Relationship of strength and endurance with skeletal muscle structure and metabolic potential.

Human skeletal muscle consists of fast and slow twitch motor units. The FT units can be further subdivided into three types on the basis of staining procedures that identify the proteins in the contractile elements. Presently it does not appear that a conversion of motor units occurs with training. Major metabolic differences exist between the different motor units. With endurance training there is an increase in the enzymes for end-terminal oxidation. This reduces the differences between fiber types. This reduction is the differences between fiber types is most dramatic when viewed from histochemical stains. Training does increase the concentration of glycogen stored in the muscle. Muscular strength is related to the total cross-sectional area of the muscle. This can be increased with heavy resistance exercise. This enlargement is due to hypertrophy of the pre-existing fibers and not to a proliferation of fibers as the result of splitting of the fibers.