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Mus­cle is a soft tis­sue found in most ani­mals. Mus­cle cells con­tain pro­tein fil­a­ments of actin and myosin that slide past one anoth­er, pro­duc­ing a con­trac­tion that changes both the length and the shape of the cell. Mus­cles func­tion to pro­duce force and motion. They are pri­mar­i­ly respon­si­ble for main­tain­ing and chang­ing pos­ture, loco­mo­tion, as well as move­ment of inter­nal organs, such as the con­trac­tion of the heart and the move­ment of food through the diges­tive sys­tem via peri­stal­sis. Mus­cle tis­sues are derived from the meso­der­mal lay­er of embry­on­ic germ cells in a process known as myo­ge­n­e­sis. There are three types of mus­cle, skele­tal or stri­at­ed, car­diac, and smooth. Mus­cle action can be clas­si­fied as being either vol­un­tary or invol­un­tary. Car­diac and smooth mus­cles con­tract with­out con­scious thought and are termed invol­un­tary, where­as the skele­tal mus­cles con­tract upon com­mand. Skele­tal mus­cles in turn can be divid­ed into fast and slow twitch fibers. Mus­cles are pre­dom­i­nant­ly pow­ered by the oxi­da­tion of fats and car­bo­hy­drates, but anaer­o­bic chem­i­cal reac­tions are also used, par­tic­u­lar­ly by fast twitch fibers. These chem­i­cal reac­tions pro­duce adeno­sine triphos­phate (ATP) mol­e­cules that are used to pow­er the move­ment of the myosin heads. The term mus­cle is derived from the Latin mus­cu­lus mean­ing “lit­tle mouse” per­haps because of the shape of cer­tain mus­cles or because con­tract­ing mus­cles look like mice mov­ing under the skin.