Skeletal muscle is composed of 2 kinds of muscle fibers- Type I, also known as slow twitch, and Type II, or fast twitch. Fast twitch fibers are also divided in two subgroups called Type IIa and IIb. Muscle fiber contraction obeys a sequential recruitment pattern where Type I muscle fibers are the first ones to be recruited. As exercise intensity increases muscle contractile demands increase and Type I muscle fibers cannot sustain the necessary demand. Type IIa muscle fibers kick in and eventually as intensity keeps increasing Type IIb will finally be recruited. Simply put, slow twitch fibers are used at slower speeds and fast twitch at faster speeds. Each muscle fiber has different biochemical properties and thus different behaviors during exercise and competition. Type I muscle fibers have the highest mitochondrial density and capacity and therefore are very efficient at utilizing fat for energy purposes. Type IIa fibers have a lower mitochondrial density and a higher capacity to utilize glucose. Type IIb muscle fibers have a little mitochondrial density and a very high capacity to use glucose as well as ATP stored in these fibers for instant anaerobic energy. Therefore, each exercise intensity implies different metabolic responses and muscle fiber recruitment patterns which also corresponds to different training zones which are summarized below:

Types of Skeletal Muscle Fibers