100 Chapter 5: Training for Strength spectrum of repetition ranges. Lighter loads with high repetitions will target and develop the type I fibers; whereas higher loads with few repetitions will target and develop the type II fibers. Therefore, it is recommended to train at a variety of load and repetition assignments in order to target and develop each muscle fiber type individually and maximize hypertrophy. Figure 5.8. depicts some of the physiological differences between type I and type II muscle fibers. Figure 5.8. Slow-Twitch vs. Fast-Twitch Muscle Fibers Muscle Hypertrophy The process of hypertrophy involves an increase in the number of myofibrils (long filaments that comprise a muscle fiber) as well as the number of contractile proteins (i.e., actin and myosin) within the myofibrils. The extent of hypertrophy is largely based on muscle fiber type. Although both type I and type II muscle fibers adapt and enlarge with chronic strength training, type II fibers generally manifest greater increases in size than type I fibers. Ultimately, an individual’s potential for hypertrophy is likely correlated to the relative percentage of type II muscle fibers (Haff & Triplett, 2016). Research suggests there are two ways in which the muscle gets bigger: sarcoplasmic hypertrophy and myofibrillar hypertrophy (Zatsiorsky & Kraemer, 2006). As the name suggests, sarcoplasmic hypertrophy is characterized by growth of the sarcoplasm (cytoplasm of a muscle fiber that contains ATP, phosphagens and various metabolic enzymes) and noncontractile proteins. Myofibrillar hypertrophy is characterized by an increased number of myofibrils as well as actin and myosin filaments. Although heavy strength training will lead to both sarcoplasmic and myofibrillar hypertrophy, Olympic weightlifting and powerlifting tend to promote more myofibrillar hypertrophy; whereas, bodybuilding tends to promote more sarcoplasmic hypertrophy. Figure 5.9 depicts the anatomy of skeletal muscle. Figure 5.10 depicts the differences between sarcoplasmic and myofibrillar hypertrophy within a muscle fiber.
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