Physiological Classes of Muscle Fibers

Not all muscle fibers are metabolically alike or adapted to perform the same task. Some respond slowly but are relatively resistant to fatigue, while others respond more quickly but also fatigue quickly (table 11.3). Each primary type of fiber goes by several names:

• Slow oxidative (SO), slow-twitch, red, or type I fibers.

These fibers have relatively abundant mitochondria, myoglobin, and blood capillaries, and therefore a relatively deep red color. They are well adapted to aerobic respiration, which does not generate lactic acid. Thus, these fibers do not fatigue easily. However, in response to a single stimulus, they exhibit a relatively long twitch, lasting about 100 milliseconds (msec). The soleus muscle of the calf and the postural

Saladin: Anatomy & I 11. Muscular Tissue I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

430 Part Two Support and Movement

Table 11.3 Classification of Skeletal Muscle Fibers

Fiber Type

Table 11.3 Classification of Skeletal Muscle Fibers

Properties

Slow Oxidative

Fast Glycolytic

Relative diameter

Smaller

Larger

ATP synthesis

Aerobic

Anaerobic

Fatigue resistance

Good

Poor

ATP hydrolysis

Slow

Fast

Glycolysis

Moderate

Fast

Myoglobin content

Abundant

Low

Glycogen content

Low

Abundant

Mitochondria

Abundant and large

Fewer and smaller

Capillaries

Abundant

Fewer

Color

Red

White, pale

Representative Muscles in Which Fiber Type Is Predominant

Soleus

Gastrocnemius

Erector spinae

Biceps brachii

Quadratus lumborum

Muscles of eye

movement movement muscles of the back are composed mainly of these slow oxidative, high-endurance fibers.

• Fast glycolytic (FG), fast-twitch, white, or type II fibers. These fibers are well adapted for quick responses but not for fatigue resistance. They are rich in enzymes of the phosphagen and glycogen-lactic acid systems. Their sarcoplasmic reticulum releases and reabsorbs Ca2+ quickly, which partially accounts for their quick, forceful contractions. They are poorer than SO fibers in mitochondria, myoglobin, and blood capillaries, so they are relatively pale (hence the expression white fibers). These fibers produce twitches as short as 7.5 msec, but because of the lactic acid they generate, they fatigue more easily than SO fibers. Thus, they are especially important in sports such as basketball that require stop-and-go activity and frequent changes of pace. The gastrocnemius muscle of the calf, biceps brachii of the arm, and the muscles of eye movement consist mainly of FG fibers.

Some authorities recognize two subtypes of FG fibers called types IIA and IIB. Type IIB is the common type just described, while IIA, or intermediate fibers, combine fast-twitch responses with aerobic fatigue-resistant metabolism. Type IIA fibers, however, are relatively rare except in some endurance-trained athletes. The three fiber types can be differentiated histologically by using stains for certain

Fast Oxidative Glycolytic
Figure 11.20 Types of Muscle Fibers. Muscle stained to distinguish fast glycolytic (FG) from slow oxidative (SO) fibers. Cross section.

Table 11.4 Proportion of Slow Oxidative (SO) and Fast Glycolytic (FG) Fibers in the Quadriceps Femoris Muscle of Male Athletes

Sample Population SO FG

Marathon runners 82% 18%

Swimmers 74 26

Average males 45 55

Sprinters and jumpers 37 63

mitochondrial enzymes and other cellular components (fig. 11.20). All muscle fibers of one motor unit belong to the same physiological type.

Nearly all muscles are composed of both SO and FG fibers, but the proportions of these fiber types differ from one muscle to another. Muscles composed mainly of SO fibers are called red muscles and those composed mainly of FG fibers are called white muscles. People with different types and levels of physical activity differ in the proportion of one fiber type to another even in the same muscle, such as the quadriceps femoris of the anterior thigh (table 11.4). It is thought that people are born with a genetic predisposition for a certain ratio of fiber types. Those who go into competitive sports discover the sports at which they can excel and gravitate toward those for which heredity has best equipped them. One person might be a "born sprinter" and another a "born marathoner."

Saladin: Anatomy & I 11. Muscular Tissue I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 11 Muscular Tissue 431

We noted earlier that sometimes two or more muscles act across the same joint and superficially seem to have the same function. We have already seen some reasons why such muscles are not as redundant as they seem. Another reason is that they may differ in the proportion of SO to FG fibers. For example, the gastrocnemius and soleus muscles of the calf both insert on the calcaneus through the same tendon, the calcaneal tendon, so they exert the same pull on the heel. The gastrocnemius, however, is a white, predominantly FG muscle adapted for quick, powerful movements such as jumping, whereas the soleus is a red, predominantly SO muscle that does most of the work in endurance exercises such as jogging and skiing.

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