Author(s): CHEN, Y., ZHANG, B., ZHANG, J., WONG, S.H.S., POON, E.T.C., Institution: THE CHINESE UNIVERSITY OF HONG KONG, Country: HONG KONG, Abstract-ID: 1503

INTRODUCTION:
Exercise in the heat (ExH) is considered an effective means of improving heat resilience, but its efficacy is often confounded by other heat acclimation strategies and moderators. Emerging evidence also indicates that exercise in thermoneutral condition (ExN) may yield similar adaptations, but rigorous comparative data are lacking. Therefore, this study aimed to evaluate the effectiveness of ExH and ExN on aerobic performance and heat-related physiological adaptations.
METHODS:
PubMed, SPORTDiscus, Cochrane Library, Scopus, and Web of Science were searched up to 20 November 2025. Randomized and non-randomized controlled trials investigating ExH and ExN were included. Multi-level meta-analyses were employed to examine the within- and between-group effects of ExH and ExN. Subgroup analyses were conducted to examine potential moderators including participants characteristics, heat induction method, intervention features, measurement conditions, and performance outcome type.
RESULTS:
Twenty-five studies (446 participants; 73.3% male) were included in the meta-analysis, with an overall moderate risk of bias. Compared with ExN, ExH demonstrated small-to-moderate improvements in aerobic performance (effect size [ES] = 0.36, 95% confidence interval [CI] 0.08 to 0.64), core temperature (ES = 0.43, 95% CI 0.18 to 0.68), skin temperature (ES = 0.59, 95% CI 0.18 to 1.00), sweat rate (ES = 0.38, 95% CI 0.02 to 0.74), and heart rate (HR) (ES = 0.43, 95% CI 0.17 to 0.70). Within-group analyses revealed small-to-moderate improvements following ExH compared with baseline, whereas ExN elicited small improvements in core temperature (ES = 0.21, 95% CI 0.02 to 0.40) and HR (ES = 0.34, 95% CI 0.08 to 0.60). ExH demonstrated superior effects compared with ExN only for time-to-exhaustion performance (ES = 0.79, 95% CI 0.22 to 1.35). In subgroup analyses of comparative effectiveness, greater magnitude of heat adaptations was observed in elite athletes (Tier 3), while no other moderators were significant (all p > 0.05).
CONCLUSION:
This review supports ExH as a more effective strategy for improving heat-related physiological adaptations, while ExN also induces modest improvements in core temperature and HR. Additional enhancements in aerobic performance were observed in time-to-exhaustion tests following ExH. These findings provide practical guidance for heat acclimation strategies in both athletic and general populations, while highlighting the importance of considering baseline training status.