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.

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