Rushall Thoughts, (1992).

There are generally three types of muscle fibers found in skeletal muscles.

1. Slow twitch fibers respond slowly to stimulation, produce the least tension, and have good aerobic endurance.

2. Fast twitch A fibers are fast twitch fibers that have been "converted" primarily through VO2max training to function in an aerobic manner. They have moderate endurance not quite reaching the endurance capacity of true slow twitch fibers.

3. Fast twitch B fibers are used for maximal contractions and have poor endurance qualities.

These fibers are used at different times depending upon the amount of force that is attempted. It is the amount of force required by the muscle which determines the type and amount of fibers that are turned on, NOT the speed at which the muscles shorten.

Force:

• is affected by the percentage of each fiber present in the activated muscle;
• is affected by the number of force developing units within the muscle, that is, myofibrils located inside each fiber, and
• most importantly, is established by the nervous system (it is a learned response).

Return to Table of Contents for this issue.