Skin temperature feedback optimizes microclimate cooling

INTRODUCTION: A novel pulsed cooling paradigm (PCskin) integrating mean skin temperature (Tsk) feedback was compared with constant cooling (CC) or time-activated pulsed cooling (PC). METHODS: Eight males exercised while wearing personal protective equipment (PPE) in a warm, dry environment (dry bulb temperature: 30 degrees C; dew-point temperature: 11 degrees C) in each of the tests. Treadmill exercise was performed (approximately 225 W x m(-2)) for 80 min. A liquid cooling garment (LCG) covered 72% of the body surface area. Core temperature (Tc), local skin temperatures, heart rate, inlet and outlet LCG perfusate temperatures, flow, and electrical power to the LCG and metabolic rate were measured during exercise. RESULTS: At 75 min of exercise Tsk was higher (33.9 +/- 0.2 degrees C) in PCskin, than in PC (33.1 +/- 0.5 degrees C) or CC (32.0 +/- 0.6 degrees C) and PC > CC. The changes in Tc and heart rate during the tests were not different. Tc at 75 min was not different among the cooling paradigms (37.6 +/- 0.3 degrees C in PCskin, 37.6 +/- 0.2 degrees C in PC and 37.6 +/- 0.2 degrees C in CC). Heart rate averaged 124 +/- 10 bpm in PCskin, 120 +/- 9 bpm in PC and 117 +/- 9 bpm in CC. Total body insulation (degrees C x W(-1) x m(-2)) was significantly reduced in PCskin (0.020 +/- 0.003) and PC (0.024 +/- 0.004) from CC (0.029 +/- 0.004). Electrical power in PCskin was reduced by 46% from CC and by 28% from PC. DISCUSSION/CONCLUSION: Real-time Tsk feedback to control cooling optimized LCG efficacy and reduced electrical power for cooling without significantly changing cardiovascular strain in exercising men wearing PPE.     

Thermoregulation and heat exchange in a nonuniform thermal environment during simulated extended EVA. Extravehicular activities

BACKGROUND: Nonuniform heating and cooling of the body, a possibility during extended duration extravehicular activities (EVA), was studied by means of a specially designed water circulating garment that independently heated or cooled the right and left sides of the body. The purpose was to assess whether there was a generalized reaction on the finger in extreme contradictory temperatures on the body surface, as a potential heat status controller. METHOD: Eight subjects, six men and two women, were studied while wearing a sagittally divided experimental garment with hands exposed in the following conditions: Stage 1 baseline--total body garment inlet water temperature at 33 degrees C; Stage 2--left side inlet water temperature heated to 45 degrees C; right side cooled to 8 degrees C; Stage 3--left side inlet water temperature cooled to 8 degrees C, right side heated to 45 degrees C. RESULTS: Temperatures on each side of the body surface as well as ear canal temperature (Tec) showed statistically significant Stage x Side interactions, demonstrating responsiveness to the thermal manipulations. Right and left finger temperatures (Tfing) were not significantly different across stages; their dynamic across time was similar. Rectal temperature (Tre) was not reactive to prevailing cold on the body surface, and therefore not informative. Subjective perception of heat and cold on the left and right sides of the body was consistent with actual temperature manipulations. CONCLUSIONS: Tec and Tre estimates of internal temperature do not provide accurate data for evaluating overall thermal status in nonuniform thermal conditions on the body surface. The use of Tfing has significant potential in providing more accurate information on thermal status and as a feedback method for more precise thermal regulation of the astronaut within the EVA space suit.     

Thermal balance effects on vigilance during 2-hour exposures to -20 degrees C

INTRODUCTION: Human mental performance is markedly affected by environmental temperature, body temperature, body heat content, and comfort, but the effects of these different factors are not entirely clear. A liquid conditioning garment (LCG) can be used to manipulate these factors independently. We hypothesized that cold exposure (coldEX) has negative effects on vigilance which can be alleviated by increasing body heat content throughout or prior to coldEX. METHODS: Subjects (n = 10) were tested for vigilance during a 130-min coldEX to -20 degrees C while warmly clothed; a period of moderate exercise occurred at minutes 64-74. An LCG was used to provide heating either before coldEX (prior heating, PH) or throughout coldEX (heating, H); the control condition involved no heating (NH). RESULTS: There were significant differences among conditions for rectal temperature (PH: 37.3 +/- 0.26 degrees C, H: 37.0 +/- 0.24 degrees C, NH: 37.05 +/- 0.26 degrees C) and mean skin temperature (PH: 33.85 +/- 1.21 degrees C, H: 35.11 +/- 1.35 degrees C, NH: 32.84 +/- 0.65 degrees C). Reaction time decreased significantly after the 45th minute of coldEX in H and NH. At the same time, signal detection criteria in all conditions demonstrated considerable alterations, indicating bias in favor of 'target-present' responses, which were associated with lower mean skin temperature and thermal comfort vote. PH was more effective than H and NH in preserving reaction time and response consistency. Relative to men, women demonstrated increased heat loss and more deteriorated vigilance and signal detection. CONCLUSION: Vigilance deteriorates in cold conditions within 45 min of exposure. Increasing body heat content prior to coldEX is efficacious in preserving vigilance performance during exposures lasting up to 2 h.     

