Muscle Terminology

There are about 640 named muscles in the human body in addition to 1000s of smaller un-named muscles.

Definitions of some words used to refer to muscles and muscle functions follow in alphabetical order.




Movement of a bone, and the limb of which it is a part, away from the mid-line of the body.
Verb: To abduct


Any muscle that moves one part of the body away from another - or away from the midline of the body.


Movement (usually of skin or bone) caused by muscle contractions.

Group Actions:

Most movements occur when several skeletal muscles act in groups, as opposed to individually.

Most skeletal muscles are arranged at joints, in opposing pairs.

The muscle that causes an action is referred to as the prime mover or agonist (= leader). Often another muscle, called the antagonist (ant = against), relaxes while the prime mover contracts.

Most movements also involve muscles called synergists, which help the prime mover function more efficiently by reducing unnecessary movement.

Some muscles in a group also act as fixators, stabilizing the origin of the prime mover so that the prime mover can act more efficiently.

Under different conditions and depending on the movement, many muscles act at various times as prime movers, antagonists, synergists, or fixators.


Movement of a bone, and the limb of which it is a part, towards the mid-line of the body.
Verb: To adduct


Any muscle that moves one part of the body towards another or towards the midline of the body.


Backward flexion of a foot or hand, or of their digits. That is, bending fingers or toes backwards towards the upper surface of the foot or hand.

Put another way: Bending of the foot (or hand) in the direction of the dorsum (superior surface), e.g. when standing on ones heels.


Increasing the angle between articulating bones, often to restore a part of the body to the anatomical position after it has been flexed.


The inability of a muscle to contract forcefully after prolonged activity.

An important factor in muscle fatigue is the lowered release of calcium ions from the sarcoplasmic reticulum, resulting in a decline of Ca2+ level in the sarcoplasm. Other factors that contribute to muscle fatigue include depletion of creatine phosphate, insufficient oxygen, depletion of glycogen and other nutrients, build-up of lactic acid and ADP, and failure of nerve impulses in the motor neuron to release enough acetyl-choline. Because increased lactic acid would cause a decrease in the pH of body fluids, muscle fatigue may be viewed as a homeostatic mechanism that prevents the pH from dropping below the normal acceptable range.


Decreasing the angle between articulating bones. For example, "Plantar Flexion" is the bending of the toes (or fingers) downwards towards the sole (or palm).


A muscle that causes a limb or other body-part to bend.
Examples include the mucles of the arms, legs, fingers, and toes.


The position at which the other end of the muscle (i.e. from its origin) is attached, by means of a tendon, to the moveable bone.


The attachment of muscle (by means of a tendon) to the stationary bone.


Flexion of a foot or hand, or of their digits. That is, bending fingers or toes towards the lower surface of the foot or hand. For example, bending the foot in the direction of the plantar surface, as when standing on your toes - as opposed to ones heels.


Movement of the forearm so that the palm is turned backward or downward.


Movement of a bone such that it revolves around its own longitudinal axis (e.g. turning the head from side to side at the joint between the atlas and axis).


Movement of the forearm so that the palm is turned forward or upward.


Even when a whole muscle is not contracting, a small number of its motor units are involuntarily activated to produce a sustained contraction of the muscle fibers. The process gives rise to muscle tone.

To sustain muscle tone, small groups of motor units are alternately active and inactive in a constantly shifting pattern. Muscle tone keeps skeletal muscles firm, but it does not result in a contraction strong enough to produce movement.

See also information about anterior muscles, posterior muscles, and muscles of facial expression.

More about Muscles

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