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Four different types of motor units - slow-twitch, fatigue resistant (type S), fast-twitch, fatigue resistant (type FR), fast-twitch, fatigue intermediate (type FInt), and fast-twitch, fatigable (type FF) - are classified based on contractile and fatigue properties of innervated muscle fibers (MyHCSlow, MyHC2A, MyHC2X, and MyHC2B). The speed of contraction varies between the motor units. Modified from Mantilla and Sieck (148), with permission.
