Muscle
277
input increases. Because the smallest motor neurons innervate
the slow-oxidative motor units (see Table 9–3), these motor
units are recruited fi rst, followed by fast-oxidative-glycolytic
motor units, and fi nally, during very strong contractions, by
fast-glycolytic motor units (see Figure 9–26).
Thus, during moderate-strength contractions, such as
those that occur in most endurance types of exercise, relatively
few fast-glycolytic motor units are recruited, and most of the
activity occurs in the more fatigue-resistant oxidative fi bers. The
large fast-glycolytic motor units, which fatigue rapidly, begin to
be recruited when the intensity of contraction exceeds about 40
percent of the maximal tension the muscle can produce.
In summary
, the neural control of whole-muscle ten-
sion involves (1) the frequency of action potentials in individ-
ual motor units (to vary the tension generated by the fi bers in
that unit) and (2) the recruitment of motor units (to vary the
number of active fi bers). Most motor neuron activity occurs
in bursts of action potentials, which produce tetanic contrac-
tions of individual motor units rather than single twitches.
Recall that the tension of a single fi ber increases only three- to
fi vefold when going from a twitch to a maximal tetanic con-
traction. Therefore, varying the frequency of action potentials
in the neurons supplying them provides a way to make only
three- to fi vefold adjustments in the tension of the recruited
motor units. The force a whole muscle exerts can be varied
over a much wider range than this, from very delicate move-
ments to extremely powerful contractions, by recruiting motor
units. Thus, recruitment provides the primary means of vary-
ing tension in a whole muscle. Recruitment is controlled by the
central commands from the motor centers in the brain to the
various motor neurons (Chapter 10).
Control of Shortening Velocity
As we saw earlier, the velocity at which a single muscle fi ber short-
ens is determined by (1) the load on the fi ber and (2) whether
the fi ber is a fast or slow fi ber. Translated to a whole muscle,
these characteristics become (1) the load on the whole muscle
and (2) the types of motor units in the muscle. For the whole
muscle, however, recruitment becomes a third very important
factor, one that explains how the shortening velocity can be
varied from very fast to very slow even though the load on the
muscle remains constant. Consider, for the sake of illustration,
a muscle composed of only two motor units of the same size
and fi ber type. One motor unit by itself will lift a 4-g load
more slowly than a 2-g load because the shortening velocity
decreases with increasing load. When both units are active and
a 4-g load is lifted, each motor unit bears only half the load,
and its fi bers will shorten as if it were lifting only a 2-g load. In
other words, the muscle will lift the 4-g load at a higher veloc-
ity when both motor units are active. Recruitment of motor
units thus leads to increases in both force and velocity.
Muscle Adaptation to Exercise
The regularity with which a muscle is used, as well as the dura-
tion and intensity of its activity, affect the properties of the
muscle. If the neurons to a skeletal muscle are destroyed or the
neuromuscular junctions become nonfunctional, the dener-
vated muscle fi
bers will become progressively smaller in diam-
eter, and the amount of contractile proteins they contain will
decrease. This condition is known as
denervation atrophy.
A muscle can also atrophy with its nerve supply intact if the
muscle is not used for a long period of time, as when a broken
arm or leg is immobilized in a cast. This condition is known
as
disuse atrophy.
In contrast to the decrease in muscle mass that results from
a lack of neural stimulation, increased amounts of contractile
activity—in other words, exercise—can produce an increase in
the size (hypertrophy) of muscle fi
bers as well as changes in
their capacity for ATP production.
Exercise that is of relatively low intensity but long duration
(popularly called “aerobic exercise”), such as running or swim-
ming, produces increases in the number of mitochondria in
the fi
bers that are recruited in this type of activity. In addition,
the number of capillaries around these fi bers also increases. All
these changes lead to an increase in the capacity for endurance
activity with a minimum of fatigue. (Surprisingly, fi ber diam-
eter decreases slightly, and thus there is a small decrease in the
maximal strength of muscles as a result of endurance training.)
As we will see in later chapters, endurance exercise produces
changes not only in the skeletal muscles but also in the respira-
tory and circulatory systems, changes that improve the delivery
of oxygen and fuel molecules to the muscle.
In contrast, short-duration, high-intensity exercise (popu-
larly called “strength training”), such as weight lifting, affects
primarily the fast-glycolytic fi bers, which are recruited during
strong contractions. These fi bers undergo an increase in fi ber
diameter (hypertrophy) due to the increased synthesis of actin
and myosin fi laments, which form more myofi brils. In addition,
glycolytic activity is increased by increasing the synthesis of gly-
colytic enzymes. The result of such high-intensity exercise is an
increase in the strength of the muscle and the bulging muscles
of a conditioned weight lifter. Such muscles, although very pow-
erful, have little capacity for endurance, and they fatigue rapidly.
Exercise produces limited change in the types of myo-
sin enzymes the fi
bers form and thus little change in the
proportions of fast and slow fi bers in a muscle. As described
previously, however, exercise does change the rates at which
metabolic enzymes are synthesized, leading to changes in the
proportion of oxidative and glycolytic fi bers within a muscle.
With endurance training, there is a decrease in the number of
fast-glycolytic fi bers and an increase in the number of fast-oxi-
dative-glycolytic fi bers as the oxidative capacity of the fi bers
increases. The reverse occurs with strength training as fast-
oxidative-glycolytic fi bers convert to fast-glycolytic fi bers.
The signals responsible for all these changes in muscle
with different types of activity are unknown. They are related
to the frequency and intensity of the contractile activity in the
muscle fi bers and thus to the pattern of action potentials pro-
duced in the muscle over an extended period of time.
Because different types of exercise produce quite different
changes in the strength and endurance capacity of a muscle, an
individual performing regular exercise to improve muscle per-
formance must choose a type of exercise compatible with the
type of activity he or she ultimately wishes to perform. Thus,
lifting weights will not improve the endurance of a long-
distance runner, and jogging will not produce the increased
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