Anatomical terms of muscle

Smooth muscle is involuntary and found in parts of the body where it conveys action without conscious intent.

The majority of this type of muscle tissue is found in the digestive and urinary systems where it acts by propelling forward food, chyme, and feces in the former and urine in the latter.

Other places smooth muscle can be found are within the uterus, where it helps facilitate birth, and the eye, where the pupillary sphincter controls pupil size.

[5] Agonist muscles are also called prime movers since they produce most of the force, and control of an action.

During the down phase of a push-up, the same triceps brachii actively controls elbow flexion while producing a lengthening (eccentric) contraction.

It is still the agonist, because while resisting gravity during relaxing, the triceps brachii continues to be the prime mover, or controller, of the joint action.

Here it is important to understand that it is common practice to give a name to a muscle group (e.g. elbow flexors) based on the joint action they produce during a shortening contraction.

However, muscles do not always work this way; sometimes agonists and antagonists contract at the same time to produce force, as per Lombard's paradox.

Also, sometimes during a joint action controlled by an agonist muscle, the antagonist will be slightly activated, naturally.

This occurs normally and is not considered to be a problem unless it is excessive or uncontrolled and disturbs the control of the joint action.

[citation needed] 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 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.

[citation needed] A muscle that fixes or holds a bone so that the agonist can carry out the intended movement is said to have a neutralizing action.

This is the case when grabbing objects lighter than the body, as in the typical use of a lat pull down machine.