Muscular characteristics of detraining in humans

Skeletal muscle is characterized by its ability to dynamically adapt to var iable levels of functional demands. During periods of insufficient training stimulus. muscular detraining occurs. This may be characterized by a decre ased capillary density, which could take place within 2-3 wk of inactivity. Arterial-venous oxygen difference declines if training stoppage continues beyond 3-8 wk. Rapid and progressive reductions in oxidative enzyme activit ies bring about a reduced mitochondrial ATP production. The above changes a re related to the reduction in V(over dot)O-2max, observed during long-term training cessation. These muscular characteristics remain above sedentary values in the detrained athlete but usually return to baseline values in re cently trained individuals. Glycolytic enzyme activities show nonsystematic changes during periods of training cessation. Fiber distribution remains u nchanged during the initial weeks of inactivity, but oxidative fibers may d ecrease in endurance athletes and increase in strength -trained athletes wi thin 8 wk of training stoppage. Muscle fiber cross-sectional area declines rapidly in strength and sprint athletes, and in recently endurance-trained subjects, whereas it may increase slightly in endurance athletes. Force pro duction declines slowly and in relation to decreased EMG activity. Strength performance in general is readily maintained for up to 4 wk of inactivity, but highly trained athletes' eccentric force and sport-specific power, and recently acquired isokinetic strength, may decline significantly.