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
|
