A Comparison of Muscle Fiber Types

 

Skeletal muscle fibers are grouped to biochemical performance characteristic into three different categories : fast-twitch fibers, intermediate fibers and slow-twitch fibers.

Each muscle contains a genetically determined percentage of fiber types. For example, one person may have more fast-twitch fiber in a particular muscle than another person.

The effect that muscle conditioning has on changing the profile of muscle types is uncertain. The percentages of fiber types, however, greatly influence muscular power and endurance.

Anaerobic fast-twitch fibers 9also call fast-glycolytic or Type IIb fibers) are able to contract very forcefully and rapidly. They are used primarily for power and speed.

Aerobic sloe-twitch fibers (also called slow-oxidative or Type I fibers) are highly resistant to fatigue. They are used primarily for endurance.

The characteristics of intermediate fibers differ somewhat from fiber to fiber, but lie on the continuum between fast-twitch and slow-twitch fibers.

A Comparison of Muscle Fiber Types

Fiber characteristic	Fast-twitch fiber	Intermediate fiber	Slow-twitch fiber
Fiber size	Large	Intermediate	Small
Glycogen content	High	Intermediate	Low
Myosin ATPase	High	High	Low
Myoglobin content	Low	High	High
Energy system	Anaerobic	Combination	Aerobic
Twitch	Fast	Fast	Slow
Primary use	Speed and power	Moderate	Endurance