Heat Acclimation Following Heat Acclimatization Elicits Additional Physiological Improvements in Male Endurance Athletes

Int J Environ Res Public Health. 2021 Apr 20;18(8):4366. doi: 10.3390/ijerph18084366.

Abstract

The purpose of this study was to assess the effectiveness of heat acclimatization (HAz) followed by heat acclimation (HA) on physiological adaptations. 25 male endurance athletes (age 36 ± 12 y, height 178.8 ± 6.39 cm, body mass 73.03 ± 8.97 kg, and VO2peak 57.5 ± 7.0 mL·kg-1·min-1) completed HAz and HA. HAz was 3 months of self-directed summer training. In the laboratory, a 5-day HA prescribed exercise to target a hyperthermic zone (HZHA) of Trec between 38.50 and 39.75 °C for 60 min. Exercise trials were 60 min of running (59% ± 2% VO2peak) in an environmental chamber (wet bulb globe temperature 29.53 ± 0.63 °C) and administered at: baseline, post-HAz, and post-HAz+HA. Measured variables included internal body temperature (Trec), heart rate (HR), and sweat rate (SR). Repeated measure ANOVAs and post hoc comparisons were used to assess statistically significant (p < 0.05) differences. Trec was lower post-HAz+HA (38.03 ± 0.39 °C) than post-HAz (38.25 ± 0.42 °C, p = 0.009) and baseline (38.29 ± 0.37 °C, p = 0.005). There were no differences between baseline and post-HAz (p = 0.479) in Trec. HR was lower post-HAz (143 ± 12 bpm, p = 0.002) and post-HAz+HA (134 ± 11 bpm, p < 0.001) than baseline (138 ± 14 bpm). HR was lower post-HAz+HA than post-HAz (p = 0.013). SR was higher post-HAz+HA (1.93 ± 0.47 L·h-1) than post-HAz (1.76 ± 0.43 L·h-1, p = 0.027). Combination HAz and HA increased physiological outcomes above HAz. This method can be used to improve performance and safety in addition to HAz alone.

Keywords: aerobic; heat illness; heat mitigation; heat tolerance; thermoregulation; training strategy.