Human Physiology, 14th edition (2016)

380 Chapter 12

are accordingly faster than the muscles of the legs. These dif-

ferences in contraction speed are associated with other differ-

ences, including different myosin ATPase isoenzymes that can

also be designated as slow or fast. For example, researchers

have measured a sixfold difference in the rate of ATP hydro-

lysis between the myosin of the slow soleus and the fast psoas

muscles. The extraocular muscles that position the eyes have

a high proportion of fast-twitch fibers and reach maximum

tension in about 7.3 msec (milliseconds—thousandths of a

second). The soleus muscle in the leg, by contrast, has a high

proportion of slow-twitch fibers and requires about 100 msec

to reach maximum tension ( fig. 12.25 ).

Muscles like the soleus are postural muscles; they are

able to sustain a contraction for a long period of time without

fatigue. The resistance to fatigue demonstrated by these mus-

cles is aided by other characteristics of slow-twitch (type I)

fibers that endow them with a high oxidative capacity for aero-

bic respiration. Hence, the type I fibers are often referred to as

slow oxidative fibers. These fibers have a rich capillary supply,

numerous mitochondria and aerobic respiratory enzymes, and a

high concentration of myoglobin. Myoglobin is a red pigment,

similar to the hemoglobin in red blood cells, that improves the

delivery of oxygen to the slow-twitch fibers. Because of their

high myoglobin content, slow-twitch fibers are also called red

fibers. Because slow, type I muscle fibers can obtain all of the

ATP they need through aerobic respiration in mitochondria,

they can contract without fatigue (discussed in the next sec-

tion) longer than can other types of muscle fibers.

The thicker fast-twitch (type II) fibers have fewer capillar-

ies and mitochondria than slow-twitch fibers and not as much

myoglobin; hence, these fibers are also called white fibers.

Fast-twitch fibers are adapted to metabolize anaerobically by a

large store of glycogen and a high concentration of glycolytic

enzymes.

In addition to the type I (slow-twitch) and type II (fast-

twitch) fibers, human muscles have an intermediate fiber type.

These intermediate fibers are fast-twitch but also have a high

oxidative capacity; therefore, they are relatively resistant to

fatigue. They are called type IIA fibers, or fast oxidative

fibers, because of their aerobic ability. The other fast-twitch

fibers are anaerobically adapted; these are called fast glycolytic

fibers because of their high rate of glycolysis. Not all fast gly-

colytic fibers are alike, however. There are different fibers in

this class, which vary in their contraction speeds and glycolytic

Slow- and Fast-Twitch Fibers

Skeletal muscle fibers can be divided on the basis of their
contraction speed (time required to reach maximum tension)
into slow-twitch, or type I, fibers, and fast-twitch, or type
II, fibers. In general, the arms have more type II fibers and

into muscle fibers), strength, and performance during short-
term, high-intensity exercise; however, creatine supplemen-
tation has not been observed to improve performance during
more sustained exercise. Studies of the long-term effects of
creatine supplements in rodents suggest possible damaging
effects to the liver and kidneys, but the health implications of
these studies to long-term creatine supplementation in humans
are not established.

Figure 12.24 The production and use of phosphocreatine in muscles. Phosphocreatine serves as a muscle reserve of
high-energy phosphate, used for the rapid formation of ATP. These reactions are catalyzed by creatine phosphokinase (CPK).

ATP Phosphocreatine

Muscle

contraction

ADP

During rest During exercise

Creatine ADP

ATP ADP + Pi

CLINICAL APPLICATION

Creatine kinase ( CK ), also called creatine phosphokinase

( CPK ), is an enzyme measured in blood samples to aid the

diagnosis of such conditions as myocardial infarction (heart

attack), muscular dystrophy, rhabdomyolysis, and others.

Rhabdomyolysis, which can produce a greatly increased

concentration of CK in the blood, refers to skeletal muscle

damage producing muscle weakness and pain. Different

drugs, various forms of trauma, and certain toxins may cause

rhabdomyolysis. If necessary for diagnosis, laboratories

can test for different isoenzymatic forms of CK ( chapter 4,

section 4.1). For example, damaged skeletal muscles

release the CK-MM isoenzyme, whereas damaged heart

muscle releases the CK-MB isoenzyme.

Clinical Investigation CLUES

Mia had a blood test for CK-MB, and the results were

normal.

• What function does this enzyme normally perform in
  the muscle cell?


• What is the clinical significance of this test result?