and is responsible for locomotor activity. <a href="/wiki/Smooth_muscle" title="Smooth muscle">Smooth muscle</a> forms <a href="/wiki/Blood_vessels" class="mw-redirect" title="Blood vessels">blood vessels</a>, the <a href="/wiki/Gastrointestinal_tract" title="Gastrointestinal tract">gastrointestinal tract</a>, and other areas in the body that produce sustained contractions. <a href="/wiki/Cardiac_muscle" title="Cardiac muscle">Cardiac muscle</a> makes up the heart, which pumps blood. Skeletal and cardiac muscles are called <a href="/wiki/Striated_muscle" class="mw-redirect" title="Striated muscle">striated muscle</a> because of their striped appearance under a microscope, which is due to the highly organized alternating pattern of A bands and I bands. </p> <div class="mw-heading mw-heading3"><h3 id="Skeletal_muscle">Skeletal muscle</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=8" title="Edit section: Skeletal muscle"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:1022_Muscle_Fibers_(small).jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/9/9c/1022_Muscle_Fibers_%28small%29.jpg/220px-1022_Muscle_Fibers_%28small%29.jpg" decoding="async" width="220" height="176" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/9/9c/1022_Muscle_Fibers_%28small%29.jpg/330px-1022_Muscle_Fibers_%28small%29.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/9/9c/1022_Muscle_Fibers_%28small%29.jpg/440px-1022_Muscle_Fibers_%28small%29.jpg 2x" data-file-width="801" data-file-height="642" /></a><figcaption>Organization of skeletal muscle</figcaption></figure> <p>Excluding reflexes, all <a href="/wiki/Skeletal_muscle" title="Skeletal muscle">skeletal muscle</a> contractions occur as a result of signals originating in the brain. The brain sends electrochemical signals through the <a href="/wiki/Nervous_system" title="Nervous system">nervous system</a> to the <a href="/wiki/Motor_neuron" title="Motor neuron">motor neuron</a> that <a href="/wiki/Nerve" title="Nerve">innervates</a> several muscle fibers.<sup id="cite_ref-16" class="reference"><a href="#cite_note-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup> In the case of some <a href="/wiki/Reflexes" class="mw-redirect" title="Reflexes">reflexes</a>, the signal to contract can originate in the <a href="/wiki/Spinal_cord" title="Spinal cord">spinal cord</a> through a feedback loop with the grey matter. Other actions such as locomotion, breathing, and chewing have a reflex aspect to them: the contractions can be initiated either consciously or unconsciously. </p> <div class="mw-heading mw-heading4"><h4 id="Neuromuscular_junction">Neuromuscular junction</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=9" title="Edit section: Neuromuscular junction"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:1009_Motor_End_Plate_and_Innervation.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/5/57/1009_Motor_End_Plate_and_Innervation.jpg/220px-1009_Motor_End_Plate_and_Innervation.jpg" decoding="async" width="220" height="370" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/5/57/1009_Motor_End_Plate_and_Innervation.jpg/330px-1009_Motor_End_Plate_and_Innervation.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/5/57/1009_Motor_End_Plate_and_Innervation.jpg/440px-1009_Motor_End_Plate_and_Innervation.jpg 2x" data-file-width="1531" data-file-height="2575" /></a><figcaption>Structure of neuromuscular junction.</figcaption></figure> <p>A <a href="/wiki/Neuromuscular_junction" title="Neuromuscular junction">neuromuscular junction</a> is a <a href="/wiki/Chemical_synapse" title="Chemical synapse">chemical synapse</a> formed by the contact between a <a href="/wiki/Motor_neuron" title="Motor neuron">motor neuron</a> and a <a href="/wiki/Muscle_fiber" class="mw-redirect" title="Muscle fiber">muscle fiber</a>.<sup id="cite_ref-Levitan_2015_17-0" class="reference"><a href="#cite_note-Levitan_2015-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup> It is the site in which a motor neuron transmits a signal to a muscle fiber to initiate muscle contraction. The sequence of events that results in the depolarization of the muscle fiber at the neuromuscular junction begins when an action potential is initiated in the cell body of a motor neuron, which is then propagated by <a href="/wiki/Saltatory_conduction" title="Saltatory conduction">saltatory conduction</a> along its axon toward the neuromuscular junction. Once it reaches the <a href="/wiki/Chemical_synapse" title="Chemical synapse">terminal bouton</a>, the action potential causes a <a href="/wiki/Calcium_in_biology" title="Calcium in biology"><span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> ion</a> influx into the terminal by way of the <a href="/wiki/Voltage-gated_calcium_channel" title="Voltage-gated calcium channel">voltage-gated calcium channels</a>. The <a href="/wiki/Calcium_signaling" title="Calcium signaling"><span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> influx</a> causes <a href="/wiki/Synaptic_vesicles" class="mw-redirect" title="Synaptic vesicles">synaptic vesicles</a> containing the neurotransmitter <a href="/wiki/Acetylcholine" title="Acetylcholine">acetylcholine</a> to fuse with the plasma membrane, releasing acetylcholine into the <a href="/wiki/Synaptic_cleft" class="mw-redirect" title="Synaptic cleft">synaptic cleft</a> between the motor neuron terminal and the neuromuscular junction of the skeletal muscle fiber. Acetylcholine diffuses across the synapse and binds to and activates <a href="/wiki/Nicotinic_acetylcholine_receptors" class="mw-redirect" title="Nicotinic acetylcholine receptors">nicotinic acetylcholine receptors</a> on the neuromuscular junction. Activation of the nicotinic receptor opens its intrinsic <a href="/wiki/Sodium" title="Sodium">sodium</a>/<a href="/wiki/Potassium" title="Potassium">potassium</a> channel, causing sodium to rush in and potassium to trickle out. As a result, the <a href="/wiki/Sarcolemma" title="Sarcolemma">sarcolemma</a> reverses polarity and its voltage quickly jumps from the resting membrane potential of -90mV to as high as +75mV as sodium enters. The membrane potential then becomes hyperpolarized when potassium exits and is then adjusted back to the resting membrane potential. This rapid fluctuation is called the end-plate potential.<sup id="cite_ref-Saladin,_Kenneth_S._2015_18-0" class="reference"><a href="#cite_note-Saladin,_Kenneth_S._2015-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> The voltage-gated ion channels of the sarcolemma next to the end plate open in response to the end plate potential. They are sodium and potassium specific and only allow one through. This wave of ion movements creates the action potential that spreads from the motor end plate in all directions.<sup id="cite_ref-Saladin,_Kenneth_S._2015_18-1" class="reference"><a href="#cite_note-Saladin,_Kenneth_S._2015-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> If action potentials stop arriving, then acetylcholine ceases to be released from the terminal bouton. The remaining acetylcholine in the synaptic cleft is either degraded by active <a href="/wiki/Acetylcholine_esterase" class="mw-redirect" title="Acetylcholine esterase">acetylcholine esterase</a> or reabsorbed by the synaptic knob and none is left to replace the degraded acetylcholine. </p> <div class="mw-heading mw-heading4"><h4 id="Excitation–contraction_coupling"><span id="Excitation.E2.80.93contraction_coupling"></span>Excitation–contraction coupling</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=10" title="Edit section: Excitation–contraction coupling"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Excitation–contraction coupling (ECC) is the process by which a <a href="/wiki/Action_potential#Muscular_action_potentials" title="Action potential">muscular action potential</a> in the muscle fiber causes <a href="/wiki/Myofibril" title="Myofibril">myofibrils</a> to contract. In skeletal muscles, excitation–contraction coupling relies on a direct coupling between two key proteins, the <a href="/wiki/Sarcoplasmic_reticulum" title="Sarcoplasmic reticulum">sarcoplasmic reticulum</a> (SR) calcium release channel identified as the <a href="/wiki/Ryanodine_receptor_1" title="Ryanodine receptor 1">ryanodine receptor 1</a> (RYR1) and the <a href="/wiki/Voltage-dependent_calcium_channel" class="mw-redirect" title="Voltage-dependent calcium channel">voltage-gated L-type calcium channel</a> identified as <a href="/wiki/Dihydropyridine_receptor" class="mw-redirect" title="Dihydropyridine receptor">dihydropyridine receptors</a>, (DHPRs). DHPRs are located on the sarcolemma (which includes the surface sarcolemma and the <a href="/wiki/Transverse_tubules" class="mw-redirect" title="Transverse tubules">transverse tubules</a>), while the RyRs reside across the SR membrane. The close apposition of a transverse tubule and two SR regions containing RyRs is described as a triad and is predominantly where excitation–contraction coupling takes place. </p> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Interactions_within_Excitation-contraction_Coupling.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/2/28/Interactions_within_Excitation-contraction_Coupling.jpg/220px-Interactions_within_Excitation-contraction_Coupling.jpg" decoding="async" width="220" height="230" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/2/28/Interactions_within_Excitation-contraction_Coupling.jpg/330px-Interactions_within_Excitation-contraction_Coupling.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/2/28/Interactions_within_Excitation-contraction_Coupling.jpg/440px-Interactions_within_Excitation-contraction_Coupling.jpg 2x" data-file-width="611" data-file-height="639" /></a><figcaption>Picture showing the different interactions within the ECC pathway</figcaption></figure> <p>Excitation–contraction coupling (ECC) occurs when depolarization of skeletal muscles (usually through neural innervation) results in a muscle action potential. This action potential spreads across the muscle's surface and into the muscle fiber's network of <a href="/wiki/T-tubule" title="T-tubule">T-tubules</a>, depolarizing the inner portion of the muscle fiber. This activates dihydropyridine receptors in the <a href="/wiki/Terminal_cisternae" title="Terminal cisternae">terminal cisternae</a>, which are in close proximity to ryanodine receptors in the adjacent <a href="/wiki/Endoplasmic_reticulum#Sarcoplasmic_reticulum" title="Endoplasmic reticulum">sarcoplasmic reticulum</a>. The activated dihydropyridine receptors physically interact with ryanodine receptors to activate them via foot processes (involving conformational changes that allosterically activates the ryanodine receptors). As ryanodine receptors open, Ca<sup>2+</sup> is released from the sarcoplasmic reticulum into the local junctional space and diffuses into the bulk cytoplasm to cause a <a href="/wiki/Calcium_spark" class="mw-redirect" title="Calcium spark">calcium spark</a>.