Essentials of Anatomy and Physiology

Joints that are capable of a variety of movements
have several sets of antagonists. Notice how many
ways you can move your upper arm at the shoulder, for
instance. Abducting (laterally raising) the arm is the
function of the deltoid. Adducting the arm is brought
about by the pectoralis major and latissimus dorsi.
Flexion of the arm (across the chest) is also a function
of the pectoralis major, and extension of the arm
(behind the back) is also a function of the latissimus
dorsi. All of these muscles are described and depicted
in the tables and figures later in the chapter. Without
antagonistic muscles, this variety of movements would
not be possible.
You may be familiar with range-of-motion (or
ROM) exercises that are often recommended for
patients confined to bed. Such exercises are designed
to stretch and contract the antagonistic muscles of a
joint to preserve as much muscle function and joint
mobility as possible.

Synergistic Muscles

Synergistic musclesare those with the same func-
tion, or those that work together to perform a partic-
ular function. Recall that the biceps brachii flexes the
forearm. The brachioradialis, with its origin on the
humerus and insertion on the radius, also flexes
the forearm. There is even a third flexor of the fore-
arm, the brachialis. You may wonder why we need
three muscles to perform the same function, and the

explanation lies in the great mobility of the hand. If

the hand is palm up, the biceps does most of the work

of flexing and may be called the prime mover. When

the hand is thumb up, the brachioradialis is in position

to be the prime mover, and when the hand is palm

down, the brachialis becomes the prime mover. If you

have ever tried to do chin-ups, you know that it is

much easier with your palms toward you than with

palms away from you. This is because the biceps is a

larger, and usually much stronger, muscle than is the

brachialis.

Muscles may also be called synergists if they help to

stabilize or steady a joint to make a more precise

movement possible. If you drink a glass of water, the

biceps brachii may be the prime mover to flex the

forearm. At the same time, the muscles of the shoul-

der keep that joint stable, so that the water gets to

your mouth, not over your shoulder or down your

chin. The shoulder muscles are considered synergists

for this movement because their contribution makes

the movement effective.

THE ROLE OF THE BRAIN

Even our simplest movements require the interaction

of many muscles, and the contraction of skeletal mus-

cles depends on the brain. The nerve impulses for

movement come from the frontal lobesof the cere-

brum. The cerebrum is the largest part of the brain;

the frontal lobes are beneath the frontal bone. The

The Muscular System 139

Radius

Ulna Humerus

Triceps (contracted)

Biceps (relaxed)

Scapula

Triceps (relaxed)

Biceps (contracted)

A Extension B Flexion

Figure 7–1. Antagonistic muscles. (A) Extension of the forearm. (B) Flexion of the

forearm.

QUESTION:When the biceps contracts, what happens to its length, and what kind of force

does it exert?