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. These differences are associated with different myosin ATPase isoenzymes, which can also be designated as "slow" and "fast." The two fiber types can be distinguished by their ATPase isoenzyme when they are appropriately stained (fig. 12.23). The extraocular muscles that position the eyes, for example, 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. 12.24).

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 muscles is aided by other characteristics of slow-twitch (type I) fibers that endow them with a high oxidative capacity for aerobic respiration. Hence, the type I fibers are often referred to as slow oxidative fibers. These fibers have a rich capillary supply, numerous mitochondria

Slow Oxidative Muscles
Figure 12.22 The production and utilization of phosphocreatine in muscles. Phosphocreatine serves as a muscle reserve of high-energy phosphate, used for the rapid formation of ATP.

Muscle 345

and aerobic respiratory enzymes, and a high concentration of myo-globin. 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.

Slow And Fast Twitch Fibres

■ Figure 12.23 Skeletal muscle stained to indicate activity of myosin ATPase. ATPase activity is greater in the type II (fast-twitch) fibers than in the type I (slow-twitch) fibers. Among the fast-twitch fibers, ATPase activity is greatest in the fast-glycolytic (IIX) fibers. The fast-oxidative (IIA) fibers show an intermediate level of activity.

The thicker, fast-twitch (type II) fibers have fewer capillaries 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 respire 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 glycolytic fibers are alike, however. There are different fibers in this class, which vary in their contraction speeds and glycolytic abilities. In some animals, the extreme fast glycolytic fibers are of the type designated type IIB fibers. In humans, these fast glycolytic fibers are currently designated type IIX fibers. The three major fiber types in humans are compared in table 12.4.

People vary tremendously in the proportion of fast- and slow-twitch fibers in their muscles (fig. 12.25). The percent 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 be primarily the result of differences in genetics.

A muscle such as the gastrocnemius contains both fastand slow-twitch fibers, although fast-twitch fibers predominate.

Figure Slow And Fast Twitch Muscles

■ Figure 12.24 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.

Table 12.4 Characteristics of Muscle Fiber Types

Feature

Slow Oxidative/Red (Type I)

Fast Oxidative/White (Type IIA)

Fast Glycolytic/White (Type IIX)

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

Fast

Fast

Myosin ATPase content

Low

High

Muscle Fiber Type Average Couch Potato

Person World-

with class spinal sprinter injury

Average Average couch active potato person

Middle- World- Extreme distance class endurance runner marathon athlete runner

Person World-

with class spinal sprinter injury

Average Average couch active potato person

Middle- World- Extreme distance class endurance runner marathon athlete runner

■ Figure 12.25 Relative abundance of different muscle fiber types in different people. The percent of slow type I fibers, fast type IIX fibers, and intermediate fast type IIA fibers in the muscles of different people varies tremendously. This is due to differences in genetics and to the effects of physical training.

A given somatic motor axon, however, innervates muscle fibers of one type only. The sizes of these motor units differ; the motors 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 respire anaerobically and thus fatigue quickly, would be used relatively infrequently and for only short periods of time.

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Responses

  • virgilia
    Is aerobic respiration slow type 1 fibers?
    5 years ago
  • alfredino
    Why does fast twitch muscles respire anaerobically?
    5 years ago
  • Michael
    Why do type 1 fibres have "fewer" myoglobin than type 2?
    5 years ago
  • James
    Why large store of glycogen in fast twitch fibres?
    5 years ago
  • Mauro Fanucci
    Why are slow fibers also called red fibers?
    5 years ago
  • aman
    Why does couch potato have 60% fast and 20% slow motor units?
    5 years ago
  • andreas
    What is the fast , slow , and intermediate twitch fibers?
    5 years ago
  • Anne
    Which of the fiber types can reach maximal tension the fastest?
    5 years ago
  • regolo
    Which muscle fiber reaches its maximum tension the fastest?
    5 years ago
  • CARAMELLA CHUBB
    How is fast and slow twitch related to respiration?
    5 years ago
  • lukas freeh
    How are fast twitch fibres adapted?
    5 years ago
  • nasih
    Where are high concentrations of slow twitch fibers?
    5 years ago
  • melanie
    What activity is aided by a large proportion of fasttwitch muscles?
    5 years ago
  • tim
    What are twitch fibers in human?
    5 years ago
  • lamorac
    What Muscles have a high myoglobin content in humans?
    4 years ago
  • Markus
    Which activity would be aided by a large proportion of slowtwitch muscles?
    4 years ago
  • Faye
    Have a high percentage of fast twitch fibers in gastrocnemius?
    4 years ago
  • becky dickson
    Are soleus muscles rich in myglobin?
    4 years ago
  • paul
    Do slow twitch musclue have myoglobin?
    4 years ago
  • aaliyah williamson
    Which muscles have the most fast twitch fibers?
    4 years ago
  • SEMRAWIT
    Is soleus muscle rich in myoglobin?
    4 years ago
  • ULRICH
    Which of the fiber type can reach maximum tension the fastest?
    2 years ago
  • beniamino rizzo
    How do slow twitch fibers take to reach maximum tension?
    1 year ago
  • ALANNAH
    Why are slow twitch muscles abundant in capillaries?
    4 months ago

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