Muscle Function And Body Movement

Muscles span joints and are attached at two or more points to the bony levers of the skeleton. The muscles provide the power that moves the body's levers. Muscles are described in terms of their origin and insertion attachment sites. The origin tends to be the more fixed, less mobile location, while the insertion refers to the skeletal site that is more mobile. Movement occurs when a muscle generates force on its attachment sites and undergoes shortening. This type of action is termed an isotonic or concentric contraction. Another form of muscular action is a controlled lengthening while still generating force. This is an eccentric contraction. A muscle may also generate force but hold its attachment sites static, as in isometric contraction.

Because muscle contraction can produce movement in only one direction, at least two muscles opposing each other at a joint are needed to achieve motion in more than one direction. When a muscle produces movement by shortening, it is an agonist. The prime mover is the muscle that contributes most to the movement. Muscles that oppose the action of the prime mover are antagonists. The quadriceps and hamstring muscles are examples of agonist-antagonist pairs in knee extension and flexion. During both simple and light-load skilled movements, the antagonist is relaxed. Contraction of the agonist with concomitant relaxation of the antagonist occurs by the nervous system function of reciprocal inhibition. Co-contraction of agonist and antagonist occurs during movements that require precise control.

A muscle functions as a synergist if it contracts at the same time as the agonist while cooperating in producing the movement. Synergistic action can aid in producing a movement (e.g., the activity of both flexor carpi ulnaris and extensor carpi ulnaris are used in producing ulnar deviation of the wrist); eliminating unwanted movements (e.g., the activity of wrist extensors prevents flexion of the wrist when finger flexors contract in closing the hand); or stabilizing proximal joints (e.g., isometric contractions of muscles of the forearm, upper arm, shoulder, and trunk accompany a forceful grip of the hand).

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