Glycolytic fibers, with few mitochondria and rich in glycogen but low in myoglobin (the reason they are whitish in colour), that undergo fast, short-lived contractions. Typical of muscles involved in sprinting etc. as they are activated by large diameter, and therefore fast, conduction, motoneurons. There are two sorts of fast-twitch fibers: Type 2a muscle fibers that fatigue at an intermediate rate, and Type 2b muscle fibers that fatigue very quickly (‘pure fast fibers’). Most striated muscles contain some mixture of Type 1 and Type 2 fibers, while a single motor unit always contains one type of fiber, but not the other. Finally, the ratio between them can be changed by endurance exercise. For example, Type 2b fibers can transition to Type 2a fibers with chronic endurance training, but it does not seem that Type 2a fibers can be changed to Type 1.
See Glycogen, Motor unit, Muscle fiber, Myoglobin, Striated (striped or voluntary) muscle, Type 1 muscle fibers