<P> Muscle fibers can only contract up to 40% of their fully stretched length . Thus the short fibers of pennate muscles are more suitable where power rather than range of contraction is required . This limitation in the range of contraction affects all muscles, and those that act over several joints may be unable to shorten sufficiently to produce the full range of movement at all of them simultaneously (active insufficiency, e.g., the fingers cannot be fully flexed when the wrist is also flexed). Likewise, the opposing muscles may be unable to stretch sufficiently to allow such movement to take place (passive insufficiency). For both these reasons, it is often essential to use other muscles, called fixators or synergists, in this type of action to fix certain of the joints so that others can be moved effectively, e.g., fixation of the wrist during full flexion of the fingers in clenching the fist . Synergists are muscles that facilitate the fixation action . </P> <P> There is an important difference between a helping synergist muscle and a true synergist muscle . A true synergist muscle is one that only neutralizes an undesired joint action, whereas a helping synergist is one that neutralizes an undesired action but also assists with the desired action . </P> <P> A muscle that fixes or holds a bone so that the agonist can carry out the intended movement is said to have a neutralising action . A good famous example of this are the hamstrings; the semitendinosus and semimembranosus muscles perform knee flexion and knee internal rotation whereas the biceps femoris carries out knee flexion and knee external rotation . For the knee to flex while not rotating in either direction, all three muscles contract to stabilize the knee while it moves in the desired way . </P> <P> Composite or hybrid muscles have more than one set of fibers that perform the same function, and are usually supplied by different nerves for different set of fibers . For example, the tongue itself is a composite muscle made up of various components like longitudinal, transverse, horizontal muscles with different parts innervated having different nerve supply . </P>

The agonist is the muscle that contracts to produce a desired movement