Type II Fibers

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Type II muscle fibers have relatively poor aerobic endurance in comparison with type I fibers. In type II fibers, ATP is formed through anaerobic, not oxidative, pathways when oxygen supply is insufficient. Type IIa motor units generate considerably more force than do type I motor units, but they also fatigue more easily. Thus type IIa fibers appear to be the primary fiber type used during shorter, high-intensity endurance events, such as the mile run or the 400-m swim.

Type IIx fibers are not easily activated by the nervous system. Thus they are used rather infrequently in low-intensity activity but are predominant in highly explosive activity, such as the 100-m dash or the 50-m swim. Current knowledge of type IIx fibers is quite limited.

TABLE 5-1 Characteristics of Fiber Types of Skeletal Muscles

Fiber Types

Type I (Slow Twitch)

Type IIa (Fast Twitch a)

Type IIx (Fast Twitch x)

Contractile speed

Slow

Fast

Fast

Fatigue resistance

High

Moderate

Low

Oxidative capacity

High

Moderately high

Low

Glycolytic capacity

Low

High

Highest

Motor unit strength

Low

High

High

The composition of a person's muscle fibers appears to be determined early in life, possibly within the first few years. Genes determine which a-motor neurons innervate individual muscle fibers. After innervation is established, the signals from a-motor neurons determine the differentiation of muscle types. Physical environment also affects the differentiation. There is some evidence that endurance training and strength training, as well as muscular inactivity, may cause a shift in the myosin isoforms. Training may induce a small change, perhaps less than 10%, in the amount of type I and type II fibers. Both endurance and resistance training have been shown to reduce the percentage of type IIx fibers and increase the proportion of type IIa fibers.

Researchers have further shown that aging alters the genetic expression of muscle fibers. Older people tend to lose type II motor units, which increases the percentage of type I fibers.

Athletes who participate in low-intensity but high-endurance sports have a high percentage of type I fibers, whereas those of high-intensity, short-term and explosive activities have more type II fibers. As anticipated, the leg muscles of distance runners, who rely on endurance, have a predominance of type I fibers (Table 5-2).1

Despite all these data, it is difficult to conclusively determine whether type I and type II fibers can be transformed into each other by prolonged intensive training. Each type of muscle fiber is controlled by a particular type of motor neuron. This innervation of a particular muscle fiber by a motor neuron may not change. It is possible that after a very long period of power training, muscle fibers that require a full oxygen supply may adapt to work with a smaller oxygen supply. It is presumed that type II muscle fibers can be trained to operate at a more consistent production of energy with improved blood flow and, therefore, better oxygen supply. As a result, less lactic acid is produced, and these type II fibers can work for a longer period of time before signs of exhaustion occur. It is, therefore, very important for athletes that they recover from, or at least mitigate, musculo-skeletal stress or symptoms of overtraining as soon as possible after each event and before attempting the next one. This can be achieved with the help of needling techniques that are introduced in this book.

It has been observed that type I fibers change after prolonged work of relatively low intensity: • The capillary net in the muscle around the type I fibers increases. This means that the ability to supply the operating fibers with oxygen and energy-providing substances also increases.

TABLE 5-2

Percentage of Muscle Fibers in Selected Muscles of Male and Female Athletes

Athlete

Sex

Muscle

% Type I

% Type II

Sprint runner

Male

Gastrocnemius

24

76

Female

Gastrocnemius

27

73

Distance runner

Male

Gastrocnemius

79

21

Female

Gastrocnemius

69

31

Cyclist

Male

Vastus lateralis

57

43

Female

Vastus lateralis

51

49

Swimmer

Male

Posterior deltoid

67

33

Weightlifter

Male

Gastrocnemius

44

56

Male

Deltoid

53

47

Triathletes

Male

Posterior deltoid

60

40

Male

Vastus lateralis

63

37

Male

Gastrocnemius

59

41

Canoeists

Male

Posterior deltoid

71

29

Shot-putters

Male

Gastrocnemius

38

62

Nonathletes

Male

Vastus lateralis

47

53

Female

Gastrocnemius

52

48

From Wilmore JH, Costill DL, Kenney WL: Physiology of sports and exercise, Champaign, IL, 2008, Human Kinetics, p 41.

From Wilmore JH, Costill DL, Kenney WL: Physiology of sports and exercise, Champaign, IL, 2008, Human Kinetics, p 41.

• The number and size of mitochondria in the fibers increase. This means that the ability to produce energy increases at the required pace.

• The muscle endurance with submaximal loads increases. This means that a person can perform the same movement many more times.

• The size of the fibers changes very little or not at all. This means that there is no increase in strength.

Training with heavy loads affects mainly type II fibers. The main changes in type II muscles are as follows:

• The size (the cross-section area) of the fibers increases, and so more force can be produced. The increase of cross-sectional area results from the creation of more fibrils in the fiber. The number of fibers does not change. Genes for fibrils are activated.

• The fiber's ability to work with less oxygen increases, and so the fiber produces less lactic acid.

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