Montain, Scott J., Michael N. Sawka, Bruce S. Cadar-ercise duration during heat stress (3-6, 10,11, 17, 18) and ette, Mark D. Quigley, and James M. McKay. Physiologi-to predict individual tolerance to heat strain (6, 10, 11, cal tolerance to uncompensable heat stress: effects of exercise 26), there remains little information to predict the inciintensity, protective clothing, and climate. J. AppL PhysioL dence of exhaustion from heat strain (7, 12,23). This 77(1): [216][217][218][219][220][221][222] 1994.-This study determined the influence of information is needed for mathematical models that preexercise intensity, protective clothing level, and climate on dict the physiological responses and work capability durphysiological tolerance to uncompensable heat stress. It also compared the relationship between core temperature and the ing heat stress. Recently, we examined the physiological incidence of exhaustion from heat strain for persons wearing strain tolerated by unclothed persons during uncompenprotective clothing to previously published data of unclothed sable heat stress (23). We found that exhaustion ocpersons during uncompensable heat stress. Seven heat-accli-curred over a broad range of core temperatures and that mated men attempted 180-min treadmill walks at metabolic there was no threshold core temperature where exhaus-VM'rates of -425 and 600 W while wearing full (clo = 1.5) or par-tion abruptly increased. We also proposed that core temin tial (clo = 1.3) protective clothing in both a desert (43°C dry perature might be a physiological index to estimate the bulb, 20% relative humidity, wind 2.2 m/s) and tropical (359C incidence of exhaustion as the relationship between core dry bulb, 50% relative humidity, wind 2.2 m/s) climate. During tern these trials, the evaporative cooling required to maintain thermal balance exceeded the maximal evaporative capacity of the ducible in a separate set of subjects (7). Additional studenvironment and core temperature continued to rise until ex-ies need to determine whether these relationships are 00 haustion from heat strain occurred. Our findings concerning valid when subjects are wearing protective clothing durexhaustion from heat strain are 1) full encapsulation in protec-ing uncompensable heat stress. The low moisture perme-S tive clothing reduces physiological tolerance as core tempera-ability and high insulation properties of protective clothture at exhaustion was lower (P < 0.05) in fully than in partially ing might result in higher skin temperatures, more wet- ___-clothed persons, 2) partial encapsulation results in physiologi-ted skin, and greater subjective discomfort compared cal tolerance similar to that reported for unclothed persons, 3) with when subjects are unclothed (23); these differences raising metabolic rate from 400 to 600 W does not alter physio-could reduce physiological tolerance to heat strain. logical tolerance when subjects are fully clothed, and 4) physiological tolerance is similar when subjects are wearing protective ur previous study a...

This study determined whether 1) exhaustion from heat strain occurs at the same body temperatures during exercise in the heat when subjects are euhydrated as when they are hypohydrated, 2) aerobic fitness influences the body temperature at which exhaustion from heat strain occurs, and 3) curves could be developed to estimate exhaustion rates at a given level of physiological strain. Seventeen heat-acclimated men [maximal oxygen uptake (VO2max) from 45 to 65 ml.kg-1.min-1] attempted two heat stress tests (HSTs): one when euhydrated and one when hypohydrated by 8% of total body water. The HSTs consisted of 180 min of rest and treadmill walking (45% VO2max) in a hot-dry (ambient temperature 49 degrees C, relative humidity 20%) environment. The required evaporative cooling (Ereq) exceeded the maximal evaporative cooling capacity of the environment (Emax); thus thermal equilibrium could not be achieved and 27 of 34 HSTs ended by exhaustion from heat strain. Our findings concerning exhaustion from heat strain are 1) hypohydration reduced the core temperature that could be tolerated; 2) aerobic fitness, per se, did not influence the magnitude of heat strain that could be tolerated; 3) curves can be developed to estimate exhaustion rates for a given level of physiological strain; and 4) exhaustion was rarely associated with a core temperature up to 38 degrees C, and it always occurred before a temperature of 40 degrees C was achieved. These findings are applicable to heat-acclimated individuals performing moderate-intensity exercise under conditions where Ereq approximates or exceeds Emax and who have high skin temperatures.

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NEUFER, P. DARRELL, MICHAEL N. SAWKA, ANDREW J. for the restoration of preexercise glycogen levels, More-YOUNG, MARK D. QUIGLEY, WILLIAM A. LATZKA, AND LESLIE over, elite endurance athletes, whose capacity for glyco-LEVINE. Hypohydration does not impair skeletal muscle glycogen gen storage may be >150 mmol glucose/kg wet muscle, resynthesis after exercise. J. Appl. Physiol. 70(4): 1490-1494, may store as much as 1 liter of water in skeletal muscle. It 1991.-The purpose of this investigation was to examine the is not known whether the amount of water available is a effects of moderate hypohydration (HY) on skeletal muscle limiting factor for complete muscle glycogen resynthesis glycogen resynthesis after exhaustive exercise. On two occasions, eight males completed 2 h of intermittent cycle ergome-after exhaustive exercise. ter exercise (4 bouts of 17 min at 60% and 3 min at 80% of It is not unusual for industrial workers, military permaximal O2 consumption/10 min rest) to reduce muscle glyco-sonnel, and athletes to incur a 3-8% reduction in body gen concentrations (control values 711 t 41 j*mol/g dry wt). weight (from water loss) during exercise in the heat (7, During one trial, cycle exercise was followed by several hours of 10, 26). This body water deficit reduces an individual's light upper body exercise in the heat without fluid replacement ability to perform aerobic exercise primarily by acting on to induce HY (-5% body wt); in the second trial, sufficient the cardiovascular and thermoregulatory systems (18, water was ingested during the upper body exercise and heat 25, 27, 28). The availability of water for muscle glycogen exposure to maintain euhydration (EU). In both trials, 400 g of resynthesis might be compromised when prior exercise in carbohydrate were ingested at the completion of exercise and the heat results in hypohydration. When hypohydration followed by 15 h of rest while the desired hydration level was maintained. Muscle biopsy samples were obtained from the IS mediated by sweating, the plasma becomes hyperosvastus lateralis immediately after intermittent cycle exercise motic, and this solute excess creates an osmotic gradient (T1) and after 15 h of rest (T2). During the HY trial, the muscle to move fluid from the intracellular to the extracellular water content was lower (P < 0.05) at TI and T2 (288 ± 9 and space to defend plasma volume (27). As a result, sweat-265 ± 5 ml/100 g dry wt, respectively; NS) than during EU (313 mediated hypohydration results in water loss from both ± 8 and 301 ± 4 ml/100 g dry wt, respectively; NS). Muscle the intrqcellular and extracellular fluid spaces. Nose and glycogen concentration was not significantly different during colleag les (21) have shown that thermal-induced hypo-EU and HY at Ti (200 ± 35 vs. 251 ± 50 pmol/g dry wt) or T2 hydration results in water redistribution from extra-and (452 ± 34 vs. 491 ± 35 ,mol/g dry wt). These data indicate that, intracellular spaces of primarily skeletal muscle and skin despite reduced water content during the first 15 h...