Thermoregulation in hot-dry and warm-humid heat stress during cycling among males.

INTRODUCTION: Hot-dry (HD) and warm-humid (WH) environments with equivalent wet-bulb globe temperature (WBGT) may be expected to produce similar physiological strain; however, previous studies have reported divergent thermoregulatory and cardiovascular responses. This study compared heat storage, thermal, and cardiovascular responses between HD and WH conditions with matched WBGT during cycling. METHODS: Ten healthy, active, non-heat-acclimatized males completed two randomized crossover trials consisting of 60 minutes of cycling at an individualized workload corresponding to 55% of peak power output in HD (39.2°C, 30% relative humidity [RH]) and WH (34.2°C, 55% RH) conditions with equivalent WBGT (~30°C). Heat storage, dry heat loss (DHL), and evaporative heat loss (EHL) were estimated using partitional calorimetry. Rectal temperature (Tre), mean skin temperature (Tsk), and cardiovascular responses were measured throughout the trials. RESULTS: No condition differences were observed for cumulative heat storage, Tre, heart rate, blood pressure, or forearm skin blood flux (all p > .050). Tsk (HD: 36.28 ± 0.67°C, WH: 35.89 ± 0.70°C; p = .030, ηp² = .465) and EHL (HD: 450 ± 53 W, WH: 319 ± 30 W; p < .001, ηp² = .932) were greater in HD, whereas DHL was greater in WH (HD: -80 ± 25 W; WH: 48 ± 25 W; p < .001, ηp² = .950). Conclusion: Despite similar heat storage and cardiovascular strain, matched-WBGT environments altered the mechanisms of heat exchange. Thus, ambient temperature, humidity, and workload should be considered alongside WBGT when monitoring heat strain and selecting heat-mitigation strategies. CONCLUSION: Despite similar heat storage and cardiovascular strain, matched-WBGT environments altered the mechanisms of heat exchange. Thus, ambient temperature, humidity, and workload should be considered alongside WBGT when monitoring heat strain and selecting heat-mitigation strategies.