Chapter 9
tion of a limb. The contraction of some muscles leads to two
types of limb movement, depending on the contractile state of
other muscles acting on the same limb. For example, contrac-
tion of the gastrocnemius muscle in the calf causes a fl exion
of the leg at the knee, as in walking (
Figure 9–28
). However,
contraction of the gastrocnemius muscle with the simultaneous
contraction of the quadriceps femoris (which causes extension
of the lower leg) prevents the knee joint from bending, leaving
only the ankle joint capable of moving. The foot is extended,
and the body rises on tiptoe.
The muscles, bones, and joints in the body are
arranged in lever systems. The basic principle of a lever is
illustrated by the fl exion of the arm by the biceps muscle
Figure 9–29
), which exerts an upward pulling tension on
the forearm about 5 cm away from the elbow joint. In this
example, a 10-kg weight held in the hand exerts a downward
load of 10 kg about 35 cm from the elbow. A law of physics
tells us that the forearm is in mechanical equilibrium when
the product of the downward load (10 kg) and its distance
from the elbow (35 cm) is equal to the product of the iso-
strength a weight lifter desires. Most exercises, however,
produce some effect on both strength and endurance.
These changes in muscle in response to repeated periods
of exercise occur slowly over a period of weeks. If regular exer-
cise ceases, the changes in the muscle that occurred as a result
of the exercise will slowly revert to their unexercised state.
The maximum force a muscle generates decreases by 30
to 40 percent between the ages of 30 and 80. This decrease
in tension-generating capacity is due primarily to a decrease
in average fi ber diameter. Some of the change is simply the
result of diminishing physical activity and can be prevented by
regular exercise. The ability of a muscle to adapt to exercise,
however, decreases with age: The same intensity and duration
of exercise in an older individual will not produce the same
amount of change as in a younger person.
This effect of aging, however, is only partial, and there
is no question that even in the elderly, exercise can produce
signifi cant adaptation. Aerobic training has received major
attention because of its effect on the cardiovascular system
(Chapter 12). Strength training to even a modest degree,
however, can partially prevent the loss of muscle tissue that
occurs with aging. Moreover, it helps maintain stronger bones
and joints.
Extensive exercise by an individual whose muscles have
not been used in performing that particular type of exercise
leads to muscle soreness the next day. This soreness is the result
of a mild infl ammation in the muscle, which occurs whenever
tissues are damaged (Chapter 18). The most severe infl amma-
tion results from lengthening contractions, indicating that the
lengthening of a muscle fi ber by an external force produces
greater muscle damage than does either shortening or isomet-
ric contraction. Thus, exercising by gradually lowering weights
will produce greater muscle soreness than an equivalent
amount of weight lifting. This explains a phenomenon well-
known to athletic trainers: the shortening contractions of leg
muscles used to run
fl ights of stairs result in far less soreness
than the lengthening contractions used for running
The effects of anabolic steroids on skeletal muscle
growth and strength are described in Chapter 17.
Lever Action of Muscles and Bones
A contracting muscle exerts a force on bones through its con-
necting tendons. When the force is great enough, the bone
moves as the muscle shortens. A contracting muscle exerts
only a pulling force, so that as the muscle shortens, the bones
it is attached to are pulled toward each other.
to the
of a limb at a joint, whereas
of a limb (
Figure 9–27
). These opposing
motions require at least two muscles, one to cause fl exion and
the other extension. Groups of muscles that produce oppo-
sitely directed movements at a joint are known as
For example, from Figure 9–27 we can see that contraction
of the biceps causes fl exion of the arm at the elbow, whereas
contraction of the antagonistic muscle, the triceps, causes the
arm to extend. Both muscles exert only a pulling force upon
the forearm when they contract.
Sets of antagonistic muscles are required not only for
fl exion-extension, but also for side-to-side movements or rota-
Figure 9–27
Antagonistic muscles for fl exion and extension of the forearm.
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