Human conscious response to thermal input is adjusted to changes in mean body temperature

OBJECTIVE AND DESIGN: To detect the dependable criteria of behavioural thermoregulation through modelling temperature fluctuations of individuals allowed to freely manipulate inlet water temperature of a liquid conditioning garment (LCG) during 130 min of passive exposure to -20 degrees C interspersed with a 10 min period of moderate exercise at the 65th minute using a double-blind experiment. PARTICIPANTS: Eleven volunteers (5 women; 23.40 (SD 2.09) years; BMI: 23.24 (SD 2.19)) who lacked previous experience with LCG and cold exposure experiments. RESULTS: Despite variations in core and skin temperatures, thermal comfort, thermal sensation, and mean body temperature did not fluctuate significantly over time. Participants were able to find a desired level of LCG inlet temperature within 25 minutes which was maintained at similar levels until the 65th minute of the cold exposure. During exercise, LCG inlet water temperature decreased significantly. Regression models demonstrated that mean skin temperature and change in mean body temperature were significantly associated with thermal comfort and thermal sensation. Subsequent models revealed that, although all temperature variables were associated with LCG inlet water temperature, the coefficient of determination mainly depended on mean skin temperature and change in mean body temperature. The involvement of skin temperature was anticipated as the liquid conditioning garment was in contact with the skin. CONCLUSIONS: Humans generate conscious thermoregulatory responses in resting and exercise conditions during exposures to cold environments that are aimed towards maintaining a threshold mean body temperature, rather than temperature changes in individual body regions.     

Efficiency of liquid cooling garments: prediction and manikin measurement

INTRODUCTION: We studied the efficiency of liquid cooling garments (LCG) and their relationship to the insulation of outer clothing, perfusate inlet temperatures, and environmental conditions by both theoretical analysis and thermal manikin (TM) testing. METHODS: An equation to estimate LCG cooling efficiency was developed on the basis of energy balance. Cooling efficiency is a function of the thermal resistance between the TM skin and perfusate in the LCG, the thermal resistance between the environment and the perfusate, and TM skin, ambient, and perfusate temperatures. Three ensembles, a cooling vest (CV) only, CV plus a battle dress uniform (CVB), and CVB plus a battle dress overgarment (CVO), were tested on a sweating TM in dry and wet conditions. The TM surface temperature was maintained at 33 degrees C and the environment was 30 degrees C and 50% RH. The LCG heat removal from the TM was calculated using the power inputs to the TM with and without perfusate flow. RESULTS: The cooling efficiency was increased from approximately 0.45 for CV to approximately 0.70 for CVO in dry experiments and from approximately 0.53 for CV to 0.78 for CVO in wet experiments. CONCLUSION: With additional outer clothing layers, higher thermal resistances increased the rate of heat removal from the TM surface, and decreased heat gain from the ambient environment, thus increasing efficiency. The perfusate inlet temperature had minimal influence on the efficiency. The equations developed can predict cooling efficiency and heat removal rates under a wider range of environmental conditions.     

Responsiveness of thermal sensors to nonuniform thermal environments and exercise

BACKGROUND: We investigated the utility of finger temperature, hand heat flux, and mean skin temperature as indices of overall thermal balance during nonuniform thermal manipulations combined with exercise, with a view to identifying useful feedback sites for input into personal thermal control systems. METHODS: There were 16 subjects who performed 4 x 30 s of 120% VO2peak cycling with a 4-min recovery. During recovery, subjects either received no cooling (CON), upper-body cooling (UC), or upper-body cooling combined with leg heating (UCLH) using a multi-zone liquid conditioning garment. Heat loss during recovery was approximately equal to heat production during exercise. Skin temperature was measured on the mid-medial phalanx of the fourth finger. Heat flux was measured on the dorsum of the hand. RESULTS: Neither hand heat flux or finger temperature distinguished between the two cooling conditions during any of the recovery periods, though hand heat flux was very sensitive to the onset and cessation of exercise. Mean skin temperature was significantly different (p < 0.05) during CON (34.0 +/- 0.1 degrees C), UC (32.5 +/- 0.2 degrees C), and UCLH (33.0 +/- 0.2 degrees C). CONCLUSION: Mean skin temperature may serve as a good indicator of overall heat exchange in the body, even when exposed to nonuniform thermal environments. As hand heat flux was very sensitive to the onset and cessation of exercise, it may be useful as a supplemental thermal feedback to modulate heat exchange in microclimate thermal control systems.     

