Exercise M aintains H ealthy Muscles
tretching prior to exercise is as important as exercise
in maintaining your muscle health. Proper stretching
(Figure a) lengthens the connective tissue associated with
the muscles and improves flexibility. Stretching should
not only be done prior to exercise but also on a regular,
daily basis. Frequent stretching can reduce muscle ten-
sion, improve agility, and increase range of joint motion.
Stretching should be done slowly, with the application of
gentle force to prevent injury to the muscle or associated
connective tissues.
Exercise does not change the total number of muscle
fibers but can affect the distribution of muscle fiber types
within working muscles. Although your ratio of FG fibers
to SO fibers is determined by genetics, you can change
the proportion of FOG fibers with exercise. For example,
aerobic/endurance exercises such as running (Figure b)
or swimming can cause some FG fibers to change into FOG
fibers. The new FOG fibers are larger and have increased
numbers of mitochondria, increased blood supply, and
strength. The increased size (hypertrophy) is caused by
increased synthesis of additional myosin, actin, troponin,
and tropomyosin.
In contrast, lack of exercise causes a loss of muscle
mass. This is especially true in bedridden patients and
astronauts in microgravity (see the opening to Chapter 5).
The muscle fibers decrease in size (atrophy) as the muscle
proteins degrade.
Think Critically^
Soft tissue injuries (sprains or m uscle pulls) are com m on
in athletes. The soft tissues generally involved in these
injuries are the ligam ents and tendons. M uscle strains
(aches) can also occur if m uscles are stretched too much.
O n e im portant part of the treatm ent for such injuries is
rest. W hy w ould rest be an integral part of the therapy for
healing soft tissue and m uscle injuries?
Sam just started an exercise regime. A t first, he was stiff,
and his muscles fatigued easily. After several weeks of exer-
cising, however, Sam notices that he is more flexible and can
tolerate longer and longer sessions. W hat is happening here?
Muscles work in groups, usual-
ly as pairs of muscles with opposing
actions. The muscle that starts the
desired action is the prime mover,
. As the agonist contracts,
the muscle with an opposing action,
called the
, relaxes. For
example, when you flex (bend) your arm at the elbow, the
biceps is the agonist and the triceps is the antagonist. When
you extend your arm, the two muscles switch roles.
Most movements involve additional muscles called
). A synergist helps the agonist
function more efficiently by reducing unnecessary move-
ments. Some muscles in a group act as
, stabilizing
one of the bones so the agonist can move the other bone
more efficiently. During arm movement, for example,
muscles associated with the scapula serve as fixators to
stabilize the scapula while the humerus moves. Depending
on the movement and the conditions, many muscles can
switch roles, acting as agonists, antagonists, synergists, or
fixators at different times. Exercise is important to main-
taining healthy muscles (see
What a Health Provider Sees).
As mentioned earlier, there are about 700 skeletal
muscles, each with a different name. For an overview of
selected superficial muscles of the human body, (
on the following pages.)
prime mover
The muscle directly
responsible for
producing a desired
motion. Also called
a g o n is t.
Skeletal Muscles Are Grouped Based on Location and Action 169
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