All skeletal muscles are composed of a blend of fast-twitch (Type IIb), slow-twitch (Type I), and intermediate muscle fibers (Type IIa). The fast-twitch and slow-twitch muscle fibers have different characteristics. Fast-twitch muscle fibers are larger, have less blood flow, have more immediate fuel sources, have larger nerves innervating them, and can contract more rapidly and with more force than slow-twitch fibers. Slow-twitch fibers are smaller, have more blood flow, have less-immediate fuel sources, have smaller nerves innervating them, and contract less quickly and with less force than fast-twitch fibers. The intermediate fibers are more like fast-twitch fibers but, like slow-twitch fibers, have more blood flow.
