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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.
Agonist muscles causes a movement to occur through its own contraction.  For example, the triceps brachii does 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. While resisting gravity during relaxing, the triceps brachii is still the prime mover, or controller, of the joint action. For both of those movements the elbow flexor muscles are the antagonists. Agonists are also referred to, interchangeably, as "prime movers", since they are the muscles being considered that are primarily responsible for generating a specific movement. This term typically describes skeletal muscles.
Antagonist 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, played depending on the motion. If the motion is reversed, agonist and antagonist swich roles. A flexor muscle is always flexor. But in flexion, it is always agonist and in extension, it is always antagonist. An extensor muscle is agonist in extension and antagonist in flexion.
Antagonist and agonist muscles often occur in pairs, called antagonistic pairs. As one muscle contracts, the other relaxes. An example of an antagonisic pair is the biceps and triceps; to contract - the triceps relaxes while the biceps contracts to lift the arm. "Reverse motions" need antagonistic pairs located in opposite sides of a joint or bone, including abductor-adductor pairs and flexor-extensor pairs. These consist of an extensor muscle, which "opens" the joint (by increasing the angle between the two bones) and a flexor muscle, which does the opposite by decreasing the angle between two bones.
Not all muscles are paired in this way. An example of exception is the deltoid.
Synergist muscles performs, or helps perform, the same set of joint motion as the agonists. Synergists muscles act on movable joints. Synergists are sometimes referred to as "neutralizers" because they help cancel out, or neutralize, extra motion from the agonists to make sure that the force generated works within the desired plane of motion.
Muscle fibres can only contract up to 40% of their fully stretched length. Thus the short fibres 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 move 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.
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.
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 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. In order to flex the knee but at the same time prevent it from rotating in either direction, all three muscles contract so that the knee is stabilized and the desired movement can be affected.
Composite or hybrid muscles are those muscles which have more than one set of fibers but perform the same function and are usually supplied by different nerves for different set of fibers.
The origin of a muscle is the bone, typically proximal, which has greater mass and is more stable during a contraction than a muscle's insertion.  For example, with the latissimus dorsi muscle, the origin site is the torso, and the insertion is the arm. Normally the distal (arm) moves due to having less mass. This is the case when grabbing objects lighter than the body (like someone beginning on a lat pull down machine). This can be reversed however, such as a gymnast doing a front lever, whose arms are stabilized by holding onto a chin up bar as the torso moves up to meet the arm.
The insertion of a muscle is the structure that it attaches to and tends to be moved by the contraction of the muscle.  This may be a bone, a tendon or the subcutaneous dermal connective tissue. Insertions are usually connections of muscle via tendon to bone. The insertion is a bone which tends to be distal, has less mass, and has greater motion than the origin during a contraction.
Muscles may also be described by the direction that the muscle fibres run in.
Hypertrophy is the increase in muscle size which is a result of an increase in size of the individual cells of the muscle. This usually occurs as a result of exercise.