<sup id="cite_ref-Lanner_a003996_19-0" class="reference"><a href="#cite_note-Lanner_a003996-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup> The action potential creates a near synchronous activation of thousands of <a href="/wiki/Calcium_sparks" title="Calcium sparks">calcium sparks</a> and causes a cell-wide increase in calcium giving rise to the upstroke of the <a href="/w/index.php?title=Calcium_transient&action=edit&redlink=1" class="new" title="Calcium transient (page does not exist)">calcium transient</a>. The Ca<sup>2+</sup> released into the cytosol binds to <a href="/wiki/Troponin_C" title="Troponin C">Troponin C</a> by the <a href="/wiki/Actin_filaments" class="mw-redirect" title="Actin filaments">actin filaments</a>. This bond allows the actin filaments to perform <a href="/wiki/Cross-bridge_cycling" class="mw-redirect" title="Cross-bridge cycling">cross-bridge cycling</a>, producing force and, in some situations, motion. </p><p>When the desired motion is accomplished, relaxation can be achieved quickly through numerous pathways. Relaxation is quickly achieved through a Ca<sup>2+</sup> buffer with various cytoplasmic proteins binding to Ca<sup>2+</sup> with very high affinity.<sup id="cite_ref-20" class="reference"><a href="#cite_note-20"><span class="cite-bracket">[</span>20<span class="cite-bracket">]</span></a></sup> These cytoplasmic proteins allow for quick relaxation in fast twitch muscles. Although slower, the <a href="/wiki/SERCA" title="SERCA">sarco/endoplasmic reticulum calcium-ATPase</a> (SERCA) actively pumps Ca<sup>2+</sup> back into the sarcoplasmic reticulum, resulting in a permanent relaxation until the next action potential arrives.<sup id="cite_ref-Lanner_a003996_19-1" class="reference"><a href="#cite_note-Lanner_a003996-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup> </p><p>Mitochondria also participate in Ca<sup>2+</sup> reuptake, ultimately delivering their gathered Ca<sup>2+</sup> to SERCA for storage in the sarcoplasmic reticulum. A few of the relaxation mechanisms (NCX, Ca2+ pumps and Ca2+ leak channels) move Ca2+ completely out of the cells as well.<sup id="cite_ref-21" class="reference"><a href="#cite_note-21"><span class="cite-bracket">[</span>21<span class="cite-bracket">]</span></a></sup> As Ca<sup>2+</sup> concentration declines to resting levels, Ca2+ releases from Troponin C, disallowing cross bridge-cycling, causing the force to decline and relaxation to occur. Once relaxation has fully occurred, the muscle is able to contract again, thus fully resetting the cycle. </p> <div class="mw-heading mw-heading4"><h4 id="Sliding_filament_theory">Sliding filament theory</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=11" title="Edit section: Sliding filament theory"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:Sarcomere.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/6/6e/Sarcomere.svg/220px-Sarcomere.svg.png" decoding="async" width="220" height="170" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/6/6e/Sarcomere.svg/330px-Sarcomere.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/6/6e/Sarcomere.svg/440px-Sarcomere.svg.png 2x" data-file-width="1350" data-file-height="1044" /></a><figcaption>Sliding filament theory: A sarcomere in relaxed (above) and contracted (below) positions</figcaption></figure> <p>The <a href="/wiki/Sliding_filament_theory" title="Sliding filament theory">sliding filament theory</a> describes a process used by <a href="/wiki/Muscle" title="Muscle">muscles</a> to contract. It is a cycle of repetitive events that cause a thin filament to slide over a thick filament and generate tension in the muscle.<sup id="cite_ref-Saladin_2012_22-0" class="reference"><a href="#cite_note-Saladin_2012-22"><span class="cite-bracket">[</span>22<span class="cite-bracket">]</span></a></sup> It was independently developed by <a href="/wiki/Andrew_F._Huxley" class="mw-redirect" title="Andrew F. Huxley">Andrew Huxley</a> and <a href="/wiki/Rolf_Niedergerke" title="Rolf Niedergerke">Rolf Niedergerke</a> and by <a href="/wiki/Hugh_Huxley" title="Hugh Huxley">Hugh Huxley</a> and <a href="/wiki/Jean_Hanson" title="Jean Hanson">Jean Hanson</a> in 1954.<sup id="cite_ref-Huxley_AF,_Niedergerke_R_1954_971–973_23-0" class="reference"><a href="#cite_note-Huxley_AF,_Niedergerke_R_1954_971–973-23"><span class="cite-bracket">[</span>23<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Huxley_H,_Hanson_J_1954_973–976_24-0" class="reference"><a href="#cite_note-Huxley_H,_Hanson_J_1954_973–976-24"><span class="cite-bracket">[</span>24<span class="cite-bracket">]</span></a></sup> Physiologically, this contraction is not uniform across the sarcomere; the central position of the thick filaments becomes unstable and can shift during contraction but this is countered by the actions of the elastic myofilament of <a href="/wiki/Titin" title="Titin">titin</a>. This fine myofilament maintains uniform tension across the sarcomere by pulling the thick filament into a central position.<sup id="cite_ref-pmid3680378_25-0" class="reference"><a href="#cite_note-pmid3680378-25"><span class="cite-bracket">[</span>25<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading5"><h5 id="Cross-bridge_cycle">Cross-bridge cycle</h5><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=12" title="Edit section: Cross-bridge cycle"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:1008_Skeletal_Muscle_Contraction.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/2/24/1008_Skeletal_Muscle_Contraction.jpg/220px-1008_Skeletal_Muscle_Contraction.jpg" decoding="async" width="220" height="342" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/2/24/1008_Skeletal_Muscle_Contraction.jpg/330px-1008_Skeletal_Muscle_Contraction.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/2/24/1008_Skeletal_Muscle_Contraction.jpg/440px-1008_Skeletal_Muscle_Contraction.jpg 2x" data-file-width="811" data-file-height="1259" /></a><figcaption>Cross-bridge cycle</figcaption></figure> <p>Cross-bridge cycling is a sequence of molecular events that underlies the sliding filament theory. A <a href="/wiki/Sliding_filament_theory#Cross-bridge_mechanism" title="Sliding filament theory">cross-bridge</a> is a myosin projection, consisting of two myosin heads, that extends from the thick filaments.<sup id="cite_ref-Widmaier_et_al_2008_1-13" class="reference"><a href="#cite_note-Widmaier_et_al_2008-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> Each myosin head has two binding sites: one for <a href="/wiki/Adenosine_triphosphate" title="Adenosine triphosphate">adenosine triphosphate</a> (ATP) and another for actin. The binding of ATP to a myosin head detaches myosin from <a href="/wiki/Actin" title="Actin">actin</a>, thereby allowing myosin to bind to another actin molecule. Once attached, the ATP is hydrolyzed by myosin, which uses the released energy to move into the "cocked position" whereby it binds weakly to a part of the actin binding site. The remainder of the actin binding site is blocked by <a href="/wiki/Tropomyosin" title="Tropomyosin">tropomyosin</a>.<sup id="cite_ref-Enoka_2013_26-0" class="reference"><a href="#cite_note-Enoka_2013-26"><span class="cite-bracket">[</span>26<span class="cite-bracket">]</span></a></sup> With the ATP hydrolyzed, the cocked myosin head now contains <a href="/wiki/Adenosine_diphosphate" title="Adenosine diphosphate">adenosine diphosphate</a> (ADP) + <a href="/wiki/Inorganic_phosphate" class="mw-redirect" title="Inorganic phosphate">P<sub>i</sub></a>. Two <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> ions bind to <a href="/wiki/Troponin_C" title="Troponin C">troponin C</a> on the actin filaments. The troponin-<span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> complex causes tropomyosin to slide over and unblock the remainder of the actin binding site. Unblocking the rest of the actin binding sites allows the two myosin heads to close and myosin to bind strongly to actin.<sup id="cite_ref-Enoka_2013_26-1" class="reference"><a href="#cite_note-Enoka_2013-26"><span class="cite-bracket">[</span>26<span class="cite-bracket">]</span></a></sup> The myosin head then releases the inorganic phosphate and initiates a <i>power stroke,</i> which generates a force of 2 pN. The power stroke moves the actin filament inwards, thereby shortening the <a href="/wiki/Sarcomere" title="Sarcomere">sarcomere</a>. Myosin then releases ADP but still remains tightly bound to actin. At the end of the power stroke, ADP is released from the myosin head, leaving myosin attached to actin in a rigor state until another ATP binds to myosin. A lack of ATP would result in the rigor state characteristic of <a href="/wiki/Rigor_mortis" title="Rigor mortis">rigor mortis</a>. Once another ATP binds to myosin, the myosin head will again detach from actin and another cross-bridge cycle occurs. </p><p>Cross-bridge cycling is able to continue as long as there are sufficient amounts of ATP and <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> in the cytoplasm.