Utility of Heat Stress Indicators in Identifying Thermoregulatory Compensability during Moderate Cycling

This study aimed to determine the ability of multiple indicators, including wet-bulb global temperature (WBGT), universal thermal climate index (UTCI), national weather service heat index (HI), and Humidex, to differentiate compensability of heat stress under moderate intensity cycling in young, healthy adults. In addition, we sought to examine the effectiveness of heat stress indicators to evaluate the physiological and perceptual strain between hot-dry (HD) and warm-humid (WH) conditions. We hypothesized that WBGT and UTCI would be useful indicators to identify heat stress compensability and appropriately track physiological and perceptual strain in varying environments. Ten healthy, active, non-heat-acclimatized males (29 ± 7y, 179 ± 11cm, 77.4 ± 9.3kg, 51.9 ± 5.3mL/kg/min VO 2peak ) performed two randomized, crossover, matched exercise trials (60-min cycling ~55% VO 2peak ) in HD (39.2°C, 30% of RH) and WH (34.2°C, 55% of RH) conditions. WBGT, UTCI, HI, and Humidex were calculated using equations based on ambient temperature, humidity, and wind speed. Metabolic heat production (H prod ), evaporation required for heat balance (E req ), and the maximum evaporative rate in the ambient environment (E max ) were calculated. The estimated compensability of heat stress was determined using E req to E max (E req / E max ≤ 1.0: compensable heat stress; E req / E max > 1.0: uncompensable heat stress). Rectal temperature (T rec ), skin temperature (T sk ), heart rate (HR), systolic blood pressure (SBP), diastolic blood pressure (DBP), forearm skin blood flux (FSBF), rate of perceived exertion (RPE), thermal sensation, thirst sensation, and muscle pain 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. WBGT, HI, and Humidex in HD and WH conditions were in level 2 category 3, ‘danger,’ and ‘extreme heat stress,’ respectively. UTCI was in ‘very strong’ heat stress category in HD condition and ‘strong ’heat stress category in WH conditions. There were no differences in WBGT ( p =.83, d=.59), HI ( p =.65, d=1.9), and Humidex ( p =.73, d=1.10) between HD and WH conditions. Whereas UTCI in HD was greater than in WH condition ( p <.001, d=1.19). H prod in HD was higher than WH condition, regardless of time point ( p =0.04, η p 2 =.38). E req in HD was higher than in WH condition, regardless of time point ( p <0.001, η p 2 =.75). E max in HD was higher than in WH condition, regardless of time point ( p <0.001, η p 2 =.94). E req /E max in both HD and WH conditions exceeded 1.0; indicating uncompensable heat stress. There were no differences in T rec ( p =.51, d=.45), T skin ( p =.35, d=1.02), HR ( p =.93, d=6.81), SBP ( p =.93, d=35.00), DBP ( p =.47, d=23.75), FSBF ( p =.21, d=42.47), RPE ( p =.70, d=2.05), thermal sensation ( p =1.00, d=.71), and thirst sensation ( p =.74, d=1.87) between HD and WH conditions at the end of the trials. In contrast, pain sensation in HD was greater than in WH condition ( p =.03, d=.89). Our data suggests HI and Humidex are more suitable for assessing thermoregulatory compensability compared to WBGT and UTCI. Further, WBGT, HI, and Humidex more appropriately identified physiological and perceptual strain in varying environments compared to UTCI. The utility of HI and Humidex for determining the magnitude of heat stress, physiological strain, and perceptual strain should protect people from excessive heat stress outcomes during exposure to extreme heat waves. This research was funded by a Graduate and Professional Student Congress Research Grant at the University of Arkansas. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.