Synergist

Muscles are described using unique anatomical terminology according to their actions and structure.

There are three types of muscle tissue in the human body: skeletal, smooth and cardiac.

Skeletal striated muscle, or Voluntary muscle, primarily links to bone through tendon, thus enabling through levers of the bones of the human skeleton posturing, upright stance under atmospheric pressure, and voluntary movements.

Smooth muscle tissue is 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.

Cardiac muscle is specific to the heart. It is also involuntary in its movement, and is additionally self-excitatory, contracting without outside stimuli.

As well as anatomical terms of motion, which describe the motion made by a muscle, unique terminology is used to describe the action of a set of muscles.

Agonist muscles and antagonist muscles refer to muscles that cause or inhibit a movement.

muscles cause a movement to occur through their own contraction. For example, the triceps brachii contracts during the up phase of a push-up (elbow extension). During the down phase of a push-up, the same triceps brachii actively controls elbow flexion while relaxing. 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. (Agonists are also interchangeably referred to as "prime movers," since they are the muscles considered primarily responsible for generating a specific movement. This term typically describes skeletal muscles.)

muscles oppose a specific movement. This controls a motion, slows it down, and returns a limb to its initial position. Antagonism is not an intrinsic property; it is a role that a muscle plays depending on the motion. If a motion is reversed, agonist and antagonist muscles switch roles. Because a flexor muscle is always a flexor, in flexion it is the agonist, and in extension it is the antagonist. Conversely, an extensor muscle is the agonist in extension and the antagonist in flexion. Using the example above of the triceps brachii during a push-up, the elbow flexor muscles are the antagonists during both the up phase and down phase of the movement.

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Wikipedia