Short term heat acclimation results in performance improvements in females

Purpose: Repeated exposures to heat stress (heat acclimation, HA) induce physiological adaptations which may improve performance in hot conditions. Guidelines recommend 14 days to achieve maximal adaptations, but short term HA (STHA) of ~7 days may result in acceptable adaptations for athletes or warfighters who have limited time before competition or deployment. Biological sex may also play a role, with females taking longer to show beneficial changes in heart rate and core temperature(T core ) compared to males. The aim of this study was to determine if improvements in performance could be discerned after a STHA protocol. Methods: 60 participants (34 F, age: 23 ± 3 years, height: 171.1 ± 10.2 cm, body mass: 68.3 ± 13.6 kg, V˙O 2max : 48.3 ± 6.5 ml·kg -1 ·min -1 ) completed an initial V˙O 2max test, followed by 2 days of heat tolerance testing (HTT1 & HTT2), 5 days of HA training, then repeated HTT1 and HTT2. For HA, participants ran at 70% V˙O 2max and 2% incline until T core reached 38.5°C which was then maintained by run/walk for a further 60min. HTT1 was a constant load test (120 min at 5 km·hr -1 , 2 % grade) while HTT2 was a time to temperature test in which participants ran at 65%VO 2max until T core 39.5 o C or volitional exhaustion. After a brief rest upon completion of HTT2, participants completed a performance battery including a BlazePod reaction test, countermovement jump, Quickfeet stepping, handgrip and either a 400 m sprint (n = 41, 20 female) or a 5 min time trial (TT) (n = 19, 13 female). All HA and HTT sessions were at 40°C and 40% relative humidity while the performance battery was in temperate conditions. Results: Overall, the end HTT2 HR was lower post the HA sessions compared to preHA (p = 0.001), an effect that was still evident when disaggregated by sex; Male (median (range)): preHA 188.5 (150-206) bpm vs postHA 179.0 (129-203) bpm, p = 0.03. Female: preHA 187.0 (151-219) bpm vs postHA 185.0 (140-208) bpm, p = 0.01. There was no difference between sexes in either preHA or postHA HR (p = 0.11). There were no differences from pre to post HA in quickfeet, handgrip or jump height either between or within sexes. However, in the choice reaction test postHA, females had more hits (p = 0.02) with a tendency to be faster (p = 0.057) compared to preHA whereas males saw no changes (p = 0.53 and p = 0.48 respectively). Males were significantly faster than females in the 400 m sprint and the 5 min TT both pre and postHA. However females improved their 400 m time by 4.7% (p = 0.009) and their TT distance by 3.3% (p = 0.02) whereas males had a nonsignificant 3.5% increase in both their 400m time (p = 0.26) and TT distance (2.5%, p = 0.59). Although both males and females significantly increased their time to temperature during HTT2 postHA, females had a greater improvement (Female mean (SD) preHA 30.61 (7.59) vs postHA 37.32 (11.29) min p = <0.001; male preHA 29.03 (5.81) vs postHA 31.71 (7.37) min p = 0.03). Conclusion: Contrary to expectations, although females did not exhibit the same heart rate adaptations as males, their running performance in the heat showed significantly more improvement following STHA. More research is required to identify the time course adaptations of females undertaking HA along with a greater understanding of molecular and cellular adaptations to delineate the effects of HA compared to regular exercise adaptations during acclimation protocols. Funding Source: DoD BA20029/ BA220010 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.