The muscular system consists of skeletal muscles and their associated connective tissues. It does not include cardiac muscle and smooth muscle, which are associated with the systems in which they are found, such as the cardiovascular, digestive, urinary, or other organ systems.
A skeletal muscle is attached to one bone and extends across a joint to attach to another bone. A muscle can also attach a bone to another structure, such as skin. When the muscle contracts, one of the structures usually remains stationary, while the other moves. The following terms refer to this characteristic of muscle contraction:
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The origin of the muscle is the muscle end that attaches to the stationary structure, usually a bone or a bony structure.
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The insertion of the muscle is the muscle end that attaches to the moving structure.
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The belly of the muscle is that part of the muscle between the origin and insertion.
Several muscles usually influence a particular body movement:
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The prime mover is the muscle that is most responsible for the movement.
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Synergists are other muscles that assist the prime mover. Synergists may stabilize nearby bones or refine the movement of the prime mover.
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Antagonists are muscles that cause a movement opposite to that of the prime mover. For example, if the prime mover raises an arm, its antagonist pulls the arm down. An antagonist is generally attached to the opposite side of the joint to which the prime mover is attached.