<sup id="cite_ref-Enoka_2013_26-2" class="reference"><a href="#cite_note-Enoka_2013-26"><span class="cite-bracket">[</span>26<span class="cite-bracket">]</span></a></sup> Termination of cross-bridge cycling can occur when <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> is <a href="/wiki/Active_transport" title="Active transport">actively pumped</a> back into the sarcoplasmic reticulum. When <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> is no longer present on the thin filament, the tropomyosin changes conformation back to its previous state so as to block the binding sites again. The myosin ceases binding to the thin filament, and the muscle relaxes. The <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> ions leave the troponin molecule to maintain the <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> ion concentration in the sarcoplasm. The active pumping of <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> ions into the sarcoplasmic reticulum creates a deficiency in the fluid around the myofibrils. This causes the removal of <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> ions from the troponin. Thus, the tropomyosin-troponin complex again covers the binding sites on the actin filaments and contraction ceases. </p> <div class="mw-heading mw-heading4"><h4 id="Gradation_of_skeletal_muscle_contractions">Gradation of skeletal muscle contractions</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=13" title="Edit section: Gradation of skeletal muscle contractions"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1237032888/mw-parser-output/.tmulti">.mw-parser-output .tmulti .multiimageinner{display:flex;flex-direction:column}.mw-parser-output .tmulti .trow{display:flex;flex-direction:row;clear:left;flex-wrap:wrap;width:100%;box-sizing:border-box}.mw-parser-output .tmulti .tsingle{margin:1px;float:left}.mw-parser-output .tmulti .theader{clear:both;font-weight:bold;text-align:center;align-self:center;background-color:transparent;width:100%}.mw-parser-output .tmulti .thumbcaption{background-color:transparent}.mw-parser-output .tmulti .text-align-left{text-align:left}.mw-parser-output .tmulti .text-align-right{text-align:right}.mw-parser-output .tmulti .text-align-center{text-align:center}@media all and (max-width:720px){.mw-parser-output .tmulti .thumbinner{width:100%!important;box-sizing:border-box;max-width:none!important;align-items:center}.mw-parser-output .tmulti .trow{justify-content:center}.mw-parser-output .tmulti .tsingle{float:none!important;max-width:100%!important;box-sizing:border-box;text-align:center}.mw-parser-output .tmulti .tsingle .thumbcaption{text-align:left}.mw-parser-output .tmulti .trow>.thumbcaption{text-align:center}}@media screen{html.skin-theme-clientpref-night .mw-parser-output .tmulti .multiimageinner img{background-color:white}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .tmulti .multiimageinner img{background-color:white}}</style><div class="thumb tmulti tright"><div class="thumbinner multiimageinner" style="width:223px;max-width:223px"><div class="trow"><div class="tsingle" style="width:221px;max-width:221px"><div class="thumbimage"><span typeof="mw:File"><a href="/wiki/File:1012_Muscle_Twitch_Myogram.jpg" class="mw-file-description"><img alt="Twitch" src="//upload.wikimedia.org/wikipedia/commons/thumb/b/b5/1012_Muscle_Twitch_Myogram.jpg/219px-1012_Muscle_Twitch_Myogram.jpg" decoding="async" width="219" height="140" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/b/b5/1012_Muscle_Twitch_Myogram.jpg/329px-1012_Muscle_Twitch_Myogram.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/b/b5/1012_Muscle_Twitch_Myogram.jpg/438px-1012_Muscle_Twitch_Myogram.jpg 2x" data-file-width="789" data-file-height="506" /></a></span></div><div class="thumbcaption">Twitch</div></div></div><div class="trow"><div class="tsingle" style="width:221px;max-width:221px"><div class="thumbimage"><span typeof="mw:File"><a href="/wiki/File:1013_Summation_Tetanus.jpg" class="mw-file-description"><img alt="Summation and tetanus" src="//upload.wikimedia.org/wikipedia/commons/thumb/d/d4/1013_Summation_Tetanus.jpg/219px-1013_Summation_Tetanus.jpg" decoding="async" width="219" height="90" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/d/d4/1013_Summation_Tetanus.jpg/329px-1013_Summation_Tetanus.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/d/d4/1013_Summation_Tetanus.jpg/438px-1013_Summation_Tetanus.jpg 2x" data-file-width="763" data-file-height="314" /></a></span></div><div class="thumbcaption">Summation and tetanus</div></div></div><div class="trow" style="display:flex"><div class="thumbcaption">Three types of skeletal muscle contractions</div></div></div></div> <p>The strength of skeletal muscle contractions can be broadly separated into <a href="/wiki/Muscle_twitch" class="mw-redirect" title="Muscle twitch">twitch</a>, summation, and <a href="/wiki/Tetanic_contractions" class="mw-redirect" title="Tetanic contractions">tetanus</a>. A twitch is a single contraction and relaxation cycle produced by an action potential within the muscle fiber itself.<sup id="cite_ref-Feher_2012_27-0" class="reference"><a href="#cite_note-Feher_2012-27"><span class="cite-bracket">[</span>27<span class="cite-bracket">]</span></a></sup> The time between a stimulus to the motor nerve and the subsequent contraction of the innervated muscle is called the <b>latent period</b>, which usually takes about 10 ms and is caused by the time taken for nerve action potential to propagate, the time for chemical transmission at the neuromuscular junction, then the subsequent steps in excitation-contraction coupling.<sup id="cite_ref-28" class="reference"><a href="#cite_note-28"><span class="cite-bracket">[</span>28<span class="cite-bracket">]</span></a></sup> </p><p>If another muscle action potential were to be produced before the complete relaxation of a muscle twitch, then the next twitch will simply sum onto the previous twitch, thereby producing a <i>summation</i>.<sup id="cite_ref-Smith_29-0" class="reference"><a href="#cite_note-Smith-29"><span class="cite-bracket">[</span>29<span class="cite-bracket">]</span></a></sup> Summation can be achieved in two ways:<sup id="cite_ref-30" class="reference"><a href="#cite_note-30"><span class="cite-bracket">[</span>30<span class="cite-bracket">]</span></a></sup> <i>frequency summation</i> and <i>multiple fiber summation</i>. In <b>frequency summation</b>, the force exerted by the skeletal muscle is controlled by varying the frequency at which <a href="/wiki/Action_potential" title="Action potential">action potentials</a> are sent to muscle fibers. Action potentials do not arrive at muscles synchronously, and, during a contraction, some fraction of the fibers in the muscle will be firing at any given time. In a typical circumstance, when humans are exerting their muscles as hard as they are consciously able, roughly one-third of the fibers in each of those muscles will fire at once<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (August 2020)">citation needed</span></a></i>]</sup>, though this ratio can be affected by various physiological and psychological factors (including <a href="/wiki/Golgi_tendon_organ" title="Golgi tendon organ">Golgi tendon organs</a> and <a href="/wiki/Renshaw_cell" title="Renshaw cell">Renshaw cells</a>). This 'low' level of contraction is a protective mechanism to prevent <a href="/wiki/Avulsion_fracture" title="Avulsion fracture">avulsion</a> of the tendon—the force generated by a 95% contraction of all fibers is sufficient to damage the body. In <b>multiple fiber summation</b>, if the central nervous system sends a weak signal to contract a muscle, the smaller <a href="/wiki/Motor_unit" title="Motor unit">motor units</a>, being more excitable than the larger ones, are stimulated first. As the <a href="/wiki/Summation_(neurophysiology)" title="Summation (neurophysiology)">strength of the signal</a> increases, more motor units are excited in addition to larger ones, with the largest motor units having as much as 50 times the contractile strength as the smaller ones. As more and larger motor units are activated, the force of muscle contraction becomes progressively stronger. A concept known as the size principle, allows for a gradation of muscle force during weak contraction to occur in small steps, which then become progressively larger when greater amounts of force are required. </p><p>Finally, if the frequency of muscle action potentials increases such that the muscle contraction reaches its peak force and plateaus at this level, then the contraction is a <i>tetanus</i>. </p> <div class="mw-heading mw-heading4"><h4 id="Length-tension_relationship">Length-tension relationship</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=14" title="Edit section: Length-tension relationship"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Further information: <a href="/wiki/Hill%27s_muscle_model" title="Hill's muscle model">Hill's muscle model</a></div> <figure class="mw-default-size mw-halign-left" typeof="mw:File/Thumb"><a href="/wiki/File:Lengthtension.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/3/3e/Lengthtension.jpg/220px-Lengthtension.jpg" decoding="async" width="220" height="163" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/3/3e/Lengthtension.jpg/330px-Lengthtension.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/3/3e/Lengthtension.jpg/440px-Lengthtension.jpg 2x" data-file-width="683" data-file-height="505" /></a><figcaption>Muscle length versus isometric force</figcaption></figure> <p>Length-tension relationship relates the strength of an isometric contraction to the length of the muscle at which the contraction occurs. Muscles operate with greatest active tension when close to an ideal length (often their resting length). When stretched or shortened beyond this (whether due to the action of the muscle itself or by an outside force), the maximum active tension generated decreases.<sup id="cite_ref-1stLT_31-0" class="reference"><a href="#cite_note-1stLT-31"><span class="cite-bracket">[</span>31<span class="cite-bracket">]</span></a></sup> This decrease is minimal for small deviations, but the tension drops off rapidly as the length deviates further from the ideal. Due to the presence of elastic proteins within a muscle cell (such as <a href="/wiki/Titin" title="Titin">titin</a>) and extracellular matrix, as the muscle is stretched beyond a given length, there is an entirely passive tension, which opposes lengthening. Combined, there is a strong resistance to lengthening an active muscle far beyond the peak of active tension. </p> <div class="mw-heading mw-heading4"><h4 id="Force-velocity_relationships">Force-velocity relationships</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=15" title="Edit section: Force-velocity relationships"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:Muscle_Force_Velocity_relationship.png" class="mw-file-description"><img alt="Force–velocity relationship of muscle contraction" src="//upload.wikimedia.org/wikipedia/en/thumb/2/22/Muscle_Force_Velocity_relationship.png/300px-Muscle_Force_Velocity_relationship.png" decoding="async" width="300" height="205" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/2/22/Muscle_Force_Velocity_relationship.png/450px-Muscle_Force_Velocity_relationship.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/2/22/Muscle_Force_Velocity_relationship.png/600px-Muscle_Force_Velocity_relationship.png 2x" data-file-width="911" data-file-height="623" /></a><figcaption>Force–velocity relationship: right of the vertical axis concentric contractions (the muscle is shortening), left of the axis eccentric contractions (the muscle is lengthened under load); power developed by the muscle in red. Since power is equal to force times velocity, the muscle generates no power at either isometric force (due to zero velocity) or maximal velocity (due to zero force). The optimal shortening velocity for power generation is approximately one-third of maximum shortening velocity.