Type I fibers are very efficient at producing ade-nosine triphosphate (ATP) from the oxidation of carbohydrate and fat. As long as oxidation occurs, type I fibers continue producing ATP, which allows the fibers to remain active. Thus the ability to maintain muscular activity for a prolonged period (muscular endurance) depends on type I fibers, which have high aerobic endurance. They are recruited most often during low-intensity events, such as marathon running, and during daily activities, such as walking, for which the muscle-force requirements are low.
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