Effects of 10 days of separate heat and hypoxic exposure on heat acclimation and temperate exercise performance

Adaptations to heat and hypoxia are typically studied in isolation, but are often encountered in combination. Whether the adaptive response to multiple stressors affords the same response as when examined in isolation is unclear. We examined: i) the influence of overnight moderate normobaric hypoxia on the time course and magnitude of adaption to daily heat exposure; ii) whether heat acclimation (HA) was ergogenic and if this was influenced by an additional hypoxic-stimulus. Eight males (V̇O_2max=58.5[8.3] mL·kg^-1·min^-1) undertook two 11-day HA programmes (balanced-crossover design), once with overnight normobaric hypoxia (8[1] h per night; 10 nights; F_IO₂=0.156; S_pO₂=91[2]% [HA_Hyp]) and once without (HA_Con). Days 1, 6, 11 were exercise-heat stress tests (HST [40°C, 50% RH]); days 2-5 and 7-10 were isothermal-strain (target rectal temperature [T_re] ~38.5°C), exercise-heat sessions. A graded exercise test and 30-minute cycle trial were undertaken pre, post and 14-days after HA in temperate-normoxia (22°C, 55% RH; F_IO₂=0.209). HA was evident on day 6 (e.g. reduced T_re, mean skin temperature [T̅_sk], heart rate, sweat [Na⁺], P<0.05) with additional adaptations on day 11 (further reduced T̅_sk, heart rate). HA increased plasma volume (+5.9[7.3]%) and erythropoietin concentration (+1.8[2.4] mIU/mL); tHb_mass was unchanged. Peak power output (+12[20] W), lactate threshold (+15[18] W) and work done (+12[20] kJ) increased following HA. The additional hypoxic-stressor did not affect these adaptations. In conclusion, a separate moderate overnight normobaric hypoxic-stimulus does not affect the time-course or magnitude of HA. Performance may be improved in temperate-normoxia following HA, but this is unaffected by an additional hypoxic stressor.