</figcaption></figure> <p>Force–velocity relationship relates the speed at which a muscle changes its length (usually regulated by external forces, such as load or other muscles) to the amount of force that it generates. Force declines in a hyperbolic fashion relative to the isometric force as the shortening velocity increases, eventually reaching zero at some maximum velocity. The reverse holds true for when the muscle is stretched – force increases above isometric maximum, until finally reaching an absolute maximum. This intrinsic property of active muscle tissue plays a role in the active damping of joints that are actuated by simultaneously active opposing muscles. In such cases, the force-velocity profile enhances the force produced by the lengthening muscle at the expense of the shortening muscle. This favoring of whichever muscle returns the joint to equilibrium effectively increases the damping of the joint. Moreover, the strength of the damping increases with muscle force. The motor system can thus actively control joint damping via the simultaneous contraction (co-contraction) of opposing muscle groups.<sup id="cite_ref-32" class="reference"><a href="#cite_note-32"><span class="cite-bracket">[</span>32<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Smooth_muscle">Smooth muscle</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=16" title="Edit section: Smooth muscle"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:1029_Smooth_Muscle_Motor_Units.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/0/09/1029_Smooth_Muscle_Motor_Units.jpg/400px-1029_Smooth_Muscle_Motor_Units.jpg" decoding="async" width="400" height="172" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/0/09/1029_Smooth_Muscle_Motor_Units.jpg/600px-1029_Smooth_Muscle_Motor_Units.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/0/09/1029_Smooth_Muscle_Motor_Units.jpg/800px-1029_Smooth_Muscle_Motor_Units.jpg 2x" data-file-width="1012" data-file-height="434" /></a><figcaption>Swellings called varicosities belonging to an autonomic neuron innervate the smooth muscle cells.</figcaption></figure> <p><a href="/wiki/Smooth_muscle" title="Smooth muscle">Smooth muscles</a> can be divided into two subgroups: <a href="/wiki/Single-unit_smooth_muscle" class="mw-redirect" title="Single-unit smooth muscle">single-unit</a> and <a href="/wiki/Multiunit_smooth_muscle" class="mw-redirect" title="Multiunit smooth muscle">multiunit</a>. Single-unit smooth muscle cells can be found in the gut and blood vessels. Because these cells are linked together by gap junctions, they are able to contract as a functional <a href="/wiki/Syncytium#Cardiac_muscle" title="Syncytium">syncytium</a>. Single-unit smooth muscle cells contract myogenically, which can be modulated by the autonomic nervous system. </p><p>Unlike single-unit smooth muscle cells, multiunit smooth muscle cells are found in the muscle of the eye and in the base of hair follicles. Multiunit smooth muscle cells contract by being separately stimulated by nerves of the autonomic nervous system. As such, they allow for fine control and gradual responses, much like <a href="/wiki/Motor_unit_recruitment" title="Motor unit recruitment">motor unit recruitment</a> in skeletal muscle. </p> <div class="mw-heading mw-heading4"><h4 id="Mechanisms_of_smooth_muscle_contraction">Mechanisms of smooth muscle contraction</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=17" title="Edit section: Mechanisms of smooth muscle contraction"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Further information: <a href="/wiki/Myogenic_mechanism" title="Myogenic mechanism">Myogenic mechanism</a> and <a href="/wiki/Myogenic_tone" title="Myogenic tone">Myogenic tone</a></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1237032888/mw-parser-output/.tmulti"><div class="thumb tmulti tright"><div class="thumbinner multiimageinner" style="width:304px;max-width:304px"><div class="trow"><div class="tsingle" style="width:302px;max-width:302px"><div class="thumbimage"><span typeof="mw:File"><a href="/wiki/File:1028_Smooth_Muscle_Contraction.jpg" class="mw-file-description"><img alt="Smooth muscle contractions" src="//upload.wikimedia.org/wikipedia/commons/thumb/5/54/1028_Smooth_Muscle_Contraction.jpg/300px-1028_Smooth_Muscle_Contraction.jpg" decoding="async" width="300" height="62" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/5/54/1028_Smooth_Muscle_Contraction.jpg/450px-1028_Smooth_Muscle_Contraction.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/5/54/1028_Smooth_Muscle_Contraction.jpg/600px-1028_Smooth_Muscle_Contraction.jpg 2x" data-file-width="1126" data-file-height="233" /></a></span></div><div class="thumbcaption">Smooth muscle contractions</div></div></div><div class="trow"><div class="tsingle" style="width:302px;max-width:302px"><div class="thumbimage"><span typeof="mw:File"><a href="/wiki/File:Actin_myosin_filaments.png" class="mw-file-description"><img alt="Frog jumping" src="//upload.wikimedia.org/wikipedia/commons/thumb/b/bc/Actin_myosin_filaments.png/300px-Actin_myosin_filaments.png" decoding="async" width="300" height="131" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/b/bc/Actin_myosin_filaments.png/450px-Actin_myosin_filaments.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/b/bc/Actin_myosin_filaments.png/600px-Actin_myosin_filaments.png 2x" data-file-width="720" data-file-height="315" /></a></span></div><div class="thumbcaption">Sliding filaments in contracted and uncontracted states</div></div></div></div></div> <p>The contractile activity of smooth muscle cells can be tonic (sustained) or phasic (transient)<sup id="cite_ref-Zhang1_33-0" class="reference"><a href="#cite_note-Zhang1-33"><span class="cite-bracket">[</span>33<span class="cite-bracket">]</span></a></sup> and is influenced by multiple inputs such as spontaneous electrical activity, neural and hormonal inputs, local changes in chemical composition, and stretch.<sup id="cite_ref-Widmaier_et_al_2008_1-14" class="reference"><a href="#cite_note-Widmaier_et_al_2008-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> This is in contrast to the contractile activity of skeletal muscle cells, which relies on a single neural input. Some types of smooth muscle cells are able to generate their own action potentials spontaneously, which usually occur following a <a href="/wiki/Pacemaker_potential" title="Pacemaker potential">pacemaker potential</a> or a <a href="/wiki/Slow_wave_potential" class="mw-redirect" title="Slow wave potential">slow wave potential</a>. These action potentials are generated by the influx of extracellular <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span>, and not <span class="chemf nowrap">Na<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span>. Like skeletal muscles, cytosolic <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> ions are also required for crossbridge cycling in smooth muscle cells. </p><p>The two sources for cytosolic <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> in smooth muscle cells are the extracellular <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> entering through calcium channels and the <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> ions that are released from the sarcoplasmic reticulum. The elevation of cytosolic <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> results in more <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> binding to <a href="/wiki/Calmodulin" title="Calmodulin">calmodulin</a>, which then binds and activates <a href="/wiki/Myosin_light-chain_kinase" title="Myosin light-chain kinase">myosin light-chain kinase</a>. The calcium-calmodulin-myosin light-chain kinase complex phosphorylates myosin on the 20 <a href="/wiki/Atomic_mass_unit" class="mw-redirect" title="Atomic mass unit">kilodalton</a> (kDa) myosin light chains on amino acid residue-serine 19, enabling the molecular interaction of myosin and actin, and initiating contraction and activating the <a href="/wiki/Myosin_ATPase" title="Myosin ATPase">myosin ATPase</a>. Unlike skeletal muscle cells, smooth muscle cells lack troponin, even though they contain the thin filament protein tropomyosin and other notable proteins – caldesmon and calponin. Thus, smooth muscle contractions are initiated by the <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span>-activated phosphorylation of myosin rather than <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> binding to the troponin complex that regulates myosin binding sites on actin like in skeletal and cardiac muscles. </p><p>Termination of crossbridge cycling (and leaving the muscle in latch-state) occurs when myosin light chain phosphatase removes the phosphate groups from the myosin heads. Phosphorylation of the 20 kDa myosin light chains correlates well with the shortening velocity of smooth muscle. During this period, there is a rapid burst of energy use as measured by oxygen consumption. Within a few minutes of initiation, the calcium level markedly decreases, the 20 kDa myosin light chains' phosphorylation decreases, and energy use decreases; however, force in tonic smooth muscle is maintained. During contraction of muscle, rapidly cycling crossbridges form between activated actin and phosphorylated myosin, generating force. It is hypothesized that the maintenance of force results from dephosphorylated "latch-bridges" that slowly cycle and maintain force. A number of kinases such as <a href="/wiki/Rho_kinase" class="mw-redirect" title="Rho kinase">rho kinase</a>, <a href="/wiki/DAPK3" title="DAPK3">DAPK3</a>, and <a href="/wiki/Protein_kinase_C" title="Protein kinase C">protein kinase C</a> are believed to participate in the sustained phase of contraction, and <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> flux may be significant. </p> <div class="mw-heading mw-heading4"><h4 id="Neuromodulation">Neuromodulation</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=18" title="Edit section: Neuromodulation"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Although smooth muscle contractions are myogenic, the rate and strength of their contractions can be modulated by the <a href="/wiki/Autonomic_nervous_system" title="Autonomic nervous system">autonomic nervous system</a>. <a href="/wiki/Postganglionic_nerve_fibers" title="Postganglionic nerve fibers">Postganglionic nerve fibers</a> of <a href="/wiki/Parasympathetic_nervous_system" title="Parasympathetic nervous system">parasympathetic nervous system</a> release the neurotransmitter acetylcholine, which binds to <a href="/wiki/Muscarinic_acetylcholine_receptors" class="mw-redirect" title="Muscarinic acetylcholine receptors">muscarinic acetylcholine receptors</a> (mAChRs) on smooth muscle cells. These receptors are <a href="/wiki/Metabotropic_receptor" title="Metabotropic receptor">metabotropic</a>, or G-protein coupled receptors that initiate a second messenger cascade. Conversely, postganglionic nerve fibers of the <a href="/wiki/Sympathetic_nervous_system" title="Sympathetic nervous system">sympathetic nervous system</a> release the neurotransmitters epinephrine and norepinephrine, which bind to adrenergic receptors that are also metabotropic. The exact effects on the smooth muscle depend on the specific characteristics of the receptor activated—both parasympathetic input and sympathetic input can be either excitatory (contractile) or inhibitory (relaxing). </p> <div class="mw-heading mw-heading3"><h3 id="Cardiac_muscle">Cardiac muscle</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=19" title="Edit section: Cardiac muscle"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:Cardiac_Muscle.