Distribution of Fiber Types

The amount of Type I and Type II muscle fibers in an individual is genetically pre-determined, and all normal skeletal muscle contains all fiber types. However, the proportion or distribution of these fiber types within and across individuals differs. Moreover, within an individual, the distribution of fibers in various muscles can vary widely. Physical training may transform musclc fiber type, and the metabolic capacity of both type I and type II muscle fibers can be modified by endurance and power training. It should be noted that performance depends not only on your fiber type composition, but also on the interplay between a variety of factors such as training, and diet, etc. Figure 1-1 represents the average percentage of slow twitch (Type I) fibers found in various subgroups of the population. As can be seen, persons whose activities are primarily endurance-related have a higher proportion of Type I fibers as compared to sprinters or wrestlers. Also in Figure 1-1 are the maximal aerobic capacities of these groups: the greater the proportion of Type I fibers, the higher the maximal aerobic capacity. Note the wide variability in the group of "recreational athletes". Specific types of training can induce changes in musclc fiber composition and characteristics.

Figure 1-1. Muscle Fiber Composition and Maximal Oxygen Uptake Values for Various Athletes by Sport

Slow Twitch Fibers Maximal Oxygen Uptake

20 40 60 80 100 40 50 60 70 80 90 100

Cross Country Skiers

Long Distance Runners Swimmers Weight Lifters Wrestlers Sprinters Recreational Athletes

Figure adapted from: Exercise Physiology: Energy Nutrition arul Unman Performance, McArdle WD. II Kalch, VI, Kalch. 4di edition. 1996. Williams and Wilkins Publishers.

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