Force Health Protection: Nutrition and Exercise Resource Manual

Which classification in Table 7-1 best matches the requirements of your
job? Which classification best describes your leisure activities?
Muscle Fiber Types
Before discussing strength training exercises and
guidelines, here is a quick review of muscle physiology. To
generate force, muscles contract. This action requires ATP
(see Chapter 4, page 31). Muscle fibers are classified
according to which energy system they use to make ATP.
The three types of skeletal muscle fibers are:
◆ Slow Twitch Oxidative (Type I) fibers are mostly involved in
endurance activities. They rely on ATP from aerobic energy
metabolism (see page 31) and are generally resistant to fatigue.
◆ Fast Twitch Glycolytic (Type IIb) fibers are involved in quick,
strong muscle contractions and rely on the anaerobic energy
systems to produce ATP. These fibers are susceptible to fatigue.
◆ Fast Twitch Oxidative-Glycolytic (Type IIa) fibers are a cross
between the slow and fast twitch fibers since they rely on both
aerobic and anaerobic systems (see page 31) for energy.
Everyone has all three muscle fiber types; however, genetics determine
which fiber type is more abundant in each person. Physical training can lead
to changes in the characteristics of the muscle fibers. For example, endurance
training makes the Type IIa fiber act more like the slow-twitch, Type I fiber.
Determinants of Muscle Size
Apart from muscle fiber type, various factors influence muscle size (see
Figure 7-1). Although some factors cannot be controlled, two factors that we
can control are exercise and nutrition habits (Chapters 3, 4, and 11).
Nutrition and Exercise Resource Manual 57