Sex-specific Responses Of Heat Acclimation On Running Performance In Cool Environmental Conditions

PURPOSE: To investigate physiological and performance adaptations in a cool environment following heat acclimation (HA) in endurance athletes. METHODS: Eleven high-level endurance runners (4 males [age: 21 ± 4 years; body mass: 78.63 ± 9.01 kg; height: 182.55 ± 3.97 cm; maximal oxygen uptake {VO2max}: 73.93 ± 9.69 ml·kg-1·min-1]; 7 females [age: 22 ± 5 years; body mass: 64.30 ± 7.97 kg; height: 168.00 ± 6.43 cm; VO2max: 58.30 ± 4.28 ml·kg-1·min-1]) participated in this study. At baseline, participants completed a combined VO2max and lactate threshold (LT) test, followed by an 8-km time-trial (TT) in cool conditions (ambient temperate [Tamb], 11 °C; relative humidity [RH], 40%). Following this, participants completed a 7-day protocol consisting of 60-min running at 50% velocity at VO2max in a hot environment (Tamb, 40 °C; RH, 35%). Physiological variables (i.e., heart rate [HR], core temperature [Tc], sweat rate [SR], plasma volume change [ΔPV]) were assessed on days 1 and 7. A post-testing visit that followed the same protocol as the pre-testing visit was conducted to determine performance changes. A two-way repeated measures ANOVA was conducted to assess physiological and performance changes from pre- to post-testing for performance variables, and from days 1 to 7 for physiological variables, between sexes. Cohen’s d classification of effect sizes was calculated. RESULTS: In a cool environment, a significant interaction between males and females from pre- to post-testing was observed for VO2max, (p = 0.005) but not for velocity at LT or 8-km TT (p > 0.05). Particularly, females demonstrated improvements in VO2max (57.62 ± 5.49 to 61.81 ± 6.16 ml·kg-1·min-1; p = 0.01; d = 0.68), but males did not (p > 0.05). Regardless of sex, velocity at LT was improved from pre- to post-testing (10.5 ± 0.3 to 11.0 ± 0.3 mi·h-1; p = 0.05; d = 1.67), whereas TT remained unchanged (p > 0.05; d = 0.14). From day 1 to 7 of HA, there were no significant interactions between males and females in maximum HR, maximum Tc, ΔPV, and SR (p > 0.05). Irrespective of sex, maximum HR was reduced (183 ± 3 to 175 ± 2 bpm; p = 0.014; d = 3.14), whereas maximum Tc, ΔPV, and SR were unchanged (p > 0.05; d = 0.38 - 1.86). CONCLUSION: The present findings indicate that HA may procure adaptations to improve endurance performance in cool temperatures in both male and female endurance runners. Supported by: This research was supported by a World Athletics Research Grant from the American College of Sports Medicine.