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The three main types of muscle fibers are slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG).  Slow oxidative (SO)  Also called type I fibers, these fibers are specialized for aerobic activity. They produce low power contractions over long periods of time. They are slow to fatigue. Fast oxidative (FO)  Also called type IIa fibers, these fibers use aerobic metabolism to produce ATP. They produce higher tension contractions than SO fibers. Fast glycolytic (FG)  Also called type IIb fibers, these fibers use anaerobic metabolism to produce powerful, high-tension contractions. They fatigue quickly. Cardiac muscle This involuntary striated muscle tissue is found only in the heart.  Cardiac muscle fibers have their own rhythm, generated by pacemaker cells.  Smooth muscle  This involuntary, non-striated muscle is common in the walls of hollow organs and blood vessels. Muscle Fibre Types -