64 Introduction to Medical Terminology
Copyright Goodheart-Willcox Co., Inc.
• elasticity (EE-las-TIS-ih-tee)—the ability of
skeletal muscle fi bers to resume their resting fi
length when a stretching force is removed; for
example, when a spring is stretched and then
released, it returns to its original position.
• excitability (also called irritability)—the ability
of a skeletal muscle to receive and respond to
a nerve impulse by contracting. For example,
nerves send a message to the heart muscle,
and the heart pumps blood through the vessels
throughout the body.
• extensibility (eks-TENS-ih-BIL-ih-tee)—the abil- y
ity of the skeletal muscles to be stretched. Extensi-
bility is evident in the hamstrings and calf muscles
during a toe touch, for example.
The ratio of muscle to total body weight is often
expressed as a percentage. Percentages can
also be expressed as fractions. (For example,
¼ is 25 percent.) To calculate a percentage,
fi rst set up a fraction. Then divide the
numerator (top number) by the denominator
(bottom number).
Directions: To practice calculating
percentages, do the word problems that follow.
Round your answers to the nearest whole
number.
1. Blake weighs 140 pounds, 60 pounds of
which are muscle. What percentage of
Blake’s body weight is muscle?
2. What is the percentage of muscle weight
for an 80-pound female with 35 pounds of
muscle?
Muscle Origin and Insertion
Skeletal muscle plays a vital role in directional
movements of the skeleton. The connecting parts
of each muscle are specific to that muscle and the fi
bone to which the muscle is connected. Skeletal
muscle is attached to bone at both ends of the
muscle.
Typically, during contraction, one end of a mus-
cle remains fi xed in its position, and the other end fi
of the muscle moves. The origin of a muscle is the
attachment site that does not move when the muscle
contracts; the insertion is the attachment site that
moves during muscular contraction. Figure  3.2
illustrates examples of origin and insertion points
in the shoulder and forearm.
Most skeletal muscle is attached to bone by
tendons. A tendon is a band of fi brous tissue that fi
connects muscle to bone (Figure 3.3). The Achilles
tendon, for example, attaches the gastrocnemius
(GAS-trok-NEE-mee-us) muscle on the calf of the
leg to the calcaneus (kal-KAY-nee-us), or heel bone.
Tendons are different from ligaments. As you
learned in Chapter 2: The Skeletal System, a
ligament is a fi brous cord of tissue that attaches fi
bone to bone. Ligaments usually impart stability
by holding structures together (Figure 3.3).
Skeletal Muscle Organization
The basic structure of muscles includes sev-
eral connective tissue layers (Figure  3.4). The
inner layer, or endomysium (EN-doh-MIZ-ee-
um), is a fine sheath of areolar connective tissue fi
around each muscle fi ber. fi Areolar tissue consists of
loosely woven fi bers and many blood vessels. The fi
perimysium (PEER-ih-MIZ-ee-um) is a sheath
of connective tissue that surrounds bundles of
muscle fi bers called fi fascicles (FAS-ih-kuhlz). The
outer layer, or epimysium (EP-ih-MIZ-ee-um), is
a dense, fibrous connective tissue that surrounds fi
the entire muscle. All of these structures are sur-
rounded by fascia (FASH-ee-uh), fibrous con- fi
nective tissue that binds muscles into functional
Origin point
Coracoid
process p
Origin point
Supraglenoid Supraglenoid
tuberosity
Biceps
muscle
Biceps
muscle
Insertion point
Tubercule of radius
Figure 3.2 The insertion is the attachment site that
moves during muscular contraction. The origin is the
muscular attachment site that does not move.