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/7/73/Cardiac_Muscle.png/220px-Cardiac_Muscle.png" decoding="async" width="220" height="220" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/7/73/Cardiac_Muscle.png/330px-Cardiac_Muscle.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/7/73/Cardiac_Muscle.png/440px-Cardiac_Muscle.png 2x" data-file-width="1200" data-file-height="1200" /></a><figcaption>Cardiac muscle</figcaption></figure> <p>There are two types of <a href="/wiki/Cardiac_muscle" title="Cardiac muscle">cardiac muscle</a> cells: autorhythmic and contractile. Autorhythmic cells do not contract, but instead set the pace of contraction for other cardiac muscle cells, which can be modulated by the autonomic nervous system. In contrast, contractile muscle cells (cardiomyocytes) constitute the majority of the heart muscle and are able to contract. </p> <div class="mw-heading mw-heading4"><h4 id="Excitation-contraction_coupling">Excitation-contraction coupling</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=20" title="Edit section: Excitation-contraction coupling"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In both skeletal and cardiac muscle excitation-contraction (E-C) coupling, depolarization conduction and Ca<sup>2+</sup> release processes occur. However, though the proteins involved are similar, they are distinct in structure and regulation. The <a href="/wiki/Dihydropyridine_receptor" class="mw-redirect" title="Dihydropyridine receptor">dihydropyridine receptors</a> (DHPRs) are encoded by different genes, and the <a href="/wiki/Ryanodine_receptor" title="Ryanodine receptor">ryanodine receptors</a> (RyRs) are distinct isoforms. Besides, DHPR contacts with <a href="/wiki/RyR1" class="mw-redirect" title="RyR1">RyR1</a> (main RyR isoform in skeletal muscle) to regulate Ca<sup>2+</sup> release in skeletal muscle, while the <a href="/wiki/L-type_calcium_channel" title="L-type calcium channel">L-type calcium channel</a> (DHPR on cardiac myocytes) and <a href="/wiki/RyR2" class="mw-redirect" title="RyR2">RyR2</a> (main RyR isoform in cardiac muscle) are not physically coupled in cardiac muscle, but face with each other by a junctional coupling.<sup id="cite_ref-34" class="reference"><a href="#cite_note-34"><span class="cite-bracket">[</span>34<span class="cite-bracket">]</span></a></sup> </p><p>Unlike skeletal muscle, E-C coupling in cardiac muscle is thought to depend primarily on a mechanism called <a href="/wiki/Calcium-induced_calcium_release" title="Calcium-induced calcium release">calcium-induced calcium release</a>,<sup id="cite_ref-Fabiato_35-0" class="reference"><a href="#cite_note-Fabiato-35"><span class="cite-bracket">[</span>35<span class="cite-bracket">]</span></a></sup> which is based on the junctional structure between T-tubule and sarcoplasmic reticulum. <a href="/wiki/JPH2" title="JPH2">Junctophilin-2</a> (JPH2) is essential to maintain this structure, as well as the integrity of <a href="/wiki/T-tubule" title="T-tubule">T-tubule</a>.<sup id="cite_ref-36" class="reference"><a href="#cite_note-36"><span class="cite-bracket">[</span>36<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-37" class="reference"><a href="#cite_note-37"><span class="cite-bracket">[</span>37<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-38" class="reference"><a href="#cite_note-38"><span class="cite-bracket">[</span>38<span class="cite-bracket">]</span></a></sup> Another protein, <a href="/wiki/REEP5" title="REEP5">receptor accessory protein 5</a> (REEP5), functions to keep the normal morphology of junctional SR.<sup id="cite_ref-39" class="reference"><a href="#cite_note-39"><span class="cite-bracket">[</span>39<span class="cite-bracket">]</span></a></sup> Defects of junctional coupling can result from deficiencies of either of the two proteins. During the process of calcium-induced calcium release, RyR2s are activated by a calcium trigger, which is brought about by the flow of Ca<sup>2+</sup> through the L-type calcium channels. After this, cardiac muscle tends to exhibit <a href="/wiki/Diad" title="Diad">diad</a> structures, rather than <a href="/wiki/Triad_(anatomy)" title="Triad (anatomy)">triads</a>. </p><p>Excitation-contraction coupling in cardiac muscle cells occurs when an action potential is initiated by pacemaker cells in the <a href="/wiki/Sinoatrial_node" title="Sinoatrial node">sinoatrial node</a> or <a href="/wiki/Atrioventricular_node" title="Atrioventricular node">atrioventricular node</a> and conducted to all cells in the heart via <a href="/wiki/Gap_junctions" class="mw-redirect" title="Gap junctions">gap junctions</a>. The action potential travels along the surface membrane into <a href="/wiki/Cardiac_striated_muscle#T-tubules" class="mw-redirect" title="Cardiac striated muscle">T-tubules</a> (the latter are not seen in all cardiac cell types) and the depolarisation causes extracellular <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> to enter the cell via L-type calcium channels and possibly <a href="/wiki/Sodium-calcium_exchanger" title="Sodium-calcium exchanger">sodium-calcium exchanger</a> (NCX) during the early part of the <a href="/wiki/Cardiac_action_potential#Phase_2" title="Cardiac action potential">plateau phase</a>. Although this Ca<sup>2+</sup> influx only count for about 10% of the Ca<sup>2+</sup> needed for activation, it is relatively larger than that of skeletal muscle. This <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> influx causes a small local increase in intracellular <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span>. The increase of intracellular <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> is detected by RyR2 in the membrane of the sarcoplasmic reticulum, which releases <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> in a <a href="/wiki/Positive_feedback" title="Positive feedback">positive feedback</a> physiological response. This positive feedback is known as <a href="/wiki/Calcium-induced_calcium_release" title="Calcium-induced calcium release">calcium-induced calcium release</a><sup id="cite_ref-Fabiato_35-1" class="reference"><a href="#cite_note-Fabiato-35"><span class="cite-bracket">[</span>35<span class="cite-bracket">]</span></a></sup> and gives rise to <a href="/wiki/Calcium_sparks" title="Calcium sparks">calcium sparks</a> (<span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> sparks<sup id="cite_ref-40" class="reference"><a href="#cite_note-40"><span class="cite-bracket">[</span>40<span class="cite-bracket">]</span></a></sup>). The spatial and temporal summation of ~30,000 <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> sparks gives a cell-wide increase in cytoplasmic calcium concentration.<sup id="cite_ref-41" class="reference"><a href="#cite_note-41"><span class="cite-bracket">[</span>41<span class="cite-bracket">]</span></a></sup> The increase in cytosolic calcium following the flow of calcium through the cell membrane and sarcoplasmic reticulum is moderated by <a href="/wiki/Calcium_buffering" title="Calcium buffering">calcium buffers</a>, which bind a large proportion of intracellular calcium. As a result, a large increase in total calcium leads to a relatively small rise in free <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span>.<sup id="cite_ref-42" class="reference"><a href="#cite_note-42"><span class="cite-bracket">[</span>42<span class="cite-bracket">]</span></a></sup> </p><p>The cytoplasmic calcium binds to Troponin C, moving the tropomyosin complex off the actin binding site allowing the myosin head to bind to the actin filament. From this point on, the contractile mechanism is essentially the same as for skeletal muscle (above). Briefly, using ATP hydrolysis, the myosin head pulls the actin filament toward the centre of the sarcomere. </p> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Cardiac_calcium_cycling_and_excitation-contraction_coupling.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/1/10/Cardiac_calcium_cycling_and_excitation-contraction_coupling.png/220px-Cardiac_calcium_cycling_and_excitation-contraction_coupling.png" decoding="async" width="220" height="165" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/1/10/Cardiac_calcium_cycling_and_excitation-contraction_coupling.png/330px-Cardiac_calcium_cycling_and_excitation-contraction_coupling.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/1/10/Cardiac_calcium_cycling_and_excitation-contraction_coupling.png/440px-Cardiac_calcium_cycling_and_excitation-contraction_coupling.png 2x" data-file-width="756" data-file-height="567" /></a><figcaption>Key proteins involved in cardiac calcium cycling and excitation-contraction coupling</figcaption></figure> <p>Following systole, intracellular calcium is taken up by the <a href="/wiki/SERCA" title="SERCA">sarco/endoplasmic reticulum ATPase</a> (SERCA) pump back into the sarcoplasmic reticulum ready for the next cycle to begin. Calcium is also ejected from the cell mainly by the <a href="/wiki/Sodium-calcium_exchanger" title="Sodium-calcium exchanger">sodium-calcium exchanger</a> (NCX) and, to a lesser extent, a plasma membrane <a href="/wiki/Calcium_ATPase" title="Calcium ATPase">calcium ATPase</a>. Some calcium is also taken up by the mitochondria.