An effective, purposeful movement of the body or any of its parts
involves considerable muscular activity in addition to that of the
muscles that are directly responsible for the movement itself. To begin
with, the muscles causing the movement must have a stable base.
This means that the bone (or bones) not engaged in the movement
but providing attachment for one end of such muscles must be stabilized
by other muscles. In some movements, such as those in which the
hands are used at a high level, the upper arms may need to be maintained
in an elevated position. This necessitates contraction of the shoulder
muscles to support the weight of the arms. Many muscles, especially
those of biaxial and triaxial joints, can cause movements involving
more than one axis, yet it may be that only one of their actions
is needed for the movement in question. A similar situation exists
in regard to the muscles of the scapula. A muscle cannot voluntarily
choose to effect one of its movements and not another; it must depend
on other muscles to contract and prevent the unwanted movement.
Even a simple movement such as threading a needle or hammering a
nail may require the cooperative action of a relatively large number
of muscles, each performing its own particular task in producing
a single, well-coordinated movement.
Muscles have various roles. Their particular roles in a given
movement depend on the requirements of that movement. These roles
are designated prime movers (or agonists), antagonists, and synergists.
Furthermore, if one concedes that the negative function of remaining
relaxed can be viewed as a role, then the muscles that are antagonistic
to the movers may also be included as participants in the total
cooperative effort. The definitions of these roles are as follows.
A mover is a muscle that is directly responsible for producing
a movement. In the majority of movements there are several movers,
some of them of greater importance than others. These are the principal,
or prime, movers. The muscles that help perform the movement but
seem to be of less importance, or contract only under certain circumstances,
are the assistant movers. Muscles that help only when an extra amount
of force is needed, as when a movement is performed against resistance,
are sometimes called emergency muscles. This distinction between
the various muscles that contribute to a movement is an arbitrary
one. There may well be some difference of opinion as to whether
a muscle is a prime or an assistant mover in a given movement.
The term synergy is often used to
describe muscles in various ways, from mutual neutralizer to stabilizer.
In this text, synergistic muscle action will be used to indicate
cooperative muscle functioning in various roles. Muscles acting
as synergists may fill one of several roles. The way in which a
muscle aids in the production of the desired motion determines its
Fixator, and Supporting Muscles
This group includes the muscles that contract statically to steady
or support some part of the body against the pull of the contracting
muscles, against the pull of gravity, or against the effect of momentum
and recoil in certain vigorous movements. One of the most common
functions of stabilizers is steadying or fixating the bone to which
a contracting muscle is attached. It is only by the stabilizing
of one of its attachments that the muscle is able to cause an effective
movement of the bone at which it has its other attachment (Figure
3.9). The term supporting may be used
when a limb or the trunk must be supported against the pull of gravity
while a distal segment such as the hand, foot, or head is engaging
in the essential movement.
If the scapula were not stabilized, the teres major would
increase the upward rotation of the scapula as it adducted the humerus.
This dual action on the humerus and scapula is shown in (a). In
(b) the scapula is stabilized by the scapular adductors and downward
rotators. This permits the teres major to concentrate its force
on the adduction of the humerus.
A neutralizer is a muscle that acts to prevent an undesired action
of one of the movers. Thus, if a muscle both flexes and abducts,
but only flexion is desired in the movement, an adductor contracts
to prevent the abduction action of the mover.
Occasionally two of the movers have one action in common but
can also perform second actions that are antagonistic to each other.
For instance, one muscle may upward rotate and adduct while the
other may downward rotate and adduct. When they contract together
to cause adduction, their rotary functions counteract each other
(Figure 3.10). Muscles that behave this way in a movement are mutual
synergists as well as movers.
The trapezius and rhomboids as mutual movers and neutralizers.
(a) The trapezius alone adducts the scapula and rotates it upward.
(b) The rhomboids alone adduct the scapula and rotate it downward.
(c) Together the trapezius and rhomboids adduct the scapula without
rotating it either upward or downward.
