274
Chapter 9
3.
Inhibition of Cross-Bridge Cycling.
The buildup
of ADP and P
i
within muscle fi bers during intense
activity may directly inhibit cross-bridge cycling (in
particular step 2) by mass action. Slowing the rate of
this step delays cross-bridge detachment from actin,
and thus slows the overall rate of cross-bridge cycling.
These changes contribute to the reduced shortening
velocity and impaired relaxation observed in muscle
fatigue resulting from high-intensity exercise.
With low-intensity, long-duration exercise, a number
of processes have been implicated in fatigue, but no single
process can completely account for it. The three factors just
listed may play minor roles in this type of exercise as well,
but it appears that depletion of fuel substrates may be more
important. Although ATP depletion is not a cause of fatigue,
the decrease in muscle glycogen, which supplies much of the
fuel for contraction, correlates closely with fatigue onset. In
addition, low blood glucose (hypoglycemia) and dehydration
have been demonstrated to increase fatigue. Thus, a certain
level of carbohydrate metabolism appears necessary to pre-
vent fatigue during low-intensity exercise, but the mecha-
nism of this requirement is unknown.
Another type of fatigue quite different from muscle
fatigue occurs when the appropriate regions of the cerebral
cortex fail to send excitatory signals to the motor neurons.
This is called
central command fatigue,
and it may cause
a person to stop exercising even though the muscles are not
fatigued. An athlete’s performance depends not only on the
physical state of the appropriate muscles but also upon the
“will to win”—that is, the ability to initiate central com-
mands to muscles during a period of increasingly distressful
sensations.
Types of Skeletal Muscle Fibers
Skeletal muscle fi bers do not all have the same mechanical
and metabolic characteristics. Different types of fi
bers can
be identifi ed on the basis of (1) their maximal velocities of
shortening—fast or slow—and (2) the major pathway they
use to form ATP—oxidative or glycolytic.
Fast and slow fi bers contain forms of myosin that dif-
fer in the maximal rates at which they split ATP. This, in
turn, determines the maximal rate of cross-bridge cycling
and thus the maximal shortening velocity. Fibers contain-
ing myosin with high ATPase activity are classifi ed as
fast
bers,
and are also sometimes referred to as type II fi
bers.
Several subtypes of fast myosin can be distinguished based
on small variations in their structure. By contrast, fi bers con-
taining myosin with lower ATPase activity are called
slow
bers,
or type I fi
bers. Although the rate of cross-bridge
cycling is about four times faster in fast fi
bers than in slow
fi bers, the force produced by both types of cross-bridges is
about the same.
The second means of classifying skeletal muscle fi bers
is according to the type of enzymatic machinery available for
synthesizing ATP. Some fi bers contain numerous mitochon-
dria and thus have a high capacity for oxidative phosphoryla-
tion. These fi bers are classifi ed as
oxidative fi
bers.
Most of
the ATP such fi bers produce is dependent upon blood fl ow to
deliver oxygen and fuel molecules to the muscle. Not surpris-
ingly, therefore, these fi
bers are surrounded by many small
blood vessels. They also contain large amounts of an oxygen-
binding protein known as
myoglobin,
which increases the
rate of oxygen diffusion within the fi ber and provides a small
store of oxygen. The large amounts of myoglobin present in
oxidative fi bers give the fi bers a dark red color, and thus oxi-
dative fi bers are often referred to as
red muscle fi
bers.
In contrast,
glycolytic fi
bers
have few mitochondria
but possess a high concentration of glycolytic enzymes and
a large store of glycogen. Corresponding to their limited use
of oxygen, these fi bers are surrounded by relatively few blood
vessels and contain little myoglobin. The lack of myoglobin
is responsible for the pale color of glycolytic fi
bers and their
designation as
white muscle fi
bers.
On the basis of these two characteristics, three types of
skeletal muscle fi bers can be distinguished:
1.
Slow-oxidative fi
bers
(Type I)combine low myosin-
ATPase activity with high oxidative capacity.
2.
Fast-oxidative-glycolytic fi
bers
(Type IIa) combine
high myosin-ATPase activity with high oxidative
capacity and intermediate glycolytic capacity.
3.
Fast-glycolytic fi
bers
(Type IIb) combine high
myosin-ATPase activity with high glycolytic capacity.
Note that the fourth theoretical possibility—slow-glycolytic
fi bers—is not found.
In addition to these biochemical differences, there are
also size differences. Glycolytic fi bers generally have much
larger diameters than oxidative fi bers (
Figure 9–24
). This fact
has signifi cance for tension development. The number of thick
and thin fi laments per unit of cross-sectional area is about the
same in all types of skeletal muscle fi bers. Therefore, the larger
the diameter of a muscle fi ber, the greater the total number
of thick and thin fi laments acting in parallel to produce force,
and the greater the maximum tension (greater strength) it
can develop. Accordingly, the average glycolytic fi ber, with its
larger diameter, develops more tension when it contracts than
does an average oxidative fi ber.
These three types of fi bers also differ in their capacity
to resist fatigue. Fast-glycolytic fi bers fatigue rapidly, whereas
slow-oxidative fi bers are very resistant to fatigue, which allows
them to maintain contractile activity for long periods with
little loss of tension. Fast-oxidative-glycolytic fi bers have an
intermediate capacity to resist fatigue (
Figure 9–25
).
Table 9–3
summarizes the characteristics of the three
types of skeletal muscle fi bers.
Whole-Muscle Contraction
As described earlier, whole muscles are made up of many mus-
cle fi bers organized into motor units. All the muscle fi bers in
a single motor unit are of the same fi ber type. Thus, you can
apply the fi ber designation to the motor unit and refer to slow-
oxidative motor units, fast-oxidative-glycolytic motor units,
and fast-glycolytic motor units.
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