Physiological Strain And Heat Storage In Different Hot Conditions Of Equivalent Wet Bulb Globe Temperature

Abstract

Varying levels of ambient temperature and humidity differently affect heat storage (S) during walking, but the quantification of S at equivalent wet bulb globe temperature (WBGT) and affects on physiological strain have yet to be determined. PURPOSE: This investigation tested the hypothesis that varying hot environments of equal WBGT result in similar cumulative S but differing levels of cardiovascular strain and ventilatory regulation. METHODS: In a randomized crossover design, 12 subjects walked for 90 min on a treadmill in hot-dry (HD; 42.2 °C, 15% relative humidity (RH)) and warm-wet (WW; 30 °C, 80% RH) environments of equal WBGT (27.8 °C). Whole body sweat rate (WBSR) was determined by measuring body mass change. Ventilation (VE), respiratory rate (f), tidal volume (TV), respiratory exchange ratio (RER), and gas volumes (O2, CO2) were collected as 5-min averages, and core temperature (Tc, ingestible pill), heart rate (HR), and four-site mean skin temperature (Tsk) were recorded every 10 min. WBSR, S, evaporative heat loss (EHL), and dry heat gain (DHG) were assessed with paired t-tests and all other dependent variables with two-way analysis of variance (condition x time: 10, 50, 90 min). RESULTS: Tc (HD: 38.5 ± 0.5 °C; WW: 38.4 ± 0.4 °C, p = 0.38) and S (HD: 159 ± 235 W; WW: 359 ± 112 W, p = 0.06) increased during exercise (p < 0.01) and were not different between groups at min 90. Tsk (HD: 36.2 ± 0.9 °C; WW: 34.4 ± 0.7 °C, p < 0.01), HR (HD: 160 ± 19 bpm; WW: 154 ± 15 bpm, p = 0.03), and WBSR (HD: 15.1 ± 4.4 mL·min-1; WW: 10.0 ± 4.1 mL·min-1, p < 0.01) were greater in HD at min 90. Cumulative EHL (HD: 1203 ± 261 W; WW: 502 ± 126 W, p < 0.01) and DHG (HD: 221 ± 69 W; WW: -260 ± 72 W, p < 0.01) were higher in HD. VE (HD: 38.1 ± 9.8 L·min-1; WW: 37.6 ± 10.5 L·min-1, p = 0.64), its components f and TV (p = 0.12; p = 0.66, respectively), RER (HD: 0.86 ± 0.04; WW: 0.83 ± 0.04, p = 0.21) and the ventilatory equivalents for CO2 (HD: 34.6 ± 5.5 VE·VCO2-1; WW: 34.7 ± 4.3 VE·VCO2-1, p = 0.95) and O2 (HD: 29.9 ± 5.3 VE·VO2-1; WW: 28.8 ± 3.3 VE·VO2-1, p = 0.29) were not different at min 90 between trials nor when VE, VE·VCO2-1, and VE·VO2-1 were adjusted for change in Tc (p = 0.59; p = 0.26; p = 0.49, respectively). CONCLUSIONS: Different hot conditions of equal WBGT elicit similar increases in Tc, S, and ventilation, but higher cardiovascular strain was observed in HD compared to WW during walking in the heat.