The effects of duration of training on VO₂(max) and citrate synthase activity in muscle during detraining

Two groups of rats were endurance trained by treadmill running for an hour a day five days a week for either 5 (n=12) or 11 (n=14) weeks. Maximal oxygen consumption (V̇O₂max) was determined at 1, 9, and 19 days, and muscle samples were taken at 3, 11, and 21 days after cessation of training. There were no differences in V̇O₂max between groups nor were there any changes in V̇O₂max within any of the groups during detraining. Citrate synthase (CS) activity in deep vastus lateralis muscle was increased 30% (P<0.01) to 39.9 and 40.9 micromol/min gram wet muscle in the 5 and 11 week trained animals, respectively, compared to the control group (29.8 micromol/min gram wet muscle) 3 days post-training. Eleven days after the cessation of training CS activity in both groups descreased to 12% above control levels. At 21 days post-training CS activity of both groups returned to control levels. Therefore, it was concluded: 1) elevated muscle oxidative capacity from endurance training (as indicated by CS activity) is maintained only by constant stimulus input; 2) the duration of training has no effect on the rate of decrease of muscle oxidative capacity during detraining; and 3) V̇O₂max and muscle oxidative capacity decline at different rates during detraining.