With moderate training (30-60 min daily at 70–80% of VO2 max, 3–5 times weekly), the trained muscles display a 40–50% increase in the content of mitochondrial oxidative enzymes. Concomitantly, the total number of muscle capillaries may increase by 50%, whereas the content of glycolytic enzymes is not, or only marginally, affected. The oxidative enzyme increase, which occurs over 6–8 wk, is lost in 4–6 wk if training is stopped. This loss occurs faster than the decrease in muscle capillarization and in the whole-body VO2 max. Trained muscles of athletes have 3–4 times higher oxidative enzyme levels and two- to threefold more capillaries per muscle fiber than untrained muscle. Extensive endurance training results in an enhanced percentage of slow-twitch fibers, but the time course of this change is not known. More extensive changes are observed in chronically stimulated rabbit muscle. In this case, enzymes of oxidation display large increases (6- to 12-fold), whereas there is a decrease of 70–90% in enzymes of glycolysis, glycogenolysis, gluconeogenesis, and high-energy phosphate transfer. There is a normal training response in mitochondrial enzyme activities in individuals with insulin-dependent and non-insulin-dependent diabetes, but the ability to form new skeletal muscle capillaries in response to physical training may be deficient in insulin-dependent diabetes. Training-induced changes in the metabolic character of skeletal muscle leads to an increased reliance on fat metabolism during exercise, with a lowered blood lactate concentration and a sparing of muscle glycogen.

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