Fast and Slow Muscle Fibers

The time delay between the occurrence of the muscle fiber action potential and the peak of the resulting tension is not constant across all muscle fibers. The delay to peak tension can be as little as 10 or as many as 200 msec. In general, muscle fibers can be grouped into two classes fast and slow on the basis of this speed. Samples of isometric contractions in fast and slow fibers are shown in Figure 11-5. Both slow and fast fibers are found together in most muscles, but slow fibers predominate in muscles that must maintain steady contraction, such as those involved in keeping us standing upright. Fast muscle fibers are more common in muscles that require rapid contraction, such as those involved in jumping and running. The fastest muscle fibers are those of muscles that move the eyes in rapid jumps, like those your eyes are making as you scan the words on this page.

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