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

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 ([Formula: see text]), 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 [Formula: see text] adaptation in thermoneutral or hot environments; (2) Conduct meta-regressions to establish the moderating effect of selected variables on [Formula: see text] 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 [Formula: see text] within-participants (pre-to-post HA); (2) Hot [Formula: see text] within-participants (pre-to-post HA); (3) Thermoneutral [Formula: see text] measurement; HA vs. control groups; (4) Hot [Formula: see text] 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 [Formula: see text] demonstrated small-to-moderate improvements in [Formula: see text] (Hedges' g = 0.42, 95% CI 0.24-0.59, P < 0.001), whereas moderate improvements were found for the equivalent analysis of hot [Formula: see text] 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 [Formula: see text] demonstrated a small improvement in [Formula: see text] 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 [Formula: see text] analysis where a higher (moderate-to-large) improvement in [Formula: see text] 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 [Formula: see text] 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 [Formula: see text] adaptation.