Muscles that have an effect opposite to that of movers, or agonists,
are labeled antagonists. Because they are located on the opposite
side of the joint from the movers, they are also called contralateral
muscles. The elbow flexors, located on the anterior arm, are antagonistic
to the elbow extensors located on the posterior arm. When the forearm
is extended at the elbow, as in doing a push-up, the extensors are
the movers and contract concentrically to provide the force for the
movement. The flexors are the antagonists and are relaxed. In the
return movement, the letdown, the joint action at the elbow is flexion
but the flexors do not contract—they remain relaxed. The
contracting muscles, once again, are the extensors, but now they
are contracting eccentrically to resist gravity and control the
speed of the elbow flexion. When a body segment is moved by muscular
effort, the contracting muscles are the movers in concentric contraction.
When movement of a segment is effected by the force of gravity and
resisted by muscle force, the contracting muscles are the antagonists
in eccentric contraction.
Now that the general rule has been stated, it is necessary to
describe what may at first appear to be a contradiction. If a movement
performed with great force and rapidity is not checked, it will
subject the ligamentous reinforcements of the joint to sudden strain.
The tissues would probably be severely damaged. This is particularly
true of quick movements of the arm or leg because of the tremendous
momentum that can be developed in a long lever. This is often referred
to as a ballistic movement, which by definition is a movement carried
on by its own momentum. To prevent injury from the momentum of the
limbs, the muscles that are antagonistic to the movers contract momentarily
to check the movement. As they contract, the movers relax, if indeed
they have not already relaxed, allowing momentum to complete the
movement. The situation is a little like taking the foot off the
accelerator to put it on the brake. At the moment when the movement
is being checked, the so-called antagonistic muscles are not truly
antagonistic. In a vigorous movement, the antagonistic muscles may
be said to perform two functions. Their first function is to relax
in order to permit the movement to be made without hindrance; their
second function is to act as a brake at the completion of the movement
and, by doing so, to protect the joint.
Summary of Muscle
Classification Based on Role in Total Movement
- Mover or agonist: A muscle
that is directly responsible for effecting a movement.
- Fixator, stabilizer, supporting muscle: Muscles
that contract statically to steady or to support some part of the
body against the pull of contracting muscles, the pull of gravity,
or any other force that interferes with the desired movement.
- Neutralizer: A muscle that acts
to prevent an undesired action of one of the movers.
- Antagonist: A muscle that causes
the opposite movement from that of the movers.
Example of the
Different Roles of Muscles in a Total Movement
Let us consider the movement of the right upper arm when playing
shuffleboard (pushing a disk with a cue). The movement of the humerus
at the shoulder joint is flexion, and this is accompanied by slight
upward rotation and abduction of the scapula. The movers of the humerus are the anterior
deltoid, the clavicular portion of the pectoralis major, and the
coracobrachialis. (Look up these terms if you are not yet familiar
with them.) The two former muscles are also inward rotators of the
humerus, but because this movement is not desired, it must be prevented. The
infraspinatus and the teres minor take care of this. Therefore,
they are serving as neutralizers in
this pushing action. Meanwhile, the scapula is rotating upward through
the action of the serratus anterior and trapezium II and IV, which
are serving as shoulder girdle movers. Trapezius
II, which is an elevator as well as an upward rotator of the scapula,
and trapezium IV, which is a depressor as well as an upward rotator,
mutually neutralize each other with respect to elevation and depression
while they are cooperating in rotating the scapula upward. Hence
they are both movers and mutual neutralizers.
Consider the movers of the humerus once again, keeping in mind
that muscles tend to pull both their distal and proximal ends toward
each other. Note that the proximal attachments of both the anterior
deltoid and the pectoralis major are side by side on the anterior
border of the clavicle. As they contract, they not only raise the
humerus forward but also tend to pull the clavicle laterally, a movement
that would put a strain on the sternoclavicular joint. They are
prevented from doing this by the action of the subclavius muscle,
which pulls the clavicle medially and thus serves as a stabilizer.