<sup id="cite_ref-43" class="reference"><a href="#cite_note-43"><span class="cite-bracket">[</span>43<span class="cite-bracket">]</span></a></sup> An enzyme, <a href="/wiki/Phospholamban" title="Phospholamban">phospholamban</a>, serves as a brake for SERCA. At low heart rates, phospholamban is active and slows down the activity of the ATPase so that <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> does not have to leave the cell entirely. At high heart rates, phospholamban is phosphorylated and deactivated thus taking most <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span> from the cytoplasm back into the sarcoplasmic reticulum. Once again, <a href="/wiki/Calcium_buffering" title="Calcium buffering">calcium buffers</a> moderate this fall in <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span>concentration, permitting a relatively small decrease in free <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span>concentration in response to a large change in total calcium. The falling <span class="chemf nowrap">Ca<span class="nowrap"><span style="display:inline-block;margin-bottom:-0.3em;vertical-align:0.8em;line-height:1em;font-size:80%;text-align:left"><sup style="font-size:inherit;line-height:inherit;vertical-align:baseline">2+</sup><br /><sub style="font-size:inherit;line-height:inherit;vertical-align:baseline"></sub></span></span></span>concentration allows the troponin complex to dissociate from the actin filament thereby ending contraction. The heart relaxes, allowing the ventricles to fill with blood and begin the cardiac cycle again. </p> <div class="mw-heading mw-heading2"><h2 id="Invertebrate">Invertebrate</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=21" title="Edit section: Invertebrate"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-heading mw-heading3"><h3 id="Circular_and_longitudinal_muscles">Circular and longitudinal muscles</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=22" title="Edit section: Circular and longitudinal muscles"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:Earthworm_movement_all.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/e/ea/Earthworm_movement_all.jpg/220px-Earthworm_movement_all.jpg" decoding="async" width="220" height="289" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/e/ea/Earthworm_movement_all.jpg/330px-Earthworm_movement_all.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/e/ea/Earthworm_movement_all.jpg/440px-Earthworm_movement_all.jpg 2x" data-file-width="1048" data-file-height="1377" /></a><figcaption>A simplified image showing earthworm movement via peristalsis</figcaption></figure> <p>In <a href="/wiki/Annelids" class="mw-redirect" title="Annelids">annelids</a> such as <a href="/wiki/Earthworms" class="mw-redirect" title="Earthworms">earthworms</a> and <a href="/wiki/Leeches" class="mw-redirect" title="Leeches">leeches</a>, circular and longitudinal muscles cells form the body wall of these animals and are responsible for their movement.<sup id="cite_ref-Hillis_2014_44-0" class="reference"><a href="#cite_note-Hillis_2014-44"><span class="cite-bracket">[</span>44<span class="cite-bracket">]</span></a></sup> In an earthworm that is moving through a soil, for example, contractions of circular and longitudinal muscles occur reciprocally while the <a href="/wiki/Coelom#Coelomic_fluid" title="Coelom">coelomic fluid</a> serves as a <a href="/wiki/Hydrostatic_skeleton" title="Hydrostatic skeleton">hydroskeleton</a> by maintaining turgidity of the earthworm.<sup id="cite_ref-Gardner_1976_45-0" class="reference"><a href="#cite_note-Gardner_1976-45"><span class="cite-bracket">[</span>45<span class="cite-bracket">]</span></a></sup> When the circular muscles in the anterior segments contract, the anterior portion of animal's body begins to constrict radially, which pushes the incompressible coelomic fluid forward and increasing the length of the animal. As a result, the front end of the animal moves forward. As the front end of the earthworm becomes anchored and the circular muscles in the anterior segments become relaxed, a wave of longitudinal muscle contractions passes backwards, which pulls the rest of animal's trailing body forward.<sup id="cite_ref-Hillis_2014_44-1" class="reference"><a href="#cite_note-Hillis_2014-44"><span class="cite-bracket">[</span>44<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Gardner_1976_45-1" class="reference"><a href="#cite_note-Gardner_1976-45"><span class="cite-bracket">[</span>45<span class="cite-bracket">]</span></a></sup> These alternating waves of circular and longitudinal contractions is called <a href="/wiki/Peristalsis" title="Peristalsis">peristalsis</a>, which underlies the creeping movement of earthworms. </p> <div class="mw-heading mw-heading3"><h3 id="Obliquely_striated_muscles">Obliquely striated muscles</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=23" title="Edit section: Obliquely striated muscles"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Invertebrates such as annelids, <a href="/wiki/Mollusca" title="Mollusca">mollusks</a>, and <a href="/wiki/Nematode" title="Nematode">nematodes</a>, possess obliquely striated muscles, which contain bands of thick and thin filaments that are arranged helically rather than transversely, like in vertebrate skeletal or cardiac muscles.<sup id="cite_ref-Alexander_2003_46-0" class="reference"><a href="#cite_note-Alexander_2003-46"><span class="cite-bracket">[</span>46<span class="cite-bracket">]</span></a></sup> In <a href="/wiki/Bivalvia" title="Bivalvia">bivalves</a>, the obliquely striated muscles can maintain tension over long periods without using too much energy. Bivalves use these muscles to keep their shells closed. </p> <div class="mw-heading mw-heading3"><h3 id="Asynchronous_muscles">Asynchronous muscles</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=24" title="Edit section: Asynchronous muscles"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:Motion_of_Insectwing.gif" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/c/c2/Motion_of_Insectwing.gif/220px-Motion_of_Insectwing.gif" decoding="async" width="220" height="183" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/c/c2/Motion_of_Insectwing.gif/330px-Motion_of_Insectwing.gif 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/c/c2/Motion_of_Insectwing.gif/440px-Motion_of_Insectwing.gif 2x" data-file-width="480" data-file-height="400" /></a><figcaption>Asynchronous muscles power flight in most insect species. a: Wings b: Wing joint c: Dorsoventral muscles power the upstroke d: Dorsolongitudinal muscles (DLM) power the downstroke. The DLMs are oriented out of the page. </figcaption></figure> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Further information: <a href="/wiki/Insect_wing#Muscles" title="Insect wing">Insect wing § Muscles</a></div> <p>Advanced insects such as <a href="/wiki/Wasp" title="Wasp">wasps</a>, <a href="/wiki/Fly" title="Fly">flies</a>, <a href="/wiki/Bee" title="Bee">bees</a>, and <a href="/wiki/Beetle" title="Beetle">beetles</a> possess <a href="/wiki/Asynchronous_muscles" title="Asynchronous muscles">asynchronous muscles</a> that constitute the flight muscles in these animals.<sup id="cite_ref-Alexander_2003_46-1" class="reference"><a href="#cite_note-Alexander_2003-46"><span class="cite-bracket">[</span>46<span class="cite-bracket">]</span></a></sup> These flight muscles are often called <i>fibrillar muscles</i> because they contain myofibrils that are thick and conspicuous.<sup id="cite_ref-47" class="reference"><a href="#cite_note-47"><span class="cite-bracket">[</span>47<span class="cite-bracket">]</span></a></sup> A remarkable feature of these muscles is that they do not require stimulation for each muscle contraction. Hence, they are called <i>asynchronous muscles</i> because the number of contractions in these muscles do not correspond (or synchronize) with the number of action potentials. For example, a wing muscle of a tethered fly may receive action potentials at a frequency of 3 Hz but it is able to beat at a frequency of 120 Hz.<sup id="cite_ref-Alexander_2003_46-2" class="reference"><a href="#cite_note-Alexander_2003-46"><span class="cite-bracket">[</span>46<span class="cite-bracket">]</span></a></sup> The high frequency beating is made possible because the muscles are connected to a <a href="/wiki/Resonance" title="Resonance">resonant</a> system, which is driven to a natural frequency of vibration. </p> <div class="mw-heading mw-heading2"><h2 id="History">History</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=25" title="Edit section: History"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:Galvani-frogs-legs-electricity.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/c/c7/Galvani-frogs-legs-electricity.jpg/220px-Galvani-frogs-legs-electricity.jpg" decoding="async" width="220" height="180" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/c/c7/Galvani-frogs-legs-electricity.jpg/330px-Galvani-frogs-legs-electricity.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/c/c7/Galvani-frogs-legs-electricity.jpg/440px-Galvani-frogs-legs-electricity.jpg 2x" data-file-width="730" data-file-height="598" /></a><figcaption>Electrodes touch a frog, and the legs twitch into the upward position<sup id="cite_ref-48" class="reference"><a href="#cite_note-48"><span class="cite-bracket">[</span>48<span class="cite-bracket">]</span></a></sup></figcaption></figure> <p>In 1780, <a href="/wiki/Luigi_Galvani" title="Luigi Galvani">Luigi Galvani</a> discovered that the muscles of dead frogs' legs twitched when struck by an electrical spark.<sup id="cite_ref-Whittaker_49-0" class="reference"><a href="#cite_note-Whittaker-49"><span class="cite-bracket">[</span>49<span class="cite-bracket">]</span></a></sup> This was one of the first forays into the study of <a href="/wiki/Bioelectricity" class="mw-redirect" title="Bioelectricity">bioelectricity</a>, a field that still studies the electrical patterns and signals in tissues such as nerves and muscles. </p><p>In 1952, the term excitation–contraction coupling was coined to describe the physiological process of converting an electrical stimulus to a mechanical response.<sup id="cite_ref-Sandow_A_1952_176–201_50-0" class="reference"><a href="#cite_note-Sandow_A_1952_176–201-50"><span class="cite-bracket">[</span>50<span class="cite-bracket">]</span></a></sup> This process is fundamental to muscle physiology, whereby the electrical stimulus is usually an action potential and the mechanical response is contraction. Excitation–contraction coupling can be dysregulated in many diseases. Though excitation–contraction coupling has been known for over half a century, it is still an active area of biomedical research. The general scheme is that an action potential arrives to depolarize the cell membrane. By mechanisms specific to the muscle type, this depolarization results in an increase in cytosolic <a href="/wiki/Calcium" title="Calcium">calcium</a> that is called a calcium transient. This increase in calcium activates calcium-sensitive contractile proteins that then use <a href="/wiki/Adenosine_triphosphate" title="Adenosine triphosphate">ATP</a> to cause cell shortening. </p><p>The mechanism for muscle contraction evaded scientists for years and requires continued research and updating.