Thermoregulation In Hot-dry & Warm-humid Heat Stress During Cycling In Matched Wbgt

Different types of heatwaves (dry versus humid) with equivalent overall heat stress indices should create similar thermal and cardiovascular strains during heat stress exposure. However, previous studies demonstrate divergent results on thermoregulatory and cardiovascular responses in varying environments with matched wet-bulb globe temperature (WBGT). PURPOSE: To compare heat storage, thermal, and cardiovascular responses between hot-dry (HD: 39.2 °C, 30% of RH) and warm-humid (WH: 34.2 °C, 55% of RH) conditions with matched WBGT (30 °C) during 60-min moderate intensity cycling. METHODS: Ten healthy, active, non-heat-acclimatized males (29 ± 7y, 179 ± 11 cm, 77.4 ± 9.3 kg, 51.9 ± 5.3 mL/kg/min VO2peak) performed two randomized, crossover, matched exercise trials (60-min cycling ~55% VO2peak) in HD and WH conditions with equivalent WBGT. Estimated heat storage, dry heat loss (DHL), and evaporative heat loss (EHL) were calculated using partitional calorimetry. Rectal temperature, skin temperature, heart rate, blood pressure, and forearm skin blood flux (FSBF) were measured at multiple timepoints during trials. Repeated measures ANOVA and paired-samples t-tests were used to compare data with significance set as p < .05. RESULTS: There were no main effect trial differences in cumulative heat storage (HD: 1057.68 ± 349.99 kJ, WH: 976.68 ± 288.49 kJ, p = .36, d = .30), rectal temperature (HD: 39.08 ± 0.53 °C, WH: 38.99 ± 0.49 °C, p = .78, ηp2 = .03), heart rate (HD: 175 ± 13 bpm, WH: 176 ± 13 bpm, p = .24, ηp2 = .25), systolic blood pressure (HD: 172 ± 30 mmHg, WH: 178 ± 49 mmHg, p = .24, ηp2 = .01), diastolic blood pressure (HD: 60 ± 21 mmHg, WH: 54 ± 19 mmHg, p = .33, ηp2 = .14), or FSBF (HD: 133 ± 50 a.u., WH: 113 ± 40 a.u., p = .39, ηp2 = .19) between HD and WH conditions at the end of trials. Skin temperature (HD: 36.35 ± 0.87 °C, WH: 36.01 ± 0.66 °C, p = .03, ηp2 = .47) and EHL (HD: 447.80 ± 52.30 W, HH: 311.70 ± 33.20 W, p < .001, ηp2 = .93) in HD were greater than in WH, whereas DHL (HD: -77.90 ± 31.19 W, WH: 42.30 ± 19.55 W, p < .001, ηp2 = .95) in WH was greater than in the HD condition, regardless of time point. CONCLUSION: Hot environments with matched WBGT altered EHL and DHL to accommodate for differences in humidity. Our data are based on greater exercise intensity with more extreme heat stress, but confirm thermoregulatory adjustments to DHL with varying levels of humidity.