Influence Of Heat Acclimation On Aerobic Performance In Cool, Hot And Hypoxic Conditions

PURPOSE: The aim of this study was to confirm the impact of heat acclimation on aerobic performance in hot conditions and elucidate the transfer of heat adaptations to cool and hypoxic environments. METHODS: Ten males (V̇O2peak: 4.50 ± 0.50 L·min-1) completed two three-week training interventions consisting of heat acclimation (HA: 36 °C and 59% RH) and exercise in temperate conditions (TEMP: 18 °C and 60% RH) in a counter-balanced crossover design. Training consisted of four work-matched controlled heart rate sessions interspersed with one intermittent sprint session, and two rest days. Before and after the interventions V̇O2peak and 20-min time trial performance were evaluated in COOL (18 °C), HOT (35 °C) and hypoxic (HYP: 18 °C and FiO2:15.4%) conditions. Data were analyzed using linear mixed effect models and are presented as mean and 95% confidence intervals. RESULTS: Following HA, V̇O2peak increased in HOT (0.24 L·min-1 [0.01, 0.47], P=0.040) but not COOL (P=0.431) or HYP (P=0.411), whereas TEMP did not influence V̇O2peak (P≥0.424; Figure 1). Mean power output during the time trial increased in HOT (20 W [11, 28], P<0.001) and COOL (12 W [4, 21], P=0.004) with a trend towards increasing HYP (7 W [-1, 16], P=0.075) after HA (Figure 1). Rectal (-0.13 °C [-0.23, -0.03], P=0.009) and skin (-0.7 °C [-1.2, -0.3], P <0.001) temperature were lower during the time trial in HOT after HA. Plasma volume expansion following HA (277 mL [62, 491], P=0.014) was correlated with time trial performance in HOT (r=0.78, P=0.012). CONCLUSIONS:HA improved aerobic performance in HOT in association with enhanced cardiovascular stability via plasma volume expansion and lower thermal strain, whereas the mechanistic pathways improving performance in COOL and HYP remain unclear.