Model of human thermoregulation for intermittent regional cooling

INTRODUCTION: A prospective approach to save energy expenditure for a liquid cooling garment (LCG) system is to provide intermittent regional cooling (IRC) to the human body instead of continuous cooling. In order to gain insight into IRC mechanisms, a mathematical model was developed to simulate thermal interaction between the human and IRC. METHODS: Human thermoregulatory responses were simulated by a previously validated six-cylinder mathematical model. Two equations were derived from the energy balance principle to estimate LCG heat removal during ON (coolant circulation) and OFF (no coolant circulation) periods. The heat removal equations were incorporated into the boundary equations of the human model. The augmented model was used to predict human thermal responses under different IRC conditions. RESULTS AND CONCLUSIONS: The model was evaluated against experimental results with IRC in warm environments. The comparison demonstrated that the model predictions of the core temperature and mean skin temperature were reliable within root mean square deviations of +/- 0.10 degrees C and 0.44 degrees C, respectively. Simulation analysis showed that IRC has the potential to reduce power requirements. Modeling is an effective alternative to predict efficacy when actual responses cannot be attempted. A systematic approach, consisting of manikin measurements, physiological experiments, and mathematical modeling can expedite the research and development of LCG systems.     

The effect of hand immersion on body temperature when wearing impermeable clothing

The effects of hand immersion on body temperature have been investigated in men wearing impermeable NBC clothing. Six men worked continuously at a rate of approximately 490 J. sec-1 in an environmental temperature of 30 degrees C. Each subject was permitted to rest for a period of 20 minutes when their aural temperature reached 37.5 degrees C, and again on reaching 38 degrees C, and for a third time on reaching 38.5 degrees C (three rest periods in total). Each subject completed three experimental conditions whereby, during the rest periods they either: a. Did not immerse their hands (control). b. Immersed both hands in a water bath set at 25 degrees C. c. Immersed both hands in water at 10 degrees C. Physiological measures of core temperature, skin temperature and heart rate were recorded at intervals throughout the experiment. Measures of mean aural temperature and mean skin temperature were significantly (P less than 0.05) reduced if hands were immersed during these rest periods, compared to non immersion. As a result, the total work time of subjects was extended when in the immersed conditions by some 10-20 minutes within the confines of the protocol. It is concluded that this technique of simple hand immersion may be effective in reducing heat stress where normal routes to heat loss are compromised.     

Thermal and metabolic responses of high and low fat women to cold water immersion.

BACKGROUND: At rest during cold exposure, the amount of body fat plays an important role in the maintenance of core temperature. High fat (HF) individuals would therefore have an advantage as compared with their low fat (LF) counterparts. Since females usually have a higher amount of body fat than males they are expected to maintain core temperature at a lower energy cost. METHODS: The purpose of the present investigation was to dichotomize female subjects by percent fat (LF = 20.5 +/- 2%, n = 6 vs. HF = 30 +/- 3%, n = 6) to elucidate the thermal and metabolic responses during acute exposure to 17 degrees C water for 120 min. The following variables were measured: rectal temperature (Tre; degrees C), mean skin temperature (Tsk; degrees C), oxygen consumption (VO2; ml x kg(-1) x min(-1)), and tissue insulation (I; degrees C x m2 x W(-1)). The experiment-wise error rate was set a priori at p = 0.05. RESULTS: Unexpectedly, only one of the variables demonstrated a main effect for fat (p < 0.05). Tre demonstrated a significant (p < 0.05) group by time interaction. However, Tsk and I demonstrated a main effect for time (p < 0.05). While VO2 demonstrated an increase across time, these changes were non-significant (p > 0.05). It appears that the HF group demonstrated a similar thermal (I and Tsk) and metabolic (VO2) response as compared with the LF counterparts. However, the LF groups maintained a lower Tre as compared with the HF subjects. Perhaps leaner subjects or colder water temperatures would elucidate the value of body fat in females, and demonstrate a differential response with respect to females varying in percent body fat.     

