Slow-Twitch and Fast-Twitch Muscle Fibers
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Slow-twitch fibers contract gradually and resist fatigue using aerobic metabolism, while fast-twitch fibers contract quickly and forcefully but fatigue sooner, relying more on anaerobic metabolism.
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A marathon runner's leg muscles favor slow-twitch fiber, while a sprinter's favor fast-twitch fiber.
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Most muscles mix both fiber types — a single muscle is rarely composed of one type exclusively.