CHANGES IN MAXIMUM OXYGEN-UPTAKE DURING PROLONGED TRAINING, OVERTRAINING, AND DETRAINING IN HORSES

Thirteen standardbred horses were trained as follows: phase 2 (enduran ce training, 7 wk), phase 2 (high-intensity training, 9 wk), phase 3 ( overload training, 18 wk), and phase 4 (detraining, 12 wk). In phase 3 , the horses were divided into ts;iio groups: overload training (OLT) and control (C). The OLT group exercised at greater intensities, frequ encies, and durations than group C, Overtraining occurred after 31 wk, of training and was defined as a significant decrease in treadmill ru n time in response to a standardized exercise test. In the OLT group, there was a significant decrease in body weight (P < 0.05). From pretr aining values of 117 +/- 2 (SE) ml .-kg(-1). min(-1), maximal O-2 upta ke (Vo2(max) increased by 15% at the end of phase 1, and when signs of overtraining were first seen in the OLT group, Vo(2max) was 29% highe r (151 +/- 2 ml . kg(-1). min(-1) in both C and OLT groups) than pretr aining values. There was no significant reduction in Vo(2max) until af ter a wk detraining when VO2max was 137 +/- 2 ml . kg(-1). min(-1). By 12 wk detraining, mean Vo(2max) was 134 +/- 2 ml . kg(-1). min(-1), s till 15% above pretraining values. When overtraining developed, Vo(2ma x) was not different between C and OLT groups, but maximal values for CO2 production (147 vs. 159 ml . kg(-1). min(-1)) and respiratory exch ange ratio (1.04 vs. 1.11) were lower in the OLT group. Overtraining w as not associated with a decrease in V-o2max and, after prolonged trai ning, decreases in V-o2max occurred slowly during detraining.