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
Authors: Waldron Mark , Fowler Rebecca , Heffernan Shane , Tallent Jamie , Kilduff Liam , Jeffries Owen 
Published: April 2021

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 ($$V{\text{O}}_{2\max }$$VO2max), 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$$V{\text{O}}_{2\max }$$VO2maxadaptation in thermoneutral or hot environments; (2) Conduct meta-regressions to establish the moderating effect of selected variables on$$V{\text{O}}_{2\max }$$VO2maxadaptation 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$$V{\text{O}}_{2\max }$$VO2maxwithin-participants (pre-to-post HA); (2) Hot$$V{\text{O}}_{2\max }$$VO2maxwithin-participants (pre-to-post HA); (3) Thermoneutral$$V{\text{O}}_{2\max }$$VO2maxmeasurement; HA vs.control groups; (4) Hot$$V{\text{O}}_{2\max }$$VO2maxmeasurement, 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$$V{\text{O}}_{2\max }$$VO2maxdemonstratedsmall-to-moderateimprovements in$$V{\text{O}}_{2\max }$$VO2max(Hedges’g = 0.42, 95% CI 0.24–0.59,P < 0.001), whereasmoderateimprovements were found for the equivalent analysis of hot$$V{\text{O}}_{2\max }$$VO2maxchanges (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$$V{\text{O}}_{2\max }$$VO2maxdemonstrated asmallimprovement in$$V{\text{O}}_{2\max }$$VO2maxin 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$$V{\text{O}}_{2\max }$$VO2maxanalysis where a higher (moderate-to-large) improvement in$$V{\text{O}}_{2\max }$$VO2maxwas 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$$V{\text{O}}_{2\max }$$VO2maxadaptation in thermoneutral or hot environments, with or without control group consideration, by at least asmalland up to amoderate–largeamount, 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$$V{\text{O}}_{2\max }$$VO2maxadaptation.