During a given rotary motion, a skeletal muscle may have one of several different roles to play. During the motion, a muscle may change from one role to another.
Prime Mover FAQ.
Of a group of muscles acting upon a moving part, the one producing the strongest and most direct force is in the prime mover role. Its force is in the direction of the motion being produced.
When another skeletal muscle produces an added force in the same general direction as the prime mover, it is referred to as a synergist.
The muscles moving a part are often arranged so that they tend to move the part at a small angle from the intended direction. In such cases, an additional muscle, the neutralizer, is present to counteract and correct the direction of pull.
Muscles whose lines of pull are opposite to the direction of motion are referred to as antagonists. Antagonists are extremely important for making a smooth, coordinated motion. They tend to adjust the actual direction, speed, and distance of the motion. Without proper antagonists, the motions of the body parts become uncontrolled and flailing. When the motion is completed, the antagonist contracts and returns the part moved to its original position.
A stabilizer is a skeletal muscle that ensures that the joint being moved is properly maintained.
When one joint is moved, the other joints of the body must be kept immobile so that the desired motion can take place normally. The skeletal muscles that hold these other joints immobile are called fixators.
Secondary Roles FAQ.
Potential secondary role of a muscle is very important to medical personnel
for two reasons:
First, during evaluation of a patient's muscular system, a muscle may only appear to be working properly. In fact, it may not be functioning. Its action may have been taken over by another muscle acting in its secondary role.
Next, one may know that a muscle is no longer functioning properly. In such a case, it may be possible to design exercises to develop the secondary role of another muscle so that it will perform the action of the first muscle as a part of a rehabilitation program.