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3. Fast (type II) Logo

Fast muscle fibres can be divided into 2 groups (divisions A & B) on the basis of their resistance to fatigue.


Division A Top

Fibres referred to as Type IIA are relatively uncommon in man and other primates. These fibers, with fast twitches (short duration), were found to maintain their force production even after a large number of contractions - they are relatively fatigue resistant (see figure 2). They tend to be high in oxidative and glycolytic enzymes and have a high myosin ATPase activity. These have been termed Fast Resistant (FR) or (histochemically) Fast Oxidative-Glycolytic (FOG).


Division B Top

Type IIB fibers display generate very rapid (short duration) twitches that are extremely forceful in comparison to type I fibres, but are unable to maintain these tensions for more than a few contractions without rest. If stimulated for an extended period the active tension declines - the muscle shows fatigue (see figure 2). Such fibres have high mysoin ATPase rates (hence fast twitch rates) and high glycolytic activities and low oxidative capacity. They have little myoglobin (and so appear pale or white), have a low density of mitochondria and the capillary density tends to be low. Fibres in this group have been termed Fast Fatiguable (FF) or Fast Glycolytic (FG) fibers.
cartoon comparison of the properties of the major histochemically distinct motor unit types.

Figure 2 - cartoon comparison of the properties of the major histochemically distinct motor unit types.

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Copyright © 1998 University of Bristol. All rights reserved.
Author: Phil Langton 		Last modified: 9 Jun 1999 20:12
Authored in CALnet