Fast-Twitch Muscle Fibers: Size, Structure, and Function

The largest skeletal muscle fibers in diameter and containing the most myofibrils are fast-twitch fibers, also called type II fibers. These fibers specialize in generating fast, powerful contractions, but they also fatigue more quickly than slow-twitch fibers. Fast-twitch fibers are essential for activities that require rapid accelerations, high speeds, and intense efforts.