Individual characteristics associated with the magnitude of heat acclimation adaptations

The magnitude of heat acclimation (HA) adaptations varies largely among individuals, but it remains unclear what factors influence this variability. This study compared individual characteristics related to fitness status and body dimensions of low-, medium-, and high responders to HA.

Twenty-four participants (9 female, 15 male; maximum oxygen uptake [[Formula: see text]O_2peak,kg] 52 ± 9 mL kg^-1 min^-1) completed 10 daily controlled-hyperthermia HA sessions. Adaptations were evaluated by heat stress tests (HST; 35 min cycling 1.5 W  kg^-1; 33 °C, 65% relative humidity) pre- and post-HA. Low-, medium-, and high responder groups were determined based on tertiles (n = 8) of individual adaptations for resting rectal temperature (T_re), exercise-induced T_re rise (ΔT_re), whole-body sweat rate (WBSR), and heart rate (HR).

Body dimensions (p > 0.3) and [Formula: see text]O_2peak,kg (p > 0.052) did not differentiate low-, medium-, and high responders for resting T_re or ΔT_re. High WBSR responders had a larger body mass and lower body surface area-to-mass ratio than low responders (83.0 ± 9.3 vs 67.5 ± 7.3 kg; 249 ± 12 vs 274 ± 15 cm² kg^-1, respectively; p < 0.005). Conversely, high HR responders had a smaller body mass than low responders (69.2 ± 6.8 vs 83.4 ± 9.4 kg; p = 0.02). [Formula: see text]O_2peak,kg did not differ among levels of responsiveness for WBSR and HR (p > 0.3).

Individual body dimensions influenced the magnitude of sudomotor and cardiovascular adaptive responses, but did not differentiate T_re adaptations to HA. The influence of [Formula: see text]O_2peak,kg on the magnitude of adaptations was limited.