CrossAcclimation and Endurance Performance Following Adaptation to Dry and Humid Heat

Purpose: To examine the influence of hot-dry and warm-humid heat acclimation (HA) on endurance performance and the cross-acclimation benefit between environments. Methods: In a cross-over design, trained male cyclists completed a 30-min time trial in hot-dry (Dry-TT; 42°C, 25% RH) and warm-humid (Humid-TT; 33°C, 80% RH) conditions before and after 8 days of HA in dry (Dry-HA, n =10) and humid (Humid-HA, n =12) conditions matched for wet-bulb globe temperature (~31°C). Bayesian multilevel models were employed to estimate posterior means and 90% credible intervals. Results: During the Dry-TT, mean power output increased by a further 8 W (2, 15) after Dry-HA (14 W [7, 21]) than Humid-HA (6 W [-1, 13]). Mean rectal temperature during the Dry-TT was lower after both HA regimens (-0.1ºC [-0.2, 0.0]) whereas heart rate was unchanged following Dry-HA (-1 beats·min -1 [-3, 2]) and lower after Humid-HA (-3 beats·min -1 [-6, 0]). Whole-body sweat rate (WBSR) during the Dry-TT increased with both Dry-HA (0.28 L·h -1 [0.14, 0.41]) and Humid-HA (0.23 L·h -1 [0.10, 0.35]). During the Humid-TT, mean power output increased similarly following Humid-HA (9 W [1, 17]) and Dry-HA (12 W [3, 20]). Mean rectal temperature during the Humid-TT was unchanged (0.0°C [-0.1, 0.1]) after Humid-HA and decreased (-0.2°C [-0.3, -0.1]) after Dry-HA, whereas heart rate was unchanged after Humid-HA (2 beats·min -1 [-3, 5]) and Dry-HA (-2 beats·min -1 [-6, 2]). WBSR during the Humid-TT increased with both Humid-HA (0.42 L·h -1 [0.32, 0.52]) and Dry-HA (0.35 L·h -1 [0.23, 0.47]). Conclusions: Our data indicate that Dry-HA and Humid-HA improved self-paced endurance performance in each respective environment, and in the opposing environment. Moreover, Dry-HA further improved performance in dry conditions than did Humid-HA, while performance in humid conditions was similarly improved following both interventions.