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 are muscles that cause or inhibit a movement.
Agonist muscles are also called
prime movers since they produce most of the force, and control of an action. Agonists cause a movement to occur through their own activation. For example, the
triceps brachii contracts, producing a
shortening (concentric) contraction, 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 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. Another example is the dumb-bell curl at the elbow. The
elbow flexor group is the agonist, shortening during the lifting phase (
elbow flexion). During the lowering phase the elbow flexor muscles lengthen, remaining the agonists because they are controlling the load and the movement (elbow extension). For both the lifting and lowering phase, the "elbow extensor" muscles are the antagonists (see below). They lengthen during the dumbbell lifting phase and shorten during the dumbbell lowering phase. 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, this naming convention does not mean they are only agonists during shortening. This term typically describes the function of
skeletal muscles.
Antagonist muscles are simply the muscles that produce an opposing joint torque to the agonist muscles. This torque can aid in controlling a motion. The opposing torque can slow movement down - especially in the case of a
ballistic movement. For example, during a very rapid (ballistic) discrete movement of the elbow, such as throwing a dart, the triceps muscles will be activated very briefly and strongly (in a "burst") to rapidly accelerate the extension movement at the elbow, followed almost immediately by a "burst" of activation to the elbow flexor muscles that decelerates the elbow movement to arrive at a quick stop. To use an automotive analogy, this would be similar to pressing the accelerator pedal rapidly and then immediately pressing the brake. Antagonism is not an intrinsic property of a particular muscle or muscle group; it is a role that a muscle plays depending on which muscle is currently the agonist. During slower joint actions that involve gravity, just as with the agonist muscle, the antagonist muscle can shorten and lengthen. Using the example of the triceps brachii during a push-up, the elbow flexor muscles are the antagonists at the elbow during both the up phase and down phase of the movement. During the dumbbell curl, the elbow extensors are the antagonists for both the lifting and lowering phases.
Antagonistic pairs Antagonist and agonist muscles often occur in pairs, called
antagonistic pairs. As one muscle contracts, the other
relaxes. An example of an antagonistic 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. 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. This is called agonist/antagonist co-activation and serves to mechanically stiffen the joint. Not all muscles are paired in this way. An example of an exception is the
deltoid.
Synergists Synergist muscles also called
fixators, act around a joint to help the action of an
agonist muscle. Synergist muscles can also act to counter or neutralize the force of an agonist and are also known as
neutralizers when they do this. As neutralizers they help to cancel out or neutralize extra motion produced from the agonists to ensure that the force generated works within the desired plane of motion. Muscle fibers can only contract up to 40% of their fully stretched length. 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. 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.
Neutralizer action 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. A good famous 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. For the knee to flex while not rotating in either direction, all three muscles contract to stabilize the knee while it moves in the desired way.
Composite muscle Composite or
hybrid muscles have more than one set of fibers that perform the same function, and are usually supplied by different nerves for different set of fibers. For example, the tongue itself is a composite muscle made up of various components like longitudinal, transverse, horizontal muscles with different parts innervated from a different nerve supply. ==Muscle naming==