Chapter 6: Training for Strength 131 Major Muscle Groups and Fiber Types Approximately two-thirds of the muscles in the body are skeletal muscles. In fact, there are over 600 skeletal muscles within the body, which account for roughly 40 percent of overall body weight (Rainey & Murray, 2005). Figure 6.5 depicts some of the major muscle groups. Figure 6.5. Major Muscle Groups Each muscle group is comprised of individual muscle fibers. There are two basic types of muscle fibers: slow-twitch (aka slow oxidative or type I) and fast-twitch (aka type II). Similarly, there are two basic subtypes of fast twitch muscle fibers: fast oxidative (aka type IIa) and fast glycolytic (aka type IIx). Fast oxidative are sometimes referred to as intermediate fibers because they possess characteristics of both slow twitch and fast twitch muscle fibers (Betts et al., 2017). Although the percentages of muscle fibers are genetically determined, transitions between subtypes can occur with regular strength training (Haff & Triplett, 2016). This process is referred to as fiber type transition (i.e., change in a fiber’s subtype classification as a result of chronic training). As previously mentioned, transitions between subtypes of the same fiber type can occur with training; however, transitions from one fiber type to another do not occur regardless of training. In other words, slow-twitch
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