Most muscles are composed of all three motor unit types
interspersed with each other (
Figure 9–26
). No muscle has
only a single fi ber type. Depending on the proportions of the
fi ber types present, muscles can differ considerably in their max-
imal contraction speed, strength, and fatigability. For example,
the muscles of the back, which must be able to maintain their
activity for long periods of time without fatigue while support-
ing an upright posture, contain large numbers of slow-oxidative
and fast-oxidative-glycolytic fi bers. In contrast, the muscles in
the arms, which may be called upon to produce large amounts
of tension over a short time period, as when lifting a heavy
object, have a greater proportion of fast-glycolytic fi bers.
We will next use the characteristics of single fi bers to
describe whole-muscle contraction and its control.
Control of Muscle Tension
The total tension a muscle can develop depends upon two fac-
tors: (1) the amount of tension developed by each fi ber, and
(2) the number of fi bers contracting at any time. By controlling
these two factors, the nervous system controls whole-muscle
tension as well as shortening velocity. The conditions that deter-
mine the amount of tension developed in a single fi ber have
been discussed previously and are summarized in
Table 9–4
The number of fi bers contracting at any time depends
on: (1) the number of fi
bers in each motor unit (motor unit
size), and (2) the number of active motor units.
Motor unit size varies considerably from one muscle to
another. The muscles in the hand and eye, which produce very
delicate movements, contain small motor units. For example,
one motor neuron innervates only about 13 fi
bers in an eye
muscle. In contrast, in the more coarsely controlled muscles
of the legs, each motor unit is large, containing hundreds and,
in some cases, several thousand fi bers. When a muscle is com-
posed of small motor units, the total tension the muscle pro-
duces can be increased in small steps by activating additional
motor units. If the motor units are large, large increases in
tension will occur as each additional motor unit is activated.
Thus, fi ner control of muscle tension is possible in muscles
with small motor units.
The force a single fi ber produces, as we have seen ear-
lier, depends in part on the fi ber diameter—the greater the
diameter, the greater the force. We have also noted that fast-
glycolytic fi bers have the largest diameters. Thus, a motor unit
composed of 100 fast-glycolytic fi
bers produces more force
than a motor unit composed of 100 slow-oxidative fi bers. In
addition, fast-glycolytic motor units tend to have more muscle
fi bers. For both of these reasons, activating a fast-glycolytic
glycolytic fiber
Figure 9–24
Muscle fi ber types in normal human muscle, prepared using ATPase
stain. Darkest fi bers are slow-oxidative type; lighter-colored fi bers
are fast-oxidative-glycolytic and fast-glycolytic fi bers.
Tension (mg)
Tension (mg)
Tension (mg)
Fast-oxidative-glycolytic fibers
Time (min)
Time (min)
Fast-glycolytic fibers
Time (min)
Slow-oxidative fibers
Figure 9–25
The rate of fatigue development in the three fi ber types. Each
vertical line is the contractile response to a brief tetanic stimulus and
relaxation. The contractile responses occurring between about 9
min and 60 min are not shown on the fi
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