HUMAN BIOLOGY

the MUSCULar SYSteM 107

What is the general structure and function

of a skeletal muscle?

• A skeletal muscle consists of muscle cells bundled together by
  connective tissue. Tendons strap skeletal muscles to bone.


• When a skeletal muscle contracts, it pulls on a bone to produce
  movement.


• In many movements, the action of one muscle opposes or
  reverses the action of another.


• Red or “slow” skeletal muscle fibers have features that support
  slow, long-lasting contractions.


• White or “fast” skeletal muscle fibers are specialized for rapid,
  strong bursts of contraction.

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Other muscles work in a synergistic, or support, role.
Their contraction adds force or helps stabilize another con-
tracting muscle. If you make a fist while keeping your wrist
straight, synergist muscles are stabilizing your wrist joint
while muscles in your hand are doing the “heavy lifting”
of closing your fingers.

skeletal muscle includes “fast”
and “slow” types

Your body has two basic types of skeletal muscle (Figure
6.6A). “Slow” or “red” muscle appears crimson because its
fibers are packed with myoglobin, a reddish protein that
binds oxygen for the cell’s use in making ATP. Red muscle
also is served by larger numbers of the tiny blood vessels
called capillaries. (Red muscle is the dark meat in chicken
and turkey.) Red muscle contracts fairly slowly, but because
its fibers are so well equipped to make lots of ATP, the
contractions can be sustained for a long time. For exam-
ple, some muscles of the back and legs—called postural
muscles because they aid body support—must contract for
long periods when a person is standing. They have a high
proportion of red muscle fibers. By contrast, the muscles
of your hand have fewer capillaries and relatively more
“fast” or “white” muscle fibers, in which there are fewer
mitochondria and less myoglobin. Fast muscle can contract
rapidly and powerfully for short periods, but it can’t sustain
contractions for long periods. This is why you get writer’s
cramp if you write longhand for an extended period.
When an athlete trains rigorously, one goal is to increase
the relative size and contractile strength of fast or slow

Figure 6.5 Animated! Arm movements demonstrate the
action of opposing muscle groups. A When the triceps relaxes
and its opposing partner (biceps) contracts, the elbow joint
flexes and the forearm bends up. b When the triceps contracts and
the biceps relaxes, the forearm is extended down. (© Cengage Learning)

Figure 6.6 A skeletal muscle has “fast” and “slow”

fibers. A This micrograph shows a cross section of the

different kinds of fibers in a skeletal muscle. The lighter

“white fibers” are fast muscle. They have little myoglobin

and fewer mitochondria than the dark red fibers, which

are slow muscle. b The shoulder muscles of a distance

swimmer can work for an extended time because they

contain many well-developed slow muscle fibers.

(A: Biophoto Associates/Science Source; B: Tim Davis/Science Source)

biceps contracts at

the same time, and

pulls forelimb up

triceps

contracts,

pulls the

forelimb

down

at the same

time, biceps

relaxes

triceps relaxes

insertion

origin

A B

b

fast

slow

A

fibers in muscles. The type of sport determines which type

of fiber is targeted. A sprinter will benefit from larger,

stronger fast muscle fibers in the thighs, while a distance

swimmer (Figure 6.6B) will train to increase the number of

mitochondria in the shoulder muscle fibers.

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