So in this simple pushing movement we see that we have muscles acting
as movers, neutralizers, and stabilizers, which, by their cooperative
action, ensure an efficient movement.
The patterns of muscle activation that can occur in the production
of human movement are vast in number. Recent research in muscle
activation has shown that the relationships between muscle actions
is, by nature, task dependent. That is, the relationships will depend
on the nature of the motion to be produced. Simultaneous cocontraction
or coactivation of agonist and antagonist muscles across one or
more joints does occur. Cocontraction is most often associated with
stabilization, either during postural loading or in dynamic, unstable
situations. The cocontraction of the abdominal muscles, for instance,
helps stabilize the trunk during lifting tasks. In a similar fashion, cocontraction
of the muscles of the arm stabilizes the arm when learning an accuracy
task (Granata and Marras 2000; Gribble et al. 2003).
Cocontraction of biarticular muscles is common. The muscles
that act as an agonist–antagonist pair for the motion of
one joint often reverse roles for the second joint. In this situation,
both muscles are active, but for different reasons. A more detailed
description of the actions of biarticular muscles follows.
Action of Biarticular
Another type of coordination of the muscular system may be seen
in the action of the biarticular muscles—that is, the muscles
that pass over and act on two joints. Examples of these are the
hamstrings (the semitendinosus, the semimembranosus, and the biceps
femoris), which flex the leg at the knee and extend the thigh at
the hip; the rectus femoris, which flexes the thigh and extends
the leg; the sartorius, which flexes both the thigh and the leg;
the gastrocnemius, which helps flex the leg in addition to its primary
function of extending the foot; and the long flexors and extensors
of the fingers. The latter are actually multijoint muscles because
they cross the wrist and at least two of the joints of the fingers.
A characteristic of all these muscles, whether they act on joints
that flex in the same direction, as in the case of the wrist and
fingers, or in the opposite direction, as in the case of the knee
and hip, is that they are not long enough to permit complete movement
in both joints at the same time. This results in the tension of
one muscle being transmitted to the other, in much the same manner
that a downward pull on a rope that passes through an overhead pulley
is transmitted in the form of a pull in the reverse direction to
the rope on the other side of the pulley. Thus, if the hamstrings
contract to help extend the hip, tension in the form of a stretch is
placed on the rectus femoris, causing it to extend the knee. Or
if the rectus femoris contracts to help flex the hip, tension in
the form of a stretch is placed on the hamstrings, causing them
to flex the knee. This is a simplified version of what in actuality
is a rather complex coordination.
When monoarticular (one-joint) muscles contract, their shortening
is accompanied by a corresponding loss of tension. In quick movements
of the limbs, monoarticular muscles rapidly lose their tension.
The advantage of biarticular muscles is that they can continue to
exert tension without shortening. The biarticular muscles have two
different patterns of action, described as concurrent and countercurrent movements.
An example of concurrent movement is seen in the simultaneous
extension of the hip and knee and also in the simultaneous flexion
of these joints. As the muscles contract, they act on each other
in such a way that they do not lose length; thus their tension is
retained. It is as though the pull traveled up one muscle and down
the other in a continuous circuit. In simultaneous extension of
the hip and knee, for instance, the rectus femoris’s loss
of tension at the distal or knee end is balanced by a gain in tension
at the proximal or hip end. Similarly, the hamstrings, which are
losing tension at their proximal end, are gaining it at their distal
The countercurrent pattern presents a different picture. In this
type of movement, while one of the biarticular muscles shortens
rapidly at both joints, its antagonist lengthens correspondingly
and thereby gains tension at both ends. An example of this action
is seen in the rapid loss of tension in the rectus femoris and corresponding
gain of tension in the hamstrings when the hip is flexed and the
knee is extended simultaneously. A vigorous kick is a dramatic illustration
of this kind of muscle action. The backward swing of the lower extremity
preparatory to kicking is a less spectacular but equally valid example.