<sup id="cite_ref-Huxley_2000_51-0" class="reference"><a href="#cite_note-Huxley_2000-51"><span class="cite-bracket">[</span>51<span class="cite-bracket">]</span></a></sup> The sliding filament theory was independently developed by <a href="/wiki/Andrew_F._Huxley" class="mw-redirect" title="Andrew F. Huxley">Andrew F. Huxley</a> and <a href="/wiki/Rolf_Niedergerke" title="Rolf Niedergerke">Rolf Niedergerke</a> and by <a href="/wiki/Hugh_Huxley" title="Hugh Huxley">Hugh Huxley</a> and <a href="/wiki/Jean_Hanson" title="Jean Hanson">Jean Hanson</a>. Their findings were published as two consecutive papers published in the 22 May 1954 issue of <i><a href="/wiki/Nature_(journal)" title="Nature (journal)">Nature</a></i> under the common theme "Structural Changes in Muscle During Contraction".<sup id="cite_ref-Huxley_AF,_Niedergerke_R_1954_971–973_23-1" class="reference"><a href="#cite_note-Huxley_AF,_Niedergerke_R_1954_971–973-23"><span class="cite-bracket">[</span>23<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Huxley_H,_Hanson_J_1954_973–976_24-1" class="reference"><a href="#cite_note-Huxley_H,_Hanson_J_1954_973–976-24"><span class="cite-bracket">[</span>24<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=26" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/Anatomical_terms_of_motion" title="Anatomical terms of motion">Anatomical terms of motion</a></li> <li><a href="/wiki/Calcium-induced_calcium_release" title="Calcium-induced calcium release">calcium-induced calcium release</a></li> <li><a href="/wiki/Cardiac_action_potential" title="Cardiac action potential">Cardiac action potential</a></li> <li><a href="/wiki/Cramp" title="Cramp">Cramp</a></li> <li><a href="/wiki/Dystonia" title="Dystonia">Dystonia</a></li> <li><a href="/wiki/Exercise_physiology" title="Exercise physiology">Exercise physiology</a></li> <li><a href="/wiki/Fasciculation" title="Fasciculation">Fasciculation</a></li> <li><a href="/wiki/Hill%27s_muscle_model" title="Hill's muscle model">Hill's muscle model</a></li> <li><a href="/wiki/Hypnic_jerk" title="Hypnic jerk">Hypnic jerk</a></li> <li><a href="/wiki/In_vitro_muscle_testing" title="In vitro muscle testing">In vitro muscle testing</a></li> <li><a href="/wiki/Lombard%27s_paradox" title="Lombard's paradox">Lombard's paradox</a></li> <li><a href="/wiki/Myoclonus" title="Myoclonus">Myoclonus</a></li> <li><a href="/wiki/Rigor_mortis" title="Rigor mortis">Rigor mortis</a></li> <li><a href="/wiki/Spasm" title="Spasm">Spasm</a></li> <li><a href="/wiki/Uterine_contraction" title="Uterine contraction">Uterine contraction</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=27" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap mw-references-columns"><ol class="references"> <li id="cite_note-Widmaier_et_al_2008-1"><span class="mw-cite-backlink">^ <a href="#cite_ref-Widmaier_et_al_2008_1-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Widmaier_et_al_2008_1-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-Widmaier_et_al_2008_1-2"><sup><i><b>c</b></i></sup></a> <a href="#cite_ref-Widmaier_et_al_2008_1-3"><sup><i><b>d</b></i></sup></a> <a href="#cite_ref-Widmaier_et_al_2008_1-4"><sup><i><b>e</b></i></sup></a> <a href="#cite_ref-Widmaier_et_al_2008_1-5"><sup><i><b>f</b></i></sup></a> <a href="#cite_ref-Widmaier_et_al_2008_1-6"><sup><i><b>g</b></i></sup></a> <a href="#cite_ref-Widmaier_et_al_2008_1-7"><sup><i><b>h</b></i></sup></a> <a href="#cite_ref-Widmaier_et_al_2008_1-8"><sup><i><b>i</b></i></sup></a> <a href="#cite_ref-Widmaier_et_al_2008_1-9"><sup><i><b>j</b></i></sup></a> <a href="#cite_ref-Widmaier_et_al_2008_1-10"><sup><i><b>k</b></i></sup></a> <a href="#cite_ref-Widmaier_et_al_2008_1-11"><sup><i><b>l</b></i></sup></a> <a href="#cite_ref-Widmaier_et_al_2008_1-12"><sup><i><b>m</b></i></sup></a> <a href="#cite_ref-Widmaier_et_al_2008_1-13"><sup><i><b>n</b></i></sup></a> <a href="#cite_ref-Widmaier_et_al_2008_1-14"><sup><i><b>o</b></i></sup></a></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite id="CITEREFWidmaierRaffStrang2010" class="citation book cs1">Widmaier, Eric P.; Raff, Hersel; Strang, Kevin T. (2010). "Muscle". <i>Vander's Human Physiology: The Mechanisms of Body Function</i> (12th ed.). New York, NY: McGraw-Hill. pp. 250–291. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-321-98122-6" title="Special:BookSources/978-0-321-98122-6"><bdi>978-0-321-98122-6</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=bookitem&rft.atitle=Muscle&rft.btitle=Vander%27s+Human+Physiology%3A+The+Mechanisms+of+Body+Function&rft.place=New+York%2C+NY&rft.pages=250-291&rft.edition=12th&rft.pub=McGraw-Hill&rft.date=2010&rft.isbn=978-0-321-98122-6&rft.aulast=Widmaier&rft.aufirst=Eric+P.&rft.au=Raff%2C+Hersel&rft.au=Strang%2C+Kevin+T.&rfr_id=info%3Asid%2Fen.wikipedia.org%3AMuscle+contraction" class="Z3988"></span></span> </li> <li id="cite_note-Silverthorn_2016-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-Silverthorn_2016_2-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFSilverthorn2016" class="citation book cs1">Silverthorn, Dee Unglaub (2016). "Muscles". <i>Human Physiology: An Integrated Approach</i> (7th ed.). San Francisco, CA: Pearson. pp. 377–416. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-321-98122-6" title="Special:BookSources/978-0-321-98122-6"><bdi>978-0-321-98122-6</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=bookitem&rft.atitle=Muscles&rft.btitle=Human+Physiology%3A+An+Integrated+Approach&rft.place=San+Francisco%2C+CA&rft.pages=377-416&rft.edition=7th&rft.pub=Pearson&rft.date=2016&rft.isbn=978-0-321-98122-6&rft.aulast=Silverthorn&rft.aufirst=Dee+Unglaub&rfr_id=info%3Asid%2Fen.wikipedia.org%3AMuscle+contraction" class="Z3988"></span></span> </li> <li id="cite_note-Biewener_1998-3"><span class="mw-cite-backlink">^ <a href="#cite_ref-Biewener_1998_3-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Biewener_1998_3-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFBiewener2003" class="citation book cs1">Biewener, Andrew A. (2003). "Muscles and skeletons: The building blocks of animal movement". <i>Animal Locomotion</i>. Oxford Animal Biology Series. New York, NY: Oxford University Press. pp. 15–45. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-198-50022-3" title="Special:BookSources/978-0-198-50022-3"><bdi>978-0-198-50022-3</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=bookitem&rft.atitle=Muscles+and+skeletons%3A+The+building+blocks+of+animal+movement&rft.btitle=Animal+Locomotion&rft.place=New+York%2C+NY&rft.series=Oxford+Animal+Biology+Series&rft.pages=15-45&rft.pub=Oxford+University+Press&rft.date=2003&rft.isbn=978-0-198-50022-3&rft.aulast=Biewener&rft.aufirst=Andrew+A.&rfr_id=info%3Asid%2Fen.wikipedia.org%3AMuscle+contraction" class="Z3988"></span></span> </li> <li id="cite_note-Aidley_1998-4"><span class="mw-cite-backlink">^ <a href="#cite_ref-Aidley_1998_4-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Aidley_1998_4-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-Aidley_1998_4-2"><sup><i><b>c</b></i></sup></a> <a href="#cite_ref-Aidley_1998_4-3"><sup><i><b>d</b></i></sup></a> <a href="#cite_ref-Aidley_1998_4-4"><sup><i><b>e</b></i></sup></a> <a href="#cite_ref-Aidley_1998_4-5"><sup><i><b>f</b></i></sup></a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFAidley1998" class="citation book cs1">Aidley, David J. (1998). "Mechanics and energetics of muscular contraction". <span class="id-lock-registration" title="Free registration required"><a rel="nofollow" class="external text" href="https://archive.org/details/physiologyofexci0000aidl_s5u8"><i>The Physiology of Excitable Cells</i></a></span> (4th ed.). New York, NY: Cambridge University Press. pp. <a rel="nofollow" class="external text" href="https://archive.org/details/physiologyofexci0000aidl_s5u8/page/323">323–335</a>. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-521-57421-1" title="Special:BookSources/978-0-521-57421-1"><bdi>978-0-521-57421-1</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=bookitem&rft.atitle=Mechanics+and+energetics+of+muscular+contraction&rft.btitle=The+Physiology+of+Excitable+Cells&rft.place=New+York%2C+NY&rft.pages=323-335&rft.edition=4th&rft.pub=Cambridge+University+Press&rft.date=1998&rft.isbn=978-0-521-57421-1&rft.aulast=Aidley&rft.aufirst=David+J.&rft_id=https%3A%2F%2Farchive.org%2Fdetails%2Fphysiologyofexci0000aidl_s5u8&rfr_id=info%3Asid%2Fen.wikipedia.org%3AMuscle+contraction" class="Z3988"></span></span> </li> <li id="cite_note-Sircar_2008-5"><span class="mw-cite-backlink">^ <a href="#cite_ref-Sircar_2008_5-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Sircar_2008_5-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-Sircar_2008_5-2"><sup><i><b>c</b></i></sup></a> <a href="#cite_ref-Sircar_2008_5-3"><sup><i><b>d</b></i></sup></a> <a href="#cite_ref-Sircar_2008_5-4"><sup><i><b>e</b></i></sup></a> <a href="#cite_ref-Sircar_2008_5-5"><sup><i><b>f</b></i></sup></a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFSircar2008" class="citation book cs1">Sircar, Sabyasachi (2008). "Muscle elasticity". <i>Principles of Medical Physiology</i> (1st ed.). New York, NY: Thieme. p. 113. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-1-588-90572-7" title="Special:BookSources/978-1-588-90572-7"><bdi>978-1-588-90572-7</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=bookitem&rft.atitle=Muscle+elasticity&rft.btitle=Principles+of+Medical+Physiology&rft.place=New+York%2C+NY&rft.pages=113&rft.edition=1st&rft.pub=Thieme&rft.date=2008&rft.isbn=978-1-588-90572-7&rft.aulast=Sircar&rft.aufirst=Sabyasachi&rfr_id=info%3Asid%2Fen.wikipedia.org%3AMuscle+contraction" class="Z3988"></span></span> </li> <li id="cite_note-Bullock_et_al_2001-6"><span class="mw-cite-backlink">^ <a href="#cite_ref-Bullock_et_al_2001_6-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Bullock_et_al_2001_6-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-Bullock_et_al_2001_6-2"><sup><i><b>c</b></i></sup></a> <a href="#cite_ref-Bullock_et_al_2001_6-3"><sup><i><b>d</b></i></sup></a> <a href="#cite_ref-Bullock_et_al_2001_6-4"><sup><i><b>e</b></i></sup></a> <a href="#cite_ref-Bullock_et_al_2001_6-5"><sup><i><b>f</b></i></sup></a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFBullockBoyleWang2001" class="citation book cs1">Bullock, John; Boyle, Joseph; Wang, Michael B. 