Abstract Heat tolerance testing (HTT) assesses responses to heat stress with rectal temperature (T rec ) and heart rate (HR) thresholds defining individuals as heat‐tolerant (HT) or heat‐intolerant (HI). To evaluate classification criteria by acclimation state and sex. Forty participants (19M/21F, mean ± SD, 23 ± 4 years) completed an HTT (120 min, 5 km·h −1 , 2% grade) before (PreHA) and after (PostHA) 5 days of repeated exercise in (40°C, 40% RH) categorized as isothermal (exercise intensity adjusted to maintain T rec within 38.5°C–39.5°C, 60 min) exercise‐heat acclimation (HA). Females had lower body surface area (vs. males) (1.7 ± 0.1 vs. 2.0 ± 0.2 m 2 , p < 0.001), mass (59.5 ± 5.4 vs. 79.5 ± 10.1 kg, p < 0.001), height (164.2 ± 5.7 vs. 179.1 ± 7.6 cm, p < 0.001), maximal oxygen consumption (V̇O 2 max, 44.5 ± 5.1 vs. 51.5 ± 5.7 mL·min −1 ·kg −1 , p < 0.001), and velocity at V̇O 2 max (12.9 ± 1.5 vs. 15.0 ± 1.7 km·h −1 , p < 0.001). PreHA 51.1% (mean % HI classification across criteria) of participants were HI (66.7% F/52.6% M), decreasing ( p < 0.001) to 23.6% PostHA (28.6% F/10.5% M), reflected across criteria ( p < 0.05). The rate of HI based on plateau in T rec during the final 60 min (ΔT rec (T120–T60) ≥ 0.45°C) was similar ( p = 0.735) PreHA (15% HI) and PostHA (10%), and HA reduced ΔT rec (T120–T60) (0.30 ± 0.19 vs. 0.19 ± 0.20°C, p = 0.001), demonstrating sensitivity to adaptation. HA reduces HI classification rate, but criteria differ in capturing presumed HT and classify more females as HI in naïve and acclimated states.
Bowie et al. (Sun,) studied this question.