Heat acclimation, aerobic fitness, and hydration effects on tolerance during uncompensable heat stress

The purpose ofthe present study was to determine the separate and combined effects ofaerobic fitness, short-term heat acclimation, and hypohydration ontolerance during light exercise while wearing nuclear, biological, andchemical protective clothing in the heat (40°C, 30% relativehumidity). Men who were moderately fit (MF); <50ml · kg¹ · min¹maximal O₂ consumption;n = 7] and highly fit(HF); >55ml · kg¹ · min¹maximal O₂ consumption;n = 8] were tested while theywere euhydrated or hypohydrated by ~2.5% of body mass throughexercise and fluid restriction the day preceding the trials. Tests wereconducted before and after 2 wk of daily heat acclimation (1-htreadmill exercise at 40°C, 30% relative humidity, while wearingthe nuclear, biological, and chemical protective clothing). Heatacclimation increased sweat rate and decreased skin temperature andrectal temperature (T_re) in HF subjects but had no effecton tolerance time (TT). MF subjects increased sweat rate but did notalter heart rate, Tre, or TT. In both MF and HF groups, hypohydration significantly increased T_re and heart rate and decreasedthe respiratory exchange ratio and the TT regardless of acclimationstate. Overall, the rate of rise of skin temperature was less, whileT_re, the rate of rise of T_re, and the TTwere greater in HF than in MF subjects. It was concluded thatexercise-heat tolerance in this uncompensable heat-stress environmentis not influenced by short-term heat acclimation but is significantlyimproved by long-term aerobic fitness.