Title : The effects of 10 days of separate heat and hypoxic exposure on heat acclimation and temperate 1 exercise performance 2

23 Adaptations to heat and hypoxia are typically studied in isolation, but are often encountered in combination. 24 Whether the adaptive response to multiple stressors affords the same response as when examined in isolation 25 is unclear. We examined: i) the influence of overnight moderate normobaric hypoxia on the time course and 26 magnitude of adaption to daily heat exposure; ii) whether heat acclimation (HA) was ergogenic and if this 27 was influenced by an additional hypoxic-stimulus. Eight males (V̇O2max=58.5[8.3] mL·kg·min) undertook 28 two 11-day HA programmes (balanced-crossover design), once with overnight normobaric hypoxia (8[1] h 29 per night; 10 nights; FIO2=0.156; SpO2=91[2]% [HAHyp]) and once without (HACon). Days 1, 6, 11 were 30 exercise-heat stress tests (HST [40°C, 50% RH]); days 2-5 and 7-10 were isothermal-strain (target rectal 31 temperature [Tre] ~38.5°C), exercise-heat sessions. A graded exercise test and 30-minute cycle trial were 32 undertaken pre, post and 14-days after HA in temperate-normoxia (22°C, 55% RH; FIO2=0.209). HA was 33 evident on day 6 (e.g. reduced Tre, mean skin temperature [T̅sk], heart rate, sweat [Na], P<0.05) with 34 additional adaptations on day 11 (further reduced T̅sk, heart rate). HA increased plasma volume (+5.9[7.3]%) 35 and erythropoietin concentration (+1.8[2.4] mIU/mL); tHbmass was unchanged. Peak power output (+12[20] 36 W), lactate threshold (+15[18] W) and work done (+12[20] kJ) increased following HA. The additional 37 hypoxic-stressor did not affect these adaptations. In conclusion, a separate moderate overnight normobaric 38 hypoxic-stimulus does not affect the time-course or magnitude of HA. Performance may be improved in 39 temperate-normoxia following HA, but this is unaffected by an additional hypoxic stressor. 40