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Biomechanics of Skeletal Muscle

Fiber Types and Muscle Adaptability

As mentioned, muscle fibers vary in type and can be differentiated by their histological, biochemical, and physiologic properties. Muscle fiber types can be divided into at least three separate entities based on these criteria. Each motor unit possesses fibers that are all of a similar fiber type, but some overlap of muscle fiber types is found within whole muscles.

Determination of fiber type is dependent on the type of myosin ATPase activity and, therefore, contraction velocity. This protein occurs in several varieties, each with different abilities to generate tension at different velocities. Large fibers tend to produce tension at a faster rate, because they tend to express rapid myosin ATPase.

Smaller fibers are inclined to produce tension more slowly, because they usually express a slower form of myosin ATPase. The rates of ATPase activity help to classify muscle fiber types into three different profiles: Type I, Type IIA, and Type IIB . Some overlap certainly occurs, which allows other types to be considered.

Type I fibers are slow oxidative fibers that have slower contraction and relaxation times compared to those of Type II fibers. They are very fatigue resistant because of the high concentration of mitochondria and myoglobin they contain.

Type IIA fibers, or fast oxidative glycolytic fibers, are an intermediate fiber type, because they contain forms of myosin ATPase that are present in both Type I and Type IIB fibers. Therefore, their contraction velocity is faster than that of Type I fibers but slower than that of Type IIB fibers.

Similarly, Type IIA fibers are more fatigue resistant than Type IIB fibers, but less so than Type I fibers. The Type IIB fibers are fast glycolytic fibers that are the least resistant to fatigue, but they have the fastest contraction velocity.

Human skeletal muscle is composed of a combination of the fiber types discussed above, with most muscles having 50% of slow-twitch fibers and 50% fast-twitch fibers. The relative percentage of Type I or Type II fibers generally is accepted to be determined genetically and, therefore, are not subject to change through training.

A study by Gollnick et al. in 1972 demonstrated a preponderance of one fiber type or another in trained athletes that would be advantageous for their event. The endurance athletes who were studied had higher percentages of fatigue-resistant, Type I fibers, whereas nonendurance athletes had higher percentages of Type II fibers.

Despite this knowledge, athletic ability is certainly dependent on more than just fiber type predominance, so muscle biopsy for selecting sport-specific athletes generally is not accepted.

Although muscle fiber type usually is fixed under normal physiologic conditions, various forms of overload stimulation can cause adaptation of muscle fibers, especially within the Type II fibers. For example, prolonged endurance training seems to increase the percentage of Type IIA fibers at the expense of Type IIB fibers, making the muscle more fatigue resistant. Conversely, strength training may increase the percentage of Type IIB fibers.

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