Hypoxic training increases maximal oxygen consumption in Thoroughbred horses well-trained in normoxia

Hypoxic training is effective for improving athletic performance in humans. It increases maximal oxygen consumption (V̇O2max) more than normoxic training in untrained horses. However, the effects of hypoxic training on well-trained horses are unclear. We measured the effects of hypoxic training on V̇O2max of 5 well-trained horses in which V̇O2max had not increased over 3 consecutive weeks of supramaximal treadmill training in normoxia which was performed twice a week. The horses trained with hypoxia (15% inspired O2) twice a week. Cardiorespiratory valuables were analyzed with analysis of variance between before and after 3 weeks of hypoxic training. Mass-specific V̇O2max increased after 3 weeks of hypoxic training (178 ± 10 vs. 194 ± 12.3 ml O2 (STPD)/(kg × min), P<0.05) even though all-out training in normoxia had not increased V̇O2max. Absolute V̇O2max also increased after hypoxic training (86.6 ± 6.2 vs. 93.6 ± 6.6 l O2 (STPD)/min, P<0.05). Total running distance after hypoxic training increased 12% compared to that before hypoxic training; however, the difference was not significant. There were no significant differences between pre- and post-hypoxic training for end-run plasma lactate concentrations or packed cell volumes. Hypoxic training may increase V̇O2max even though it is not increased by normoxic training in well-trained horses, at least for the durations of time evaluated in this study. Training while breathing hypoxic gas may have the potential to enhance normoxic performance of Thoroughbred horses.