Anatomical terms of muscle

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

Action
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.

Agonists and antagonists
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 lengthening. It is still the agonist. While resisting gravity during lengthening, 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 muscles which are 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.

Agonist-antagonist pairs
Antagonist and agonist muscles often occur in pairs, called antagonistic pairs. As one muscle contracts, the other relaxes. An example of antagonisic pairs is the Bicep and Tricep as to contract your tricep lengthens(relaxes) while the Bicep contracts to lift your 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 (i.e., increasing the angle between the two bones) and a flexor muscle, which does the opposite to an extensor muscle.

Not all muscles are paired in this way. An example of exception is Sphincter ani externus muscle.

Synergistic action
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.

Insertion and origin
In anatomy, the insertion is a point at which a muscle attaches to a bone, a tendon or the subcutaneous dermal connective tissue.

The origin of a muscle is the bone, typically proximal, which has greater mass and is more stable during a contraction than its counterpart called the 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. 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.

Muscle fibres


Muscles may also be described by the direction that the muscle fibres run in.
 * Unipennate muscles have fibres that run the entire length of only one side of a muscle, like a quill pen. For example, the the fibularis muscle.
 * Bipennate muscles consist of two rows of oblique muscle fibres, facing in opposite diagonal directions, converging on a central tendon. Bipennate muscle is stronger than unipennate muscle and fusiform muscle, due to a larger physiological cross-sectional area. Bipennate muscle shortens less than unipennate muscle but develops greater tension when it does, translated into greater power but less range of motion. Pennate muscles generally also tire easily. Examples of bipennate muscle in the human body are the rectus femoris muscle of the thigh, and the stapedius muscle of the middle ear.
 * Fusiform muscles have fibres that run parallel to the length of the muscle. For example, the pronator teres muscle of the forearm.