Post-exercise hot water immersion elicites heat acclimation responses in endurance and recreational athletes. A systematic review and meta-analysis

Aims  This meta-analysis aims to evaluate the effectiveness of heat acclimation (HA) via hot water immersion protocols and their effect on time trial (TT) performance, heart rate (HRE, HRM and HRTT), rectal temperature (Tre), Rate of perceived exertion (RPE), psychological stress index (PhSI), thermal comfort (Tcomf), thermal sensation (Tsen) and maximum oxygen uptake (Vo2max).  Methods  Pubmed, Scopus, Sportdiscus and Web of Science databases were used alongside the grey matter sites Google Scholar and Researchgate. The databases were then searched for randomised control trials and mixed-method design studies. Two RTCs, six repeated measure design studies and one randomised crossover design study were included after screening a total of 50 titles and 28 full-text articles. Sample sizes range from 1 - 13 with all participants having not participated in any form of heat training 6 months before their inclusion in the study.  Results  The mean difference (MD) for Heart rate (HR) was -9.1125 BPM (95% CI p = 0.026) and was considered to be statistically significant. The MD for Rectal temperature (Tre) effect size was -0.3814 Tre (°C) (95% CI; p = 0.05). The MD for sweat rate was 0.085; (96% CI; p = 0.0179) The changes in RPE, PhSI, Tcomf and Tsen were too small to be considered statistically significant. There was no significant difference between pre and post HA for Vo2max and PV.  This meta-analysis implies that HA via HWI may improve tolerance to discomfort during heat exposure and thus subsequently improve physical performance during exercise in hot conditions. Conclusion  The primary finding of this meta-analysis is that athletic performance is improved with post-exercise hot water immersion heat acclimation training.  HWI HA protocols should focus on the following guidelines:  40-50 minutes of submaximal exercise (>65% of Vo2max) should be followed directly (within 10 minutes) by 40 minutes of hot water immersion at 40°C with the individual immersed up to their neck.  The HA protocol should last between 6-9 days with a single bout of HWI every day for this period.