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(15 September 2000). <a rel="nofollow" class="external text" href="http://jeb.biologists.org/content/203/18/2713">"Asynchronous muscle: a primer"</a>. <i>Journal of Experimental Biology</i>. <b>203</b> (18): 2713–2722. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1242%2Fjeb.203.18.2713">10.1242/jeb.203.18.2713</a>. <a href="/wiki/ISSN_(identifier)" class="mw-redirect" title="ISSN (identifier)">ISSN</a> <a rel="nofollow" class="external text" href="https://search.worldcat.org/issn/0022-0949">0022-0949</a>. <a href="/wiki/PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/10952872">10952872</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Journal+of+Experimental+Biology&rft.atitle=Asynchronous+muscle%3A+a+primer&rft.volume=203&rft.issue=18&rft.pages=2713-2722&rft.date=2000-09-15&rft.issn=0022-0949&rft_id=info%3Apmid%2F10952872&rft_id=info%3Adoi%2F10.1242%2Fjeb.203.18.2713&rft.aulast=Josephson&rft.aufirst=R.+K.&rft.au=Malamud%2C+J.+G.&rft.au=Stokes%2C+D.+R.&rft_id=http%3A%2F%2Fjeb.biologists.org%2Fcontent%2F203%2F18%2F2713&rfr_id=info%3Asid%2Fen.wikipedia.org%3AMuscle+contraction" class="Z3988"></span></span> </li> <li id="cite_note-48"><span class="mw-cite-backlink"><b><a href="#cite_ref-48">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFWells1859" class="citation book cs1">Wells, David Ames (1859). <a rel="nofollow" class="external text" href="https://archive.org/details/sciencecommonth01wellgoog/page/n298">"How galvanic electricity was discovered"</a>. <i>The Science of Common Things: A Familiar Explanation of the First Principles of Physical Science</i>. New York: Ivison & Phinney. p. 290.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=bookitem&rft.atitle=How+galvanic+electricity+was+discovered&rft.btitle=The+Science+of+Common+Things%3A+A+Familiar+Explanation+of+the+First+Principles+of+Physical+Science&rft.place=New+York&rft.pages=290&rft.pub=Ivison+%26+Phinney&rft.date=1859&rft.aulast=Wells&rft.aufirst=David+Ames&rft_id=https%3A%2F%2Farchive.org%2Fdetails%2Fsciencecommonth01wellgoog%2Fpage%2Fn298&rfr_id=info%3Asid%2Fen.wikipedia.org%3AMuscle+contraction" class="Z3988"></span></span> </li> <li id="cite_note-Whittaker-49"><span class="mw-cite-backlink"><b><a href="#cite_ref-Whittaker_49-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFWhittaker1951" class="citation cs2"><a href="/wiki/E._T._Whittaker" title="E. T. Whittaker">Whittaker, E. T.</a> (1951), <i><a href="/wiki/A_History_of_the_Theories_of_Aether_and_Electricity" title="A History of the Theories of Aether and Electricity">A History of the Theories of Aether and Electricity. Vol 1</a></i>, Nelson, London</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=A+History+of+the+Theories+of+Aether+and+Electricity.+Vol+1&rft.pub=Nelson%2C+London&rft.date=1951&rft.aulast=Whittaker&rft.aufirst=E.+T.&rfr_id=info%3Asid%2Fen.wikipedia.org%3AMuscle+contraction" class="Z3988"></span></span> </li> <li id="cite_note-Sandow_A_1952_176–201-50"><span class="mw-cite-backlink"><b><a href="#cite_ref-Sandow_A_1952_176–201_50-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFSandow1952" class="citation journal cs1">Sandow, A (1952). <a rel="nofollow" class="external text" href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2599245">"Excitation-Contraction Coupling in Muscular Response"</a>. <i>Yale J Biol Med</i>. <b>25</b> (3): 176–201. <a href="/wiki/PMC_(identifier)" class="mw-redirect" title="PMC (identifier)">PMC</a> <span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2599245">2599245</a></span>. <a href="/wiki/PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/13015950">13015950</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Yale+J+Biol+Med&rft.atitle=Excitation-Contraction+Coupling+in+Muscular+Response&rft.volume=25&rft.issue=3&rft.pages=176-201&rft.date=1952&rft_id=https%3A%2F%2Fwww.ncbi.nlm.nih.gov%2Fpmc%2Farticles%2FPMC2599245%23id-name%3DPMC&rft_id=info%3Apmid%2F13015950&rft.aulast=Sandow&rft.aufirst=A&rft_id=https%3A%2F%2Fwww.ncbi.nlm.nih.gov%2Fpmc%2Farticles%2FPMC2599245&rfr_id=info%3Asid%2Fen.wikipedia.org%3AMuscle+contraction" class="Z3988"></span></span> </li> <li id="cite_note-Huxley_2000-51"><span class="mw-cite-backlink"><b><a href="#cite_ref-Huxley_2000_51-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFHuxley2000" class="citation journal cs1">Huxley, H. E. (April 2000). <a rel="nofollow" class="external text" href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1692762">"Past, Present and Future Experiments on Muscle"</a>. <i>Philosophical Transactions: Biological Sciences</i>. <b>355</b> (1396): 539–543. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1098%2Frstb.2000.0595">10.1098/rstb.2000.0595</a>. <a href="/wiki/JSTOR_(identifier)" class="mw-redirect" title="JSTOR (identifier)">JSTOR</a> <a rel="nofollow" class="external text" href="https://www.jstor.org/stable/3066716">3066716</a>. <a href="/wiki/PMC_(identifier)" class="mw-redirect" title="PMC (identifier)">PMC</a> <span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1692762">1692762</a></span>. <a href="/wiki/PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/10836507">10836507</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Philosophical+Transactions%3A+Biological+Sciences&rft.atitle=Past%2C+Present+and+Future+Experiments+on+Muscle&rft.volume=355&rft.issue=1396&rft.pages=539-543&rft.date=2000-04&rft_id=https%3A%2F%2Fwww.ncbi.nlm.nih.gov%2Fpmc%2Farticles%2FPMC1692762%23id-name%3DPMC&rft_id=info%3Apmid%2F10836507&rft_id=https%3A%2F%2Fwww.jstor.org%2Fstable%2F3066716%23id-name%3DJSTOR&rft_id=info%3Adoi%2F10.1098%2Frstb.2000.0595&rft.aulast=Huxley&rft.aufirst=H.+E.&rft_id=https%3A%2F%2Fwww.ncbi.nlm.nih.gov%2Fpmc%2Farticles%2FPMC1692762&rfr_id=info%3Asid%2Fen.wikipedia.org%3AMuscle+contraction" class="Z3988"></span></span> </li> </ol></div></div> <div class="mw-heading mw-heading2"><h2 id="Further_reading">Further reading</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=28" title="Edit section: Further reading"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li>Krans, J. L. (2010) The Sliding Filament Theory of Muscle Contraction. Nature Education 3(9):66</li> <li>Saladin, Kenneth S., Stephen J. Sullivan, and Christina A. Gan. (2015). Anatomy & Physiology: The Unity of Form and Function. 7th ed. New York: McGraw-Hill Education.</li></ul> <div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Muscle_contraction&action=edit&section=29" title="Edit section: External links"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a rel="nofollow" class="external text" href="http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__myofilament_contraction.html">Animation: Myofilament Contraction</a> <a rel="nofollow" class="external text" href="https://web.archive.org/web/20130521075303/http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__myofilament_contraction.html">Archived</a> 21 May 2013 at the <a href="/wiki/Wayback_Machine" title="Wayback Machine">Wayback Machine</a></li> <li><a rel="nofollow" class="external text" href="http://cstl-csm.semo.edu/trautwein/BS113Fall2003/Sliding%20Filament.ppt">Sliding Filament Model of Muscle Contraction</a></li></ul> <div class="navbox-styles"><style data-mw-deduplicate="TemplateStyles:r1129693374">.mw-parser-output .hlist dl,.mw-parser-output .hlist ol,.mw-parser-output .hlist ul{margin:0;padding:0}.mw-parser-output .hlist dd,.mw-parser-output .hlist dt,.mw-parser-output .hlist li{margin:0;display:inline}.mw-parser-output .hlist.inline,.mw-parser-output .hlist.inline dl,.mw-parser-output .hlist.inline ol,.mw-parser-output .hlist.inline ul,.mw-parser-output .hlist dl dl,.mw-parser-output .hlist dl ol,.mw-parser-output .hlist dl ul,.mw-parser-output .hlist ol dl,.mw-parser-output .hlist ol ol,.mw-parser-output .hlist ol ul,.mw-parser-output .hlist ul dl,.mw-parser-output .hlist ul ol,.mw-parser-output .hlist ul ul{display:inline}.mw-parser-output .hlist .mw-empty-li{display:none}.mw-parser-output .hlist dt::after{content:": "}.mw-parser-output .hlist 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class="navbox-group" style="width:1%">Tissue</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Muscle_tissue" class="mw-redirect" title="Muscle tissue">Muscle tissue</a> <ul><li><a href="/wiki/Cardiac_muscle" title="Cardiac muscle">Cardiac muscle</a></li> <li><a href="/wiki/Skeletal_muscle" title="Skeletal muscle">Skeletal muscle</a></li> <li><a href="/wiki/Smooth_muscle" title="Smooth muscle">Smooth muscle</a></li></ul></li> <li><a href="/wiki/Fascia" title="Fascia">Fascia</a> <ul><li><a href="/wiki/Superficial_fascia" class="mw-redirect" title="Superficial fascia">Superficial</a></li> <li><a href="/wiki/Deep_fascia" title="Deep fascia">Deep</a></li> <li><a href="/wiki/Visceral_fascia" class="mw-redirect" title="Visceral fascia">Visceral</a></li></ul></li> <li><a href="/wiki/Fascial_compartment" title="Fascial compartment">Fascial compartment</a> <ul><li><a href="/wiki/Fascial_compartments_of_arm" title="Fascial compartments of arm">Arm</a></li> <li><a href="/wiki/Fascial_compartments_of_forearm" class="mw-redirect" title="Fascial compartments of forearm">Forearm</a></li> <li><a href="/wiki/Fascial_compartments_of_thigh" title="Fascial compartments of thigh">Thigh</a></li> <li><a href="/wiki/Fascial_compartments_of_leg" title="Fascial compartments of leg">Leg</a></li></ul></li> <li><a href="/wiki/Tendon" title="Tendon">Tendon</a>/<a href="/wiki/Aponeurosis" title="Aponeurosis">Aponeurosis</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Shape</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Fusiform_muscle" class="mw-redirect" title="Fusiform muscle">Fusiform</a></li> <li><a href="/wiki/Pennate_muscle" title="Pennate muscle">Pennate muscle</a> <ul><li><a href="/wiki/Unipennate_muscle" class="mw-redirect" title="Unipennate muscle">Unipennate</a></li> <li><a href="/wiki/Bipennate_muscle" class="mw-redirect" title="Bipennate muscle">Bipennate</a></li></ul></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Other</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Anatomical_terms_of_muscle" title="Anatomical terms of muscle">Anatomical terms of muscle</a> <ul><li><a href="/wiki/Origin_(anatomy)" class="mw-redirect" title="Origin (anatomy)">Origin</a></li> <li><a href="/wiki/Insertion_(anatomy)" class="mw-redirect" title="Insertion (anatomy)">Insertion</a></li></ul></li> <li><a href="/wiki/List_of_muscles_of_the_human_body" class="mw-redirect" title="List of muscles of the human body">List of muscles of the human body</a></li> <li><a href="/wiki/Composite_muscle" title="Composite muscle">Composite muscle</a></li></ul> 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