What Are the Different Classes of Muscle Cells

Muscle cells are grouped into two general categories or "types" (Type I and II). Type II muscle fibers are often subclassified as Ila, lib, and Ilc. For this book, it is enough to only distinguish between the two main types. Skeletal muscle is actually bundles of a mixture of Type I and II muscle fibers. In fact, the average person will tend to have about a 50/50 mixture of Types I and II muscle fibers. Meanwhile, highly successful athletes tend to have a significant imbalance one way or the other which, as will soon be discussed, will allow them to excel at a particular sport.

Table 11.1 Performance and Metabolic Properties of Muscle Fibers

Type I Muscle Fibers

Develop force more slowly than Type II muscle fibers

Have more mitochondria and capillaries and thus are more aerobic Generate very little lactic acid (lactate) Do not fatigue quickly

Type II Muscle Fibers

Develop force more quickly

(more powerful)

Have fewer mitochondria and capillaries and thus are more anaerobic

Generate more lactate

Fatigue quickly

Type I muscle fibers are more aerobic and can perform longer than Type II muscle fibers.

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