Time course of loss of adaptations after stopping prolonged intense endurance training

Seven endurance exercise-trained subjects were studied 12, 21, 56, and 84 days after cessation of training. Maximal O₂ uptake (V̇(O₂max)) declined 7% (P < 0.05) during the first 21 days of inactivity and stabilized after 56 days at a level 16% (P < 0.05) below the initial trained value. After 84 days of detraining the experimental subjects still had a higher V̇(O₂max) than did eight sedentary control subjects who had never trained (50.8 vs. 43.3 ml·kg^-1·min^-1), due primarily to a larger arterial-mixed venous O₂ (a-vΔO₂) difference. Stroke volume (SV) during exercise was high initially and declined during the early detraining period to a level not different from control. Skeletal muscle capillarization did not decline with inactivity and remained 50% above (P < 0.05) sedentary control. Citrate synthase and succinate dehydrogenase activities in muscle declined with a half-time of 12 days and stabilized at levels 50% above sedentary control (P < 0.05). The initial decline in V̇(O₂max) was related to a reduced SV and the later decline to a reduced a-vΔO₂ difference. Muscle capillarization and oxidative enzyme activity remained above sedentary levels and this may help explain why a-vΔO₂ difference and V̇(O₂max) after 84 days of detraining were still higher than in untrained subjects.