The forward swing in walking is another. In both patterns of movement
the monoarticular and biarticular muscles appear to supplement each
other and thus, by their cooperative action, produce smooth, coordinated,
efficient movements (Figure 3.11).
(a) A concurrent muscle action of the rectus femoris
on top of the thigh and the hamstrings on the underside of the thigh
occurs when each of the antagonist muscles lengthens at one joint
and shortens at the other. (b) A countercurrent muscle action occurs
as the rectus femoris shortens concentrically at both joints while
the hamstrings are stretched at both joints.
When a biarticular muscle contracts, it acts on both joints it
crosses. If action is desired in only one joint, the other joint
must be stabilized by another muscle or by some external force.
In such circumstances, Basmajian (Basmajian and DeLuca 1985) found
that the rectus femoris shows maximum activity in either pure hip
flexion or pure knee extension, and the medial hamstrings, similarly,
are most active in either hip extension or knee flexion. In the
biarticular countercurrent movements of hip flexion and knee extension,
the rectus femoris exhibited strong activity. The medial hamstrings
exhibited similar strong activity during hip extension and knee
flexion. When the movement was concurrent hip and knee flexion,
however, the rectus femoris showed no activity, but the hamstring
did. Similarly, during knee and hip extension, the hamstrings were
inhibited and the rectus femoris was active. The activity of the
rectus femoris and hamstrings was thus inhibited when they were
Types of Bodily
Movements may be passive or active, and if active, they may be
slow or rapid. They may involve the constant application of force,
or after the initial impetus has been given, they may continue without
further muscular effort.
A passive movement requires no effort
on the part of the person involved. It is performed by another person such
as a therapist giving a treatment or an instructor or partner stretching
tight ligaments, fascia, and muscles in an attempt to increase the
range of motion of a particular joint. In some cases, it is a movement
that has been started by the subject’s own effort but is
continued by momentum. It might also be caused by the force of gravity
if the subject remained relaxed and used no muscular effort to aid,
restrain, or guide the moving part. To avoid injury, care must be
taken that the subject does not resist the movement and that the
person applying the force to cause the movement is not too aggressive.
An active movement is produced by
the subject’s own muscular activity. It is usually performed
volitionally, but it may be a reflex reaction to an external or
internal stimulus. It may be rapid or slow. In slow movements muscular
tension is maintained throughout the range of motion. Pushing a
heavy piece of furniture across the room is an example of this type
of movement. In rapid movements, tension could also be maintained
throughout the range of motion, but this would be an inefficient way
of performing. For efficiency, rapid movements should be performed
ballistically. The concept of ballistic movement
was introduced a number of years ago by experimental psychologists.
They used the term for movements that were initiated by vigorous
muscular contraction and completed by momentum. This type of movement
is characteristic of throwing, striking, and kicking. It is also seen
in the finger movements used for typing and piano playing. When
such movements are performed nonballistically—that is,
with constant muscular contraction—they are uneconomical
and hence not skillful. In fact, they are characteristic of the
way in which beginners tend to attempt new coordinations, especially
if they are concentrating on accuracy of aim rather than on a ballistic
type of motion. They need to be encouraged in the early stages of
learning a skill to concentrate on form rather than accuracy if
they are to master the skill of moving ballistically.
Ballistic movements may be terminated by one of three methods:
(1) by contracting antagonistic muscles, as in the forehand drive
in tennis; (2) by allowing the moving part to reach the limit of motion,
in which case it will be stopped by the passive resistance of ligaments,
other tissues, or the braking action of antagonistic muscles, as
in the case of the forceful overarm throw; or (3) by the interference
of an obstacle, as when chopping wood.
In many movements found in both sports and activities of living,
three types of muscular action cooperate to produce a single act.
This kind of cooperation is seen especially in striking activities that
require the use of an implement, such as a tennis racket, golf club,
or ax. Movements such as these involve (1) fixation to support the
moving part and to maintain the necessary position, (2) ballistic
movement of the active limb, and (3) fixation in the fingers as
they grasp the implement.