Effect of 8 days of exercise-heat acclimation on aerobic exercise performance of men in hypobaric hypoxia

Exercise-heat acclimation (EHA) induces ad-aptations that improve tolerance to heat exposure. Whether adapta-tions from EHA can also alter responses to hypobaric hypoxia (HH) conditions remains unclear. This study assessed whether EHA can alter time-trial performance and/or incidence of acute mountain sick-ness (AMS) during HH exposure. Thirteen sea-level (SL) resident men [SL peak oxygen consumption ((V)over dotO(2peak)) 3.19 +/- 0.43 L/min] completed steady-state exercise, followed by a 15-min cycle time trial and assessment of AMS before (HH1; 3,500 m) and after (HH2) an 8-day EHA protocol [120 min; 5 km/h; 2% incline; 40 degrees C and 40% relative humidity (RH)]. EHA induced lower heart rate (HR) and core temperature and plasma volume expansion. Time-trial performance was not different between HH1 and HH2 after 2 h (106.3 +/- 23.8 vs. 101.4 +/- 23.0 kJ, P = 0.71) or 24 h (107.3 +/- 23.4 vs. 106.3 +/- 20.8 kJ, P 0.9). From HH1 to HH2, HR and oxygen saturation, at the end of steady-state exercise and time-trial tests at 2 h and 24 h, were not different (P > 0.05). Three of 13 volunteers developed AMS during HH1 but not during HH2, whereas a fourth volunteer only developed AMS during HH2. Heat shock protein 70 was not different from HH1 to HH2 at SL [1.9 +/- 0.7 vs. 1.8 +/- 0.6 normalized integrated intensi-ties (NII), P = 0.97] or after 23 h (1.8 +/- 0.4 vs. 1.7 +/- 0.5 NII, P = 0.78) at HH. Our results indicate that this EHA protocol had little to no effect-neither beneficial nor detrimental-on exercise perfor-mance in HH. EHA may reduce AMS in those who initially developed AMS; however, studies at higher elevations, having higher incidence rates, are needed to confirm our findings.