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Effects of Heat Acclimation and Acclimatisation on Maximal Aerobic Capacity Compared to Exercise Alone in Both Thermoneutral and Hot Environments: A Meta-Analysis and Meta-Regression

Lookup NU author(s): Dr Owen JeffriesORCiD



This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).


© 2021, The Author(s). Background: Heat acclimation and acclimatisation (HA) is typically used to enhance tolerance to the heat, thereby improving performance. HA might also confer a positive adaptation to maximal oxygen consumption (VO 2 max), although this has been historically debated and requires clarification via meta-analysis. Objectives: (1) To meta-analyse all studies (with and without control groups) that have investigated the effect of HA on VO 2 max adaptation in thermoneutral or hot environments; (2) Conduct meta-regressions to establish the moderating effect of selected variables on VO 2 max adaptation following HA. Methods: A search was performed using various databases in May 2020. The studies were screened using search criteria for eligibility. Twenty-eight peer-reviewed articles were identified for inclusion across four separate meta-analyses: (1) Thermoneutral VO 2 max within-participants (pre-to-post HA); (2) Hot VO 2 max within-participants (pre-to-post HA); (3) Thermoneutral VO 2 max measurement; HA vs. control groups; (4) Hot VO 2 max measurement, HA vs. control groups. Meta-regressions were performed for each meta-analysis based on: isothermal vs. iso-intensity programmes, days of heat exposure, HA ambient temperature (°C), heat index, HA session duration (min), ambient thermal load (HA session x ambient temperature), mean mechanical intensity (W) and the post-HA testing period (days). Results: The meta-analysis of pre–post differences in thermoneutral VO 2 max demonstrated small-to-moderate improvements in VO 2 max (Hedges’ g = 0.42, 95% CI 0.24–0.59, P < 0.001), whereas moderate improvements were found for the equivalent analysis of hot VO 2 max changes (Hedges’ g = 0.63, 95% CI 0.26–1.00, P < 0.001), which were positively moderated by the number of days post-testing (P = 0.033, β = 0.172). Meta-analysis of control vs. HA thermoneutral VO 2 max demonstrated a small improvement in VO 2 max in HA compared to control (Hedges’ g = 0.30, 95% CI 0.06–0.54, P = 0.014) and this effect was larger for the equivalent hot VO 2 max analysis where a higher (moderate-to-large) improvement in VO 2 max was found (Hedges’ g = 0.75, 95% CI 0.22–1.27, P = 0.005), with the number of HA days (P = 0.018; β = 0.291) and the ambient temperature during HA (P = 0.003; β = 0.650) positively moderating this effect. Conclusion: HA can enhance VO 2 max adaptation in thermoneutral or hot environments, with or without control group consideration, by at least a small and up to a moderate–large amount, with the larger improvements occurring in the heat. Ambient heat, number of induction days and post-testing days can explain some of the changes in hot VO 2 max adaptation.

Publication metadata

Author(s): Waldron M, Fowler R, Heffernan S, Tallent J, Kilduff L, Jeffries O

Publication type: Article

Publication status: Published

Journal: Sports Medicine

Year: 2021

Volume: 51

Pages: 1509-1525

Print publication date: 01/07/2021

Online publication date: 03/04/2021

Acceptance date: 06/03/2021

Date deposited: 05/03/2021

ISSN (print): 0112-1642

ISSN (electronic): 1179-2035

Publisher: Springer Science and Business Media Deutschland GmbH


DOI: 10.1007/s40279-021-01445-6


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