b. Synergist. When another skeletal muscle produces an added force in the
same general direction as the prime mover, it is referred to as a synergist.
c. Neutralizer. 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
d. Antagonist. 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.
e. Stabilizer. A stabilizer is a skeletal muscle that ensures that the joint being
moved is properly maintained.
f. Fixator. 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.
5-18. SECONDARY ROLES OF SKELETAL MUSCLES
Most skeletal muscles are not directly aligned with the desired motions of the
joints. This means that they are potentially able to produce secondary motions at these
joints. This potential secondary role of a muscle is very important to medical personnel
for two reasons:
a. 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.
b. 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
5-19. OTHER FUNCTIONS OF SKELETAL MUSCLES
Besides moving the body parts around joints, skeletal muscles also perform other
purposes in the human body.
a. Some muscles are specially designed to maintain the erect posture of the