action and cause movement in the opposite direction—the
antagonist of the prime mover that raises the upper limb can
lower the upper limb, or the antagonist of the prime mover
that bends the upper limb can straighten it. If both a prime
mover and its antagonist contract simultaneously, the struc-
ture they act upon remains rigid. Similarly, smooth body
This is reﬂ ected in its name
meaning “two heads.” As
F gure 9.22 shows, one head of the muscle is attached to the
coracoid process of the scapula, and the other head arises
from a tubercle above the glenoid cavity of the scapula. The
muscle extends along the anterior surface of the humerus
and is inserted by a single tendon on the radial tuberosity of
the radius. When the biceps brachii contracts, its insertion is
pulled toward its origin, and the elbow bends.
Interaction of Skeletal Muscles
Skeletal muscles almost always function in groups. As a
result, when a particular body part moves, a person must do
more than contract a single muscle; instead, after learning to
make a particular movement, the person wills the movement
to occur, and the nervous system stimulates the appropriate
group of muscles.
By carefully observing body movements, it is possible to
determine the roles of particular muscles. ±or instance, abduc-
tion of the arm requires contracting the
muscle, said to
A prime mover is the muscle
primarily responsible for producing an action. However, while
a prime mover is acting, certain nearby muscles also contract.
When a deltoid muscle contracts, nearby muscles help hold
the shoulder steady and in this way make the action of the
prime mover more effective. Muscles that contract and assist
a prime mover are called
Still other muscles act as
to prime movers. These muscles can resist a prime mover’s
The biceps brachii has two heads that originate on the
scapula. A tendon inserts this muscle on the radius.
Levers and movement. (
) When the forearm bends at the elbow or (
) when the forearm straightens at the elbow, the bones and
muscles function as a lever.