Skeletal Muscle

Mitochondria

Endurance training significantly increases muscle cell oxidative capacity and mitochondrial enzymes. Furthermore, electron microscopy studies have shown both the number and size of the mitochondria increase with endurance training. Mitochondrial enzyme content is also affected by endurance training. Increased levels of enzymes responsible for ketone, fatty acid, NADH and succinate oxidation occur after endurance training. Muscle homogenate and isolated mitochondrial studies document that endurance training increases a muscle's ability to oxidize fatty acids, ketones, and pyruvate.

Resistance training may decrease mitochondrial volume density in muscle. These changes, however, reflect increases in myofibril volume, whereas mitochondrial volume appears to remain unchanged.

Myoglobin Content

With endurance training, the myoglobin content of muscle can increase by as much as 80%. These changes increase the amount of oxygen in a muscle cell and facilitate mitochondrial diffusion. Eased diffusion is consistent with a muscle cell's increased oxidative capabilities.

Muscle Fiber Types

Typically, humans have about 50% type I (slow-twitch) fibers and 50% type II (fast-twitch) skeletal muscle fibers. While sprinters tend to have a higher percentage of type II fibers and endurance athletes a higher percentage of type I fibers, these differences are likely genetically determined predisposing an athlete to one or the other sporting conditions. Conversion of type II fibers to type I fibers because of exercise training has not been shown to happen, but, conversion of IIb (fast-twitch white, glycolytic) to IIa (fast-twitch red, oxidative) can happen with endurance training. Some forms of endurance training can produce complete conversion of type IIb fibers to type IIa fibers, and the adaptive increases in oxidative capacity appear greater in the type II fibers than in the type I fibers.

Resistance training appears to selectively increase type I fiber areas faster than type II fiber areas. Furthermore, resistance athlete specialists appear to have differences in fiber type composition. For example, world class body builders, who utilize high volume low intensity resistance programs, have a relatively greater number of type II fibers than Olympic and Power lifters. These differences, however, may also be related to genetic factors.

References

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