The fundamental aim of this meta-analysis was to evaluate the effectiveness of post-exercise hot-water immersion as a method of eliciting heat-acclimation responses. The secondary aim was to identify if an intervention period of 5 to 9 days of post-exercise hot-water immersion had any effect on time-trial finishing times in hot environmental conditions. Four databases were used along with two academic search engines to search for studies that satisfied the inclusion criteria. In order to fulfil the inclusion criteria, studies had to be either randomised-control trials or mixed-method design in nature, focusing on hot-water immersion or heat acclimation, with the full text publicly available. Three repeated-measures design studies, two randomised-control trials and one randomised cross-over design study were included. Statistical analysis took place by calculating effect sizes comparing pre- and post-intervention testing for all of the main outcome measures. The main outcome measures were time to complete a predetermined treadmill-based time trial, during-exercise heart rate and post-exercise heart rate, rectal temperature, physiological strain index and thermal sensation.
Once the effect sizes had been calculated, they were graphically represented using forest plots heterogeneity tests in Meta-Essentials Excel software package.
The results of the meta-analysis indicate that a 5- to 9-day protocol of post-exercise hot-water immersion reduces: time taken to complete a treadmill-based time trial, heart rate (-9 BPM ± 1.3), rectal temperature (-0.38 ± 0.03 °C). Additionally, sweat rate increased by 0.09 litres. Changes in ratings of perceived exertion, physiological heat strain index and thermal sensation were considered statistically significant. There was no difference between endurance-trained and recreationally trained individuals in relation to changes in ratings of perceived exertion as a result of heat training. However, the time delay between when exercise was conducted and when hot-water immersion occurred significantly impacted measures of thermal sensation.
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