At one extreme are slow-twitch fibers that do not contract rapidly or powerfully but use energy aerobically and don’t fatigue easily. These type I fibers are colloquially known as red muscle because of their darker tinge.28 At the other extreme are fast-twitch (type II) fibers, which come in two types: white and pink. White muscle (type IIX) fibers burn sugar to generate powerful and rapid forces but fatigue rapidly. Pink muscle (type IIA) fibers produce moderately powerful forces aerobically and thus fatigue at an intermediate rate.
