Hydration of wildland firefighters (WLFF) during heat acclimation trials may impact performance by reducing physiological strain. By inducing dehydration in a controlled setting, physiological adaptations such as increased sweat rate and cardiovascular strain may be enhanced. PURPOSE: To investigate the impact hydration status (dehydrated/ euhydrated) has, through mitigated fluid delivery, on levels of hydration (% dehydration), sweat rate (SR), heart rate (HR), and rating of perceived exertion (RPE). METHODS: Ten aerobically fit male subjects (age=23.0±0.9, weight=82.1±4.6kg) underwent three heat acclimation trials on alternate days in an environmental chamber (40°C, 30%RH) for 90 min at 50%VO 2max. Subjects wore WLFF Nomex pants, yellow button up and 100% cotton t-shirt. Fluid delivery was controlled to induce either dehydration (DEH=0.5 ml*kg -1 *15 min -1 ) or euhydration (EUH=2.0 ml*kg -1 *15 min -1 ). After a five week washout period three subsequent trials were held in the alternate hydrated state in a random crossover design. Rating of perceived exertion (RPE) and heart rate (HR) were measured every 15 minutes throughout the 90 minute trial. Pre and post bodyweight were measured to determine sweat rate and percent dehydration. Statistics were performed as a 2 (trt) x 3 (day) analysis of variance (ANOVA) with repeated measures. RESULTS: There was a significant interaction for treatment (p<0.001) in percent dehydration (DEH=2.16±0.5, 2.36±0.5, 2.32±0.4, EUH=1.32±0.4, 1.44±0.5, 1.48±0.3%) between DEH and EUH over the three days, but no interaction by day. There was no significant main effects in peak HR (DEH=167.8±11.2, 162.4±11.7, 165.0±14.2, EUH= 165.2±11.4, 163.4±18.3, 163.0±17.2 bpm), SR (DEH=1.34±0.26, 1.37±0.23, 1.33±0.22, EUH=1.32±0.25, 1.37±0.26, 1.3±0.21 L*min - 1 ) or RPE (DEH=15±1.00, 14.33±1.12, 14.00±1.66, EUH=14.44±1.24, 14.11±1.27, 14.00±1.50) for acclimation days 1, 2, and 3 respectively. CONCLUSIONS: Our data suggests that controlled dehydration during 3 days of short term heat acclimation results in elevated percent dehydration, but does not jeopardize HR, SR, or RPE. ABSTRACT PURPOSE To investigate the impact of hydration status on physiological variables through heat acclimation in a simulated wild land firefighters work environment. METHODOLOGY Acclimation Protocol Time: HR (Resting ) HR RPE HR RPE HR RPE HR RPE HR RPE HR RPE PRE Measurements Nude Body Weight Urine Specific Gravity POST Measurements Nude Body Weight Urine Specific Gravity Environment= 40°C, 30% RH EUHYDRATION: 2 ml/kg every 15 min DEHYDRATION: 0.5 ml/kg every 15 min _________________________________________ ______________________ _____________________ ______________________ RPE RESULTS H EART R ATE Ten aerobically fit males (age=23.0±0.9, weight=82.1±4.6kg) performed in three heat acclimation trials on alternating days in a heat chamber. They walked at 50% VO 2max for 90 minutes. WLFF Nomex pants, a yellow button up and 100% cotton t-shirt were worn. Rating of perceived exertion and heart rate were recorded every 15 minutes. Pre- and Post- body weight were measured to calculate percent dehydration and sweat rate. Statistics were ran as a 2 x 3 ANOVA with repeated measures. EFFECTS OF HYDRATION ON PHYSIOLOGICAL INDICATORS DURING HEAT ACCLIMATION N. Mickelson, M.Schleh, B. Ruby FACSM, C.L. Dumke FACSM University of Montana, Montana Center for Work Physiology and Exercise Metabolism, Missoula, MT CONCLUSION Mitigated fluid delivery, while training in the heat, doesn’t have effects on heart rate, RPE, or sweat rate but does effect the percentage of dehydration. PEAK HEART RATE TRTp=0.63 DAYp=0.59 TRT*DAYp=0.69 PEAK RPE TRTp=0.45 DAYp=0.27 TRT*DAYp=0.66 SWEAT RATE TRTp=0.76 DAYp=0.62 TRT*DAYp=0.53 %DEH TRTp<.001 DAYp=0.11 TRT*DAYp=0.76