(05) THE INDUCTION OF THERMOREGULATORY ADAPTATIONS INDUCED BY HEAT ACCLIMATION/ACCLIMATIZATION IN ENDURANCE ATHLETES: A SYSTEMATIC REVIEW AND META-ANALYSIS

Global warming alongside elite sport globalization conspire to produce increasingly frequent episodes of extreme heat at major sporting events, challenging athlete performance and safety. Heat acclimation/acclimatization (HA) improves thermoregulatory mechanisms relevant for endurance performance in the heat and can provide some protection against exertional-heat illness. However, concerns transferring findings from untrained populations to endurance-athletes remains.

Purpose: This review aims to assess and quantify the induction of HA-induced thermoregulatory adaptations from studies incorporating endurance-trained athletes only.

Methods: A literature search was conducted in PubMed, SPORTDiscus, Scopus, and Cochrane-Library, with data from 23 studies extracted for analysis. Subgroup analysis distinguished differences in internal core body temperature, skin temperature, and sweat rate adaptations between short- (≤7 days), medium- (8-13 days), and long-term HA (≥14 days) interventions. Cohen’s classification of effect size magnitude was used for interpretation, whereby: g ≤ 0.19 is trivial/negligible, g = 0.20-0.49 is small, g = 0.50-0.79 is moderate, and g ≥ 0.8 is large.

Results: HA displayed moderate reductions in mean core temperature during endurance exercise in the heat (Hedges’ g [95% confidence intervals] = 0.66 [0.40–0.92] p < 0.05), with long-term HA protocols most effective for decreasing mean core temperature (-0.60 ± 0.40°C). Reductions in mean core temperature were lower for medium-term HA (-0.30 ± 0.22°C), with short-term protocols presenting the smallest mean core temperature reductions (- 0.18 ± 0.09°C). HA demonstrated a significant, small reduction in mean skin temperature during exercise in the heat (0.34 [0.05–0.63], p < 0.05). Medium-term HA protocols displayed the greatest reductions in mean skin temperature (-0.35 ± 0.22°C) compared to short-term HA (-0.01 ± 0.30°C). No long-term HA investigations measured mean skin temperature. Overall, HA had a significant, moderate effect on increasing sweat rate during exercise in the heat (0.57 [0.36–0.78], p < 0.05). Long-term HA and medium-term HA interventions produced similar, large increases in sweat rate (0.27 ± 0.14 L and 0.36 ± 0.17 L.hr^-1, respectively).

Conclusion: Results indicate that longer duration HA protocols are typically required to maximize thermoregulatory adaptations in endurance-trained athletes. Taken together, these adaptations are critical for developing thermoregulatory effector responses of sweating and alterations in peripheral skin blood flow, subsequently improving heat dissipation efficiency and endurance performance in the heat. PRACTICAL APPLICATION: The current findings are relevant for endurance athletes and their support teams to evidence-inform and individualize HA prescription prior to performance in the heat. As such, competitive athletes should spend an extended time period exposed to wet-bulb globe temperatures replicating predicted performance conditions, with the thermal stimulus progressively increased to balance HA and training requirements.

Acknowledgements: None