Andrew J. Young

After several days of exhaustive exercise in mild-warm environments, cutaneous vasoconstrictor responses to cold exposure are less effective in conserving body heat than in the rested condition. Hypothalamic-pituitary-adrenal and hypothalamic-pituitary-thyroid axes hormones could mediate this response since they may affect vasoconstriction. The effects of exertional fatigue on pituitary-adrenal hormones and thyroid hormone responses to exercise-cold stress are unknown. We hypothesized that 7 consecutive days of exercise would decrease adrenocorticotropin (ACTH) and cortisol, while elevating thyroid stimulating hormone (TSH), triiodothyronine (T3), and thyroxine (T4) and that these hormones would be related to a blunted vasoconstrictor response to cold. Nine male volunteers walked, completely wetted, for up to 6 h in 5 degrees C air, when rested (day 0, control) and after 7 consecutive days (day 7) of exhaustive exercise (4 h each day of mixed aerobic and anaerobic activities in thermoneutral conditions). Blood was sampled on day 0 and day 7 at baseline (0700 hours), and before and immediately after cold exposure. At 0700 hours, ACTH and cortisol were elevated (p < 0.05) after 7 d of exercise, compared with control conditions. Following exercise-cold exposure, cortisol, T3, and T4 increased (p < 0.05) similarly on both day 0 and day 7. ACTH and TSH did not increase as a result of exercise-cold exposure on either day. These data indicate that 7 d of exercise elevates basal (0700 hours) pituitary-adrenal stress hormones (ACTH, cortisol). However, 7 d of exercise did not modify adrenal or thyroid hormone responses, relative to the day 0 cold exposure, suggesting that they are not responsible for the blunted vasoconstriction during exercise-cold exposure following 7 consecutive days of exercise.

Whether or not the thermoregulatory system fatigues (i.e., causes blunted shivering thermogenesis and vasoconstriction) during cold exposure is unknown. Thus, a series of experiments were executed to determine if serial cold water immersion or acute exertional fatigue would cause thermoregulatory fatigue during subsequent cold exposure. This information will be used in developing thermoregulatory models during cold exposure. During these studies several unanswered questions regarding thermoregulation in the cold were also addressed: (1) do thermoregulatory responses to cold show fatigue when prolonged cold water exposures are serially repeated? (2) are thermoregulatory effector responses to cold affected by time of day? (i.e., circadian influences); (3) does a single acute, fatiguing exercise bout impair thermoregulatory responses to subsequent cold exposure? and (4) does fatigue induced by severely strenuous physical activity compromise the immune response to cold exposure? The results of these studies suggest that: (1) serial cold water blunts shivering leading to lower core temperatures, (2) thermoregulatory effector responses are not affected by time of day, (3) exercise increases peripheral heat loss and lowers core temperature during subsequent cold exposure, and (4) that acute cold exposure has immununostimulating effects.

This report considers how thermal balance of soldiers wearing nuclear, biological and chemical (NBC) protective clothing in combination with the Extreme Cold Weather Clothing System (ECWCS) is affected during work in cold weather. A review of published reports concerning physiological consequences of wearing NBC protective clothing during cold exposure was completed. The findings reported in the experimental literature were too limited to adequately forecast the effects of adding NBC clothing to ECWCS. To remedy the information gap, simulation modeling was employed to predict body temperature changes during alternating bouts of exercise and rest throughout 8 h of exposure to three different severely cold conditions. Published findings indicate that NBC protective clothing may inadequately protect against hand and finger cooling, especially during rest following strenuous activity. No evidence substantiates suggestions that wearing NBC protective masks increases susceptibility to facial frostbite. Collectively, the limited experimental work and the results of simulation modeling argue against any increased risk of hypothermia associated with wearing NBC protective clothing while working in the cold. However, wearing NBC protective clothing during strenuous activity in cold weather may increase the risk of hyperthermia, and cause sweat accumulation in clothing which may compromise insulation and increase the risk of hypothermia during subsequent periods of inactivity.

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°C) immediately after (re) was lower before and during cold air exposure for A than for SR and LR. T_re declined during cold exposure in A and SR but not LR. Mean weighted skin temperature (T _sk) 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 T_re + 0.33T _sk) 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.

In January and February, 1998, fifty Marine Recruits (thirty men, twenty women) were studied during the Crucible, a 54 hour training exercise which culminates Marine Corps Recruit Basic Training. The purpose of this study was to measure body core temperature and energy expenditure during the Crucible to: 1) assess the risk of hypothermia and 2) determine if ration restrictions should be eased. Core temperature was measured with a lightweight (3 oz) Body Core Temperature Monitor (BCTM) and an FDA-approved ingestible temperature sensor. Energy expenditure was measured using the doubly labeled water (D2180) technique. Energy intake was estimated by collecting Meal Ready to Eat (MRE) ration wrappers saved by each volunteer during the exercise. Weather data were obtained from portable stations. Results: 1) mean maximum core temperature was 38.51 deg C (range, 38.15-39.14 deg C); 2) mean minimum core temperature was 35.97 deg C (range, 35.28-36.64 deg C); 3) mean energy expenditure (kcal) over 54-h was 13,790 plus or minus 410 (6128 kcal/day) and 10,635 plus or minus 315 (4727 kcal/day) for the men and women, respectively; 4) mean energy intake for 54-h was 3,200 and 2,600 kcals for the men and women, respectively; 5) mean air temperature was 10.8 deg C (range, 3.6-18.8 deg C) in January and 13.8 deg C (range, 6.3-21.4 deg C) in February. In conclusion, mean core temperature did not fall to hypothermic levels (<35.5 deg C) and the energy deficit was approximately 10,100 kcal in the men and 7,700 kcal in the women.

This study investigated whether autologous erythrocyte infusion would ameliorate the decrement in maximal oxygen uptake_(VO(2)MAX) experienced by lowlanders when they ascend to high altitude. Sixteen men's _(VO(2)MAX) was measured (treadmill running) at sea leve. (Sfl), and on the first (HAl) and ninth (HA9) day of high altitude (4300 m) residence. After VO(2)MAX was measured at SL, subjects were divided into two matched groups (N = 8).