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Revised:   22 Nov 05

MUSCLE FIBER TYPES

All athletes possess three types of muscle fiber:

• Slow-twitch red (type I) muscle fiber develops force slowly, has a ling twitch time and a low power output, is fatigue-resistant (high endurance), and has a high aerobic capacity for energy supply, but it has limited potential for rapid force development and anaerobic power.

• Fast twitch red (type IIa) is an intermediate fiber type that can contribute to both anaerobic and aerobic activity.  It develops force moderately quickly and has a moderate twitch time, power output, fatigability, aerobic power, force development, and anaerobic power.

• Fast-twitch white (type IIb) fiber develops force rapidly and has a short twitch time, a high power output, fatigability (low endurance), low aerobic power, and high anaerobic power.

Although slow-twitch fibers cannot be changed into fast-twitch fibers, most of the intermediate fibers (type IIa) can be converted to the faster type IIb fibers through training.  This conversion aids explosive movements such as sprinting.

Source:  Sports Speed, Third Edition