The Navy SEAL Physical Fitness Guide - Uniformed Services ...

Table 1-1. Distinguishing Characteristics of MajorMuscle Fiber TypesFiberCharacteristicSlowTwitchType II aFast TwitchType II bFast TwitchAerobic Capacity High Moderate/High LowAnaerobic Capacity Low High HighContraction Speed Slow Fast FastFatigue Resistance High Moderate/High LowMyoglobin Content High High LowGlycogen Content Low Moderate HighColor Red Reddish-white WhiteDistribution of Fiber TypesThe amount of Type I and Type II muscle fibers in an individual is genetically predetermined,and all normal skeletal muscle contains all fiber types. However, theproportion or distribution of these fiber types within and across individuals differs.Moreover, within an individual, the distribution of fibers in various muscles can varywidely. Physical training may transform muscle fiber type, and the metabolic capacityof both type I and type II muscle fibers can be modified by endurance and powertraining. It should be noted that performance depends not only on your fiber typecomposition, but also on the interplay between a variety of factors such as training, anddiet, etc. Figure 1-1 represents the average percentage of slow twitch (Type I) fibers foundin various subgroups of the population. As can be seen, persons whose activities areprimarily endurance-related have a higher proportion of Type I fibers as compared tosprinters or wrestlers. Also in Figure 1-1 are the maximal aerobic capacities of thesegroups: 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 trainingcan induce changes in muscle fiber composition and characteristics.The Navy SEAL Physical Fitness Guide 9