Abstract
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.
Highlights
Acute exercise in hot environmental conditions increases physiological strain; inducing an increase in whole-body temperature leading to thermal discomfort, increased cardiovascular strain, perception of exertion and impairment of aerobic capabilities (Casa et al, 2015; Racinais et al, 2015; Young et al, 1985;)
To counteract the impacts of thermal strain and the cardiovascular limitations associated with acute exposure to exercising in hot conditions (Chong and Zhu, 2017), athletes and individuals often partake in heat-acclimation protocols aimed to improve exercise performance in hot environmental conditions (Guy et al, 2016)
Extensive research has been conducted evaluating the different methods to overcome the physiological limitations encountered while exercising in hot conditions. (Altareki et al, 2009; Cheuvront et al, 2010; Jentjens et al, 2002; Schulze et al, 2015; Tatterson et al, 2000) Heat acclimatisation is the process by which physiological adaptations occur in response to naturally occurring heat stress – for example, seasonal heat adaptation
Summary
Acute exercise in hot environmental conditions increases physiological strain; inducing an increase in whole-body temperature leading to thermal discomfort, increased cardiovascular strain, perception of exertion and impairment of aerobic capabilities (Casa et al, 2015; Racinais et al, 2015; Young et al, 1985;). Long-term exposure to heat stress intensifies thermoregulatory responses, enhances submaximal exercise performance, improves maximal oxygen uptake ( VO2max ) (Guy et al, 2016; Tyler et al, 2016), improves skin blood flow response, and increases sweat rate and plasma expansion (Racinais et al, 2015; Chong and Zhu, 2017) while mitigating the effect of thermal discomfort. To counteract the impacts of thermal strain and the cardiovascular limitations associated with acute exposure to exercising in hot conditions (Chong and Zhu, 2017), athletes and individuals often partake in heat-acclimation protocols aimed to improve exercise performance in hot environmental conditions (Guy et al, 2016). Emerging research suggests that the ergogenic benefits of exercise-focused heat acclimation may extend further than just improving exercise performance in hot conditions. While pre-exercise cooling strategies can often provide a much more economically viable solution when compared to more expensive heat-acclimation strategies, the overall successfulness of pre-exercise cooling is often sub-par (Quod et al, 2008; Ross et al, 2011)
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