Introduction: Exercise performance and capacity are impaired in hot and humid compared to temperate conditions [1], [3] and so researchers, coaches and athletes are interested in strategies to attenuate this impairment. Heat acclimation (or acclimatisation) (HA) is one approach that may reduce the thermal strain of exercising in hot conditions and benefit exercise performance [1], [2]. This meta-analysis quantified the magnitude of effect that HA has on exercise performance and capacity in the heat, and tested whether the magnitude of effect is related to the volume or intensity of heat stress experienced. Methods: The PubMed database was searched (09/01/15) using the first-order search terms acclimation, acclimatization, acclimatisation and adaptation and second-order search terms heat, exercise, performance, capacity and training. Using the four-stage process identified in the PRISMA statement the initial number of results (9,369) was reduced to 93. Data (N, mean, SD) were extracted from these articles in duplicate or triplicate. A subset of the data (n = 29 manuscripts) are presented here; manuscripts were included if exercise performance (Time to complete a fixed amount of work: E.g., time trial) and/or capacity (Open ended tests: E.g., Maximal aerobic power, time to exhaustion at a fixed workload), were measured and reported. All HA protocols regardless of duration, frequency, ambient conditions or exercise modality were used. Hedge's g (± 95 % CI) were calculated and correlation analyses were performed between the effect size and total HA time (HAtime), and HA temperature (HAtemp). Results: The 29 manuscripts reviewed used a mean (± SD) of 9.8 ± 4.0 (range: 4 - 24) HA sessions separated by 0.2 ± 0.4 (0 - 2) days. Total HAtime was 1055 ± 746 min (190 - 3,120), and the HAtemp and HAhumidity were 39.9 ± 5.9 °C (30 - 49) and 34 ± 17 % rh (14 - 87), respectively. Conclusion: HA is an effective way to improve both exercise capacity and performance in the heat. The magnitude of the effect appears to be independent of either HAtime or HAtemp for capacity, and independent of HAtemp for performance. However, the magnitude of benefit on exercise performance may be dependent upon HAtime.
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