Moderate-intensity training in hypoxia improves exercise performance and glycolytic capacity of skeletal muscle in horses.

We investigated whether moderate-intensity training of horses in moderate hypoxia for 4 weeks elicits greater adaptations in exercise performance, aerobic capacity, and glycolytic/oxidative metabolism in skeletal muscle compared to normoxic training. In a randomized crossover study design, seven untrained Thoroughbred horses (5.9 ± 1.1 years, 508 ± 9 kg) completed 4 weeks (3 sessions/week) of two training protocols consisting of 3-min cantering at 70% of maximal oxygen consumption ( V ˙ O 2 max ) in hypoxia (HYP; F I O 2  = 14.7%) and normoxia (NOR; F I O 2  = 21.0%) with a 4-month washout period. Normoxic incremental exercise tests (IET) were conducted before and after training. Biopsy samples were obtained from the middle gluteal muscle before IET and monocarboxylate transporter (MCT) protein expression and glycolytic/mitochondrial enzyme activities were analyzed. Data were analyzed using mixed models (p < 0.05). Running speed was 7.9 ± 0.2 m/s in both groups and arterial oxygen saturation during training in NOR and HYP were 92.9 ± 0.9% and 75.7 ± 3.9%, respectively. Run time in HYP (+9.7%) and V ˙ O 2 max in both groups (NOR, +6.4%; HYP, +4.3%) at IET increased after 4 weeks of training. However, cardiac output, arterial-mixed venous O 2 difference, and hemoglobin concentration at exhaustion were unchanged in both conditions. While MCT1 protein and citrate synthase activity did not increase in both conditions after training, MCT4 protein (+13%), and phosphofructokinase activity (+42%) increased only in HYP. In conclusion, 4 weeks of moderate-intensity hypoxic training improves exercise performance and glycolytic capacity of skeletal muscle in horses.