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...

The influence of heat acclimation on skeletal muscle metabolism during submaximal exercise was studied in 13 healthy men. The subjects performed 30 min of cycle exercise (70% of individual maximal O2 uptake) in a cool [21 degrees C, 30% relative humidity (rh)] and a hot (49 degrees C, 20% rh) environment before and again after they were heat acclimated. Aerobic metabolic rate was lower (0.1 l X min-1; P less than 0.01) during exercise in the heat compared with the cool both before and after heat acclimation. Muscle and plasma lactate accumulation with exercise was greater (P less than 0.01) in the hot relative to the cool environment both before and after acclimation. Acclimation lowered (P less than 0.01) aerobic metabolic rate as well as muscle and plasma lactate accumulation in both environments. The amount of muscle glycogen utilized during exercise in the hot environment did not differ from that in the cool either before or after acclimation. These findings indicate that accumulation of muscle lactate is increased and aerobic metabolic rate is decreased during exercise in the heat before and after heat acclimation; increased muscle glycogen utilization does not account for the increased muscle lactate accumulation during exercise under extreme heat stress; and heat acclimation lowers the aerobic metabolic rate and muscle and blood lactate accumulation during exercise in a cool as well as a hot environment.

No abstract

The purpose of this study was to determine how chronic exertional fatigue and sleep deprivation coupled with negative energy balance affect thermoregulation during cold exposure. Eight men wearing only shorts and socks sat quietly during 4-h cold air exposure (10 degreesC) immediately after (<2 h, A) they completed 61 days of strenuous military training (energy expenditure approximately 4,150 kcal/day, energy intake approximately 3,300 kcal/day, sleep approximately 4 h/day) and again after short (48 h, SR) and long (109 days, LR) recovery. Body weight decreased 7.4 kg from before training to A, then increased 6.4 kg by SR, with an additional 6.4 kg increase by LR. Body fat averaged 12% during A and SR and increased to 21% during LR. Rectal temperature (Tre) was lower before and during cold air exposure for A than for SR and LR. Tre declined during cold exposure in A and SR but not LR. Mean weighted skin temperature (Tsk) during cold exposure was higher in A and SR than in LR. Metabolic rate increased during all cold exposures, but it was lower during A and LR than SR. The mean body temperature (0.67 Tre + 0.33 Tsk) threshold for increasing metabolism was lower during A than SR and LR. Thus chronic exertional fatigue and sleep loss, combined with underfeeding, reduced tissue insulation and blunted metabolic heat production, which compromised maintenance of body temperature. A short period of rest, sleep, and refeeding restored the thermogenic response to cold, but thermal balance in the cold remained compromised until after several weeks of recovery when tissue insulation had been restored.