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.