Human Physiology, 14th edition (2016)

Muscle 381

Figure 12.25 A comparison of the rates at which maximum tension is developed in three muscles. These are ( a ) the
relatively fast-twitch extraocular and ( b ) gastrocnemius muscles, and ( c ) the slow-twitch soleus muscle.

ab c

Time (msec)

Tension

abilities. In some animals, the extreme fast glycolytic fibers are
of the type designated type IIB fibers. In humans, these fast
glycolytic fibers have a different myosin protein and an even
lower oxidative ability than the type IIB fibers, and so are des-
ignated as type IIX fibers. During maximal exercise, the type
IIX fibers have the greatest rate of ATP consumption and the
fastest rate of ATP and phosphocreatine depletion. The three
major fiber types in humans are compared in table 12.3.
People vary tremendously in the proportion of fast- and
slow-twitch fibers in their muscles ( fig. 12.26 ). The percentage
of slow-twitch, type I fibers in the quadriceps femoris muscles
of the legs, for example, can vary from under 20% (in people
who are excellent sprinters) to as high as 95% (in people who
are good marathon runners). These differences are believed to
primarily result from differences in genetics, although physical
training is also an important factor. This is demonstrated by the
complete disappearance of type 1 fibers from human muscle
after long-term spinal cord injury ( fig. 12.26 ).
A muscle such as the gastrocnemius contains both fast and
slow-twitch fibers, although fast-twitch fibers predominate.

A given somatic motor axon, however, innervates muscle fibers

of one type only. The sizes of these motor units differ; the

motor units composed of slow-twitch fibers tend to be smaller

(have fewer fibers) than the motor units of fast-twitch fibers.

As mentioned earlier, motor units are recruited from smaller

to larger when increasing effort is required; thus, the smaller

motor units with slow-twitch fibers would be used most often

in routine activities. Larger motor units with fast-twitch fibers,

which can exert a great deal of force but which metabolize

anaerobically and thus fatigue quickly, would be used rela-

tively infrequently and for only short periods of time.

Muscle Fatigue

Muscle fatigue may be defined as a reversible, exercise-

induced reduction in the ability of a muscle to generate force.

Fatigue during a sustained maximal contraction, when all the

motor units are used and the rate of neural firing is maximal—

as when lifting an extremely heavy weight—appears to be due

to an accumulation of extracellular K^1. Concentrations of K^1

Feature Slow Oxidative/Type I (Red) Fast Oxidative/Type IIA (Red) Fast Glycolytic/Type IIX (White)

Diameter Small Intermediate Large

Z-line thickness Wide Intermediate Narrow

Glycogen content Low Intermediate High

Resistance to fatigue High Intermediate Low

Capillaries Many Many Few

Myoglobin content High High Low

Respiration Aerobic Aerobic Anaerobic

Oxidative capacity High High Low

Glycolytic ability Low High High

Twitch rate Slow Faster Fastest

Myosin ATPase rate Low Higher Highest

Table 12.3 | Characteristics of Muscle Fiber Types