Effects of Heat Acclimation and Acclimatisation on Maximal Aerobic Capacity Compared to Exercise Alone in Both...

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.

(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.

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).

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.

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.