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 VO_2peak 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 T_rec between 38.50 and 39.75 °C for 60 min. Exercise trials were 60 min of running (59% ± 2% VO_2peak) 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 (T_rec), heart rate (HR), and sweat rate (SR). Repeated measure ANOVAs and post hoc comparisons were used to assess statistically significant (p < 0.05) differences. T_rec 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 T_rec. 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.

The purpose of this study was to examine the changes in metabolic heat production (H_prod), evaporative heat loss (H_evap), and dry heat loss (H_dry), following heat acclimatization (HAz) and heat acclimation (HA). Twenty-two male endurance athletes (mean ± standard deviation; age, 37 ± 12 y; body mass, 73.4 ± 8.7 kg; height, 178.7 ± 6.8 cm; and VO_2max, 57.1 ± 7.2 mL·kg⁻¹·min⁻¹) completed three trials (baseline; post-HAz; and post-HA), which consisted of 60 min steady state exercise at 59 ± 2% velocityVO_2max in the heat (ambient temperature [T_amb], 35.2 ± 0.6 °C; relative humidity [%rh] 47.5 ± 0.4%). During the trial, VO₂ and RER were collected to calculate H_prod, H_evap, and H_dry. Following the baseline trial, participants completed self-directed outdoor summer training followed by a post-HAz trial. Then, five days of HA were completed over eight days in the heat (T_amb, 38.7 ± 1.1 °C; %rh, 51.2 ± 2.3%). During the HA sessions, participants exercised to maintain hyperthermia (38.50 °C and 39.75 °C) for 60 min. Then, a post-HA trial was performed. There were no differences in H_prod between the baseline (459 ± 59 W·m⁻²), post-HAz (460 ± 61 W·m⁻²), and post-HA (464 ± 55 W·m⁻², p = 0.866). However, H_evap was significantly increased post-HA (385 ± 84 W·m⁻²) compared to post-HAz (342 ± 86 W·m⁻², p = 0.043) and the baseline (332 ± 77 W·m⁻², p = 0.037). Additionally, H_dry was significantly lower at post-HAz (125 ± 8 W·m⁻², p = 0.013) and post-HA (121 ± 10 W·m⁻², p < 0.001) compared to the baseline (128 ± 7 W·m⁻²). H_dry at post-HA was also lower than post-HAz (p = 0.049). H_prod did not change following HAz and HA. While H_dry was decreased following HA, the decrease in H_dry was smaller than the increases in H_evap. Adaptations in body heat exchange can occur by HA following HAz.