ABSTRACT
A muscle attaches to a bone and then extends across one or two body joints to another bone. When contracting, the muscle pulls on the body's internal bone framework. Smooth muscle surrounds vessels, mostly as sphincters that control the opening of blood vessels. The human body has several hundred skeletal muscles, known by their Latin names; for example, the biceps flexes the elbow by pulling the upper and lower arms together. Thousands of individual muscle fibers run in bundles, essentially parallel, along the length of the muscle. Contraction is the only active action that a muscle can take. A strong force opposing the muscle contraction can lengthen the muscle beyond its relaxed length. Muscles usually appear in pairs: one muscle turns a bone around an articulation in one direction while the other muscle turns the opposite way. The strength of a muscle can be measured objectively and reliably if removed from the body, as has been done with animal models.
