Effects of different oxygen concentrations during intermittent hyperoxic training on endurance capacity in well-trained male mice.

While training under intermittent hyperoxia (INT) enhances endurance capacity, the effects of specific oxygen concentrations remain unclear. This study compared 50% O2 (INT50) and 75% O2 (INT75) in well-trained male mice, which had increased maximal work values by 7.7-fold through a 7-week voluntary running program, during a 4-week treadmill program. Results showed that INT75, but not INT50, significantly improved endurance capacity compared to the normoxic-trained (ET) group (Bayes factor, BF ≥3). The enhanced exercise capacity in the INT75 group was linked to higher levels of metabolic enzymes (citrate synthase, 3-hydroxyacyl-CoA-dehydrogenase, carnitine palmitoyl transferase-2, and phosphofructokinase) in the diaphragm and increased pyruvate dehydrogenase complex activity in the red gastrocnemius (Gr) and plantaris (PL) muscles (BF ≥3). Glutathione peroxidase-1 protein levels in the PL muscle were significantly lower in INT75 (vs ET, 0.76-fold, BF ≥3). Additionally, INT75 mice exhibited increased PGC1α levels in the left ventricle, despite decreased cytochrome-c oxidase activity. In contrast, the INT50 group showed a shift toward fast-twitch muscle fibers in the Gr. In conclusion, the ergogenic effects of intermittent hyperoxia depend on oxygen concentration. INT at 75% O2 significantly enhances endurance capacity in well-trained mice by optimizing metabolic enzyme activity across skeletal and respiratory muscles.