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Depends on oxygen delivery and aerobic mechanisms. Have very fast-acting myosin ATPases and depend upon anaerobic metabolism during contraction. Red fibers, the smallest of the fiber types. Contain abundant amounts of glycogen. Abundant in muscles used to maintain posture. A relatively high percentage are found in successful marathon runners. A. Slow (oxidative), fatigue-resistant fibers B. Fast (oxidative or glycolytic), fatigable fibers