Sex-specific heat tolerance criteria are necessary for accurate return-to-activity assessments

Heat tolerance testing (HTT), an essential component of return-to-activity guidelines following exertional heat injury (EHI), assesses tolerance to standardized conditions with rectal temperature (T rec ) and heart rate (HR) thresholds defining heat tolerant (HT) and intolerant (HI). Purpose: To compare sex-based differences in HTT classification before (pre) and after (post) heat acclimation (HA). Methods: 40 healthy participants without history of EHI (18M/22F, mean±SD, 23±4 years old) completed an HTT (120min at 5km·hr -1 , 2%grade) before and after five days of repeated exercise in (40°C, 40% relative humidity) categorized as isothermal (exercise intensity adjusted to maintain T rec at 38.5-39.5°C for 60min) exercise-HA. Results: We observed females to have lower body surface area (BSA; 1.7±0.1 vs. 2.0±0.2m 2 , p<0.001), body mass (59.5±5.4 vs. 79.5±10.1 kg, p<0.001), height (164.2±5.7 vs. 179.1±7.6cm, p<0.001), [Formula: see text]O 2 max (44.5±5.1 vs. 51.5±5.7 mL·min -1 ·kg -1 , p<0.001), and vVo2max (8.0±1.0 vs. 9.3±1.1 mph, p<0.001). Across criteria, 51.1% of participants were HI pre (66.7%F/52.6%M), improving to 23.6%HI post (28.6%F/10.5%M), potentially reflecting an elevated false positive rate. The best-performing criterion was a plateau in rectal temperature during the last 60min (ΔT rec 60) ≥0.45°C improving from 15%HI pre-HA (9.5%F vs. 21.1%M) to 10%HI post (9.5%F vs. 10.5%M), indicating effective thermoregulation in both sexes. The most stringent criterion ΔT rec 60≥0.25°C & HR≥120 bpm & T rec ≥38.2°C), classified 90%HI pre (100%F/78.9%M) and 67.5%HI post (76.2%F/57.9%M). The probability of heat tolerance (PHT) criterion, which includes a borderline category (BHT), showed 60% HI and 15% BHT pre (66.7%F/52.6%M HI, 19%F/10.5%M BHT) and 20%HI and 12.5%BHT post (28.6%F/10.5%M HI, 9.5%F/15.8 %M BHT). The T rec ≥38.5°C criterion showed a significant improvement, with 40%HI pre (42.9%F vs. 36.8%M) reducing to 7.5%HI post (9.5%F vs. 5.3%M). The HR≥150bpm criterion significantly improved with 37.6%HI pre (61.9%F vs. 10.5%M) reducing to 5.0%HI post (9.5%F vs. 0.0%M). The thermal circulatory ratio (TCR) at 120 minutes classified 50%HI pre (71.4%F vs. 26.3%M) and 15%HI post (23.8%F vs. 5.3%M). TCR at 60min (T60) similarly classified 65%HI pre (85.7%F vs. 42.1%M) and 40%HI post (46.9%F vs. 15.8%M), illustrating that females have higher HR for given T rec than males at both timepoints. Physiologically, females had higher resting HR (80±14 vs. 71±12bpm, p=0.046), pre-exercise resting HR (92±18 vs. 84±10bpm, p=0.007) and T rec (37.03±0.44 vs. 36.8±0.35°C, p=0.012) than males, and lower BSA adjusted whole body sweat loss (0.76±0.25 vs. 0.88±0.22L·m -2 , p=0.020). Conclusion: HA improves HT, but guidelines still misclassify participants pre- and post-HA, disproportionately affecting females, reflecting the inadequacy of criteria developed without female participants. DOD BA200299 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.