Differing Physiological Adaptations Induced by Dry and Humid Short-Term Heat Acclimation

in International Journal of Sports Physiology and Performance
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Purpose: To investigate the effect of a 5-day short-term heat acclimation (STHA) protocol in dry (43°C and 20% relative humidity) or humid (32°C and 80% relative humidity) environmental conditions on endurance cycling performance in temperate conditions (21°C). Methods: In a randomized, cross-over design, 11 cyclists completed each of the two 5-day blocks of STHA matched for heat index (44°C) and total exposure time (480 min), separated by 30 days. Pre- and post-STHA temperate endurance performance (4-min mean maximal power, lactate threshold 1 and 2) was assessed; in addition, a heat stress test was used to assess individual levels of heat adaptation. Results: Differences in endurance performance were unclear. Following dry STHA, gross mechanical efficiency was likely reduced (between-condition effect size dry vs humid −0.59; 90% confidence interval, −1.05 to −0.15), oxygen uptake was likely increased for a given workload (0.64 [0.14 to 1.07]), and energy expenditure likely increased (0.59 [0.17 to 1.03]). Plasma volume expansion at day 5 of acclimation was similar (within-condition outcome 4.6% [6.3%] and 5.3% [5.1%] dry and humid, respectively) but was retained for 3 to 4 days longer after the final humid STHA exposure (−0.2% [8.1%] and 4.5% [4.2%] dry and humid, respectively). Sweat rate was very likely increased during dry STHA (0.57 [0.25 to 0.89]) and possibly increased (0.18 [−0.15 to 0.50]) during humid STHA. Conclusion: STHA induced divergent adaptations between dry and humid conditions, but did not result in differences in temperate endurance performance.

Tebeck, Bellenger, and Stanley are with the Dept of Sports Science, South Australian Sports Inst, Kidman Park, SA, Australia. Tebeck, Buckley, Bellenger, and Stanley are with the Alliance for Research in Exercise, Nutrition and Activity (ARENA), University of South Australia, Adelaide, SA, Australia. Bellenger is also with the Dept of Physiology, Australian Inst of Sport, Bruce, ACT, Australia. Stanley is also with the Australian Cycling Team, Cycling Australia, Gepps Cross, SA, Australia.

Tebeck (Samuel.Tebeck@mymail.unisa.edu.au) is corresponding author.
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