Influence Of Dry And Humid Heat Acclimation On Adaptation And Cross-adaptation

INTRODUCTION: Heat acclimation (HA) induces adaptations that alleviate physiological strain during exercise in hot and/or humid conditions. However, it is unclear if adaptations are influenced by environmental characteristics (dry/humid) and whether cross-adaptation differs between environments. Thus, this study compared the effects of dry and humid HA on thermoregulatory and cardiovascular adaptations and the cross-adaptation afforded by each environment. METHODS: Ten trained male cyclists (age 43.2 ± 9.3 y, body mass 80.8 ± 7.3 kg, O2max 59 ± 7 mL.kg-1.min-1) completed a 45-min heat response test at 65% O2max in hot-dry (DRY-HRT: 42 °C, 25% RH) and warm-humid (HUM-HRT: 33 °C, 80% RH) conditions before and after 8 days of controlled heart rate (HR) dry (DRY-HA) or humid (HUM-HA) HA in a counterbalanced crossover design. A Bayesian multi-level model was used to determine posterior means, 90% credible intervals and probability of direction (%). RESULTS: Sweat rate increased following HUM-HA during DRY-HRT (0.08 L·h-1 [-0.03, 0.18]; 90%) and HUM-HRT (0.19 L·h-1 [0.08, 0.29]; 99%). After DRY-HA, sweat rate increased in HUM-HRT (0.21 L·h-1 [0.09, 0.32]; 99%) but not DRY-HRT (0.00 L·h-1 [-0.11, 0.12]; 51%). There was a slight difference between DRY-HRT and HUM-HRT after HUM-HA (0.08 L·h-1 [-0.06, 0.22]; 82%). After HUM-HA, end-exercise rectal temperature (Trec) decreased by -0.28 (-0.40, -0.16; 99%) in DRY-HRT and -0.11 (-0.26, -0.04; 89%) in HUM-HRT. Following DRY-HA, end-exercise Trec decreased by -0.21 (-0.34, -0.08; 72%) in DRY-HRT and -0.29 (-0.45, -0.12; >99%) in HUM-HRT. During the HUM-HRT, DRY-HA slightly lowered end-exercise Trec compared to HUM-HA (-0.18 [-0.40, 0.03]; 92%). DRY-HA lowered end-exercise HR in DRY-HRT (-7 bpm [-11, -1]; 99%) and HUM-HRT (-9 bpm 1 [-12, -6]; 99%), as did HUM-HA in DRY-HRT (-8 bpm [-12, -1]; 99%) and HUM-HRT (-2 bpm [-5, 1]; 91%). A greater reduction in end-exercise HR of 7 bpm (-3, -11; 99%) was estimated during HUM-HRT following DRY-HA compared to HUM-HA. CONCLUSION: DRY-HA lowered end-exercise HR more than HUM-HA during HUM-HRT, whereas HA conditions appeared to have minimal influence on other parameters. Our data indicates that some HA adaptations may be influenced by environmental characteristics, although additional research is required to fully elucidate this influence.