M.W. Schleh, M. Johannsen, B.C. Ruby FACSM, C.L. Dumke FACSM

The University of Montana, Missoula, MT

Heat acclimation improves thermal tolerance and attenuates the physiological and perceptual strain associated with exercise in the heat. A limited fluid delivery strategy during heat acclimation may promote further adaptations to attenuate heat stress over long periods, decreasing the probability of heat related illness. PURPOSE: To determine i) the effect of fluid delivery by inducing dehydration (DEH=0.5 ml*kg-1*min-15) compared to euhydration (EUH=2.0 ml * kg-1 * 15 min-1) following three heat acclimation bouts on heat stress factors, and ii) to determine the effect of aerobic performance in the heat following DEH and EUH acclimation trials. METHODS: Eight aerobically fit males (23.0±0.9 yrs, 82.1 ± 2.0 kg, 184.0 ± 2.5 cm, 53.2 ± 1.5 ml*kg-1*min-1 VO2 max) completed 90-minute heat stress test (HST) in hot conditions (T=40°C, RH=30%) at 50% VO2 max prior to and following three-days of EUH and DEH acclimation trials. Acclimation trials consisted of three heat exposures in the same environmental conditions, intensity, and duration as previously stated on alternating days. Participants wore standard wildland firefighter (WLFF) Nomex: yellow shirt, green pants, and a cotton T-shirt. Following each HST, aerobic performance was assessed by a graded ramp protocol by increasing treadmill grade by 1% until 15%, and then increased 1 mph every minute until volitional exhaustion. Core temperature (Tc) and Skin temperature (Tsk) were measured continuously, and reported as peak TC and TSk at 90 minutes. Sweat rate by ∆nude weight, and post acclimation performance measured in time to exhaustion. All data was analyzed by a two-way analysis of variance (ANOVA) 2 (trt) x 2 (time). RESULTS: Peak Tc and Tsk during HST significantly decreased in effect for time in DEH & EUH groups (p<0.001, p=0.003) respectively. Sweat rate was significantly greater in DEH compared to EUH (trt*time: p=0.034). Aerobic performance increased greater in EUH compared to DEH (trt*time: p= 0.034). CONCLUSION: Short-term heat acclimation is an effective strategy to attenuate physiological strain associated with heat stress, and also improve aerobic performance in the heat. Fluid delivery strategies during acclimation do not affect thermal strain, but may increase sweat capacity. Controlled dehydration during acclimation did not however increase performance.

This project was supported by the US Forest Service.

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