Heat to hypoxia cross-adaptation: Effects of 6-week post-exercise hot-water immersion on exercise performance in acute hypoxia

Cross-adaptation occurs when exposure to one environmental stressor (e.g., heat) induces protective responses to another (e.g., hypoxia). Although post-exercise hot-water immersion (HWI) induces heat acclimation, its potential to elicit cross-adaptation remains unclear. This study evaluated the effectiveness of a 6-week post-exercise HWI intervention on exercise performance in hypoxia (O2 = 13%). Twenty healthy volunteers (28 ± 5 years; V^•_O2peak 47.4 ± 8.9 mL kg−1 min−1; 12 males, 8 females) completed interval cycling (4×4 min at 90 ± 5% maximal heart rate, 3×/week) followed by water immersion at either 34.5°C (control) or 42°C (HWI) for 40–50 min, five times per week. Following the 6-week intervention, the post-exercise HWI group exhibited lower resting heart rate (P < 0.01, q = 0.02; d = −1.32) and core temperature (P < 0.01, q = 0.001; d = −1.88) and elevated haemoglobin concentration (P < 0.01, q = 0.02; d = 1.38). Compared to the control group, the HWI group also showed greater improvements in time-to-exhaustion (TTE) trial (P and q < 0.01; d = 1.2) under hypoxia, but not in aerobic peak power (P = 0.03, q = 0.08; d = 0.86) or peak oxygen consumption (V^•_O2peak) (P = 0.04, q = 0.10; d = 0.82). Throughout the TTE, lower core temperature and tidal volume, with increased oxygen saturation and V^•_O2 were observed (P and q < 0.05). During hypoxic steady-state exercise at 60% of V^•_O2peak, the HWI group exhibited lower core temperature and higher peripheral oxygen saturation in hypoxia. No between-group differences were observed in mean V^•_O2, respiratory exchange ratio, heart rate or rate of perceived exertion, nor in V^•_O2peak and aerobic peak power under normoxia (P and q > 0.05). In conclusion, post-exercise HWI enhances maximal exercise performance under acute hypoxia, likely due to increased haemoglobin concentration, lower core temperature and improved respiratory efficiency.