Thermoregulation during exercise in individuals with spinal cord injuries

The increased participation in wheelchair sports in conjunction with environmental challenges posed by the most recent Paralympic venues has stimulated interest into the study of thermoregulation of wheelchair users. This area is particularly pertinent for the spinal cord injured as there is a loss of vasomotor and sudomotor effectors below the level of spinal lesion. Studies within this area have examined a range of environmental conditions, exercise modes and subject populations. During exercise in cool conditions (15-25 degrees C), trained paraplegic individuals (thoracic or lumbar spinal lesions) appear to be at no greater risk of thermal injury than trained able-bodied individuals, although greater heat storage for a given metabolic rate is evident. In warm conditions (25-40 degrees C), trained subjects again demonstrate similar core temperature responses to the able-bodied for a given relative exercise load but elicit increased heat storage within the lower body and reduced whole-body sweat rates, increasing the risk of heat injury. The few studies examining a wide range of lesion levels have noted that, for paraplegic individuals where heat production is matched by available sweating capacity, excessive heat strain may be offset. Studies relating to tetraplegic subjects (cervical spinal lesions) are fewer in number but have consistently shown this population to elicit much faster rates of core and skin temperature increase and thermal imbalance in both cool and warm conditions than paraplegic individuals. These responses are due to the complete absence or severely reduced sweating capacity in tetraplegic subjects. During continuous exercise protocols, the main thermal stressor for tetraplegic subjects appears to be environmental heat gain, whereas during an intermittent-type exercise protocol it appears to be metabolic heat production. Fluid losses during exercise and heat retention during passive recovery from exercise are related to lesion level. Future research is recommended to focus on the specific role of absolute and relative metabolic rates, sweating responses, training status and more sport- and vocation-specific exercise protocols.     

载人航天器预冷温度的热生理学探讨

目的探讨载人舱室适宜的预冷温度 ,以预防或减缓发射、返回段航天器内高温对人体的不利影响。方法 5名健康男性青年按着航天服时不通风和以通风流率 1 0 0L/min(STPD)通风等不同着装条件 ,在舱温 1 5、l0、5℃环境中进行 2 5人次实验。测量直肠温度 (Tr)、平均皮温 (Tsk)和平均体温 (Tb)等热生理指标。结果在舱温 1 5℃航天服通风和不通风状态 ,实验 2h内人体直肠温度降低不显著 (从初始值 37.0± 0 .2℃降为 36.7± 0 .3℃ ) ,平均体温、平均皮温显著降低 (P <0 .0 5) ,受试者有局部的冷紧张 ;而在舱温和通风温度 1 0℃时 ,受试者热生理指标随时间延长不断降低 ,直肠温度从 37.0± 0 .3℃显著下降至 36.3± 0 .3℃ ,Tsk、Tb 显著低于初始值 (P <0 .0 5) ,受试者有全身性冷紧张。结论按人体热舒适状态无显著改变的要求 ,航天器座舱预冷后维持 1 5℃气温对人体较为适宜。     

Responses of children with cerebral palsy to arm-crank exercise in the heat

PURPOSE: In response to passive heating, adults with hemispheric brain infarction demonstrate lower skin temperatures (Tsk) and higher sweating rates (SR) on the affected side. It is unknown whether children with similar conditions demonstrate a similar response and whether this response is advantageous to defending body temperature during exercise in the heat. The purpose of this study was to determine whether children with spastic cerebral palsy (CP) demonstrate less thermal strain than healthy peers during short (10 min each) bouts of arm cranking, a mode of exercise where metabolic rate can be matched between the two groups. METHODS: Eleven young people (8.3-18.3 yr) with spastic CP and 11 individually matched (body size, age, and maturity) healthy controls (CON) performed 3 x 10-min arm-cranking bouts (40 rpm) in 35 degrees C, 50% RH. Body mass, metabolic and heart rate (HR) responses, and body temperatures were periodically measured. Individuals within each CP-CON pair worked at the same intensity (0.55 +/- 0.18 W.kg-1 body mass). Data were analyzed using a repeated measures ANOVA (alpha = 0.05). RESULTS: Subjects with CP showed no difference from CON in metabolic and HR responses, or SR (as inferred from body mass changes corrected for fluid intake and output). There were also no differences between the groups in the rectal temperature change from room temperature (21-23 degrees C). The increase in Tsk from room temperature, however, was slightly (0.6 degrees C) but significantly lower (P < 0.0001; 95% CI = 0.5-0.7 degrees C) in the subjects with CP compared with CON. CONCLUSION: Subjects with CP demonstrate thermal strain responses similar to CON during upper-body exercise at relatively low intensities for short duration in a warm climate.     

Investigation of rifle marksmanship on simulated targets during thermal discomfort

BACKGROUND: Marksmanship has been reported to deteriorate during exposure to heat or cold, yet the effect of the resultant thermal strain in isolation of the inflicting thermal stress is not entirely clear. In most studies, the debilitating effects on marksmanship are confounded by an evolving thermal strain (in terms of body heat storage/debt). HYPOTHESIS: Thermal strain (hot and cold) should degrade marksmanship, more with moving targets than with conventional pop-up targets that are deemed less challenging. METHODS: The 12 rifle-trained military reservists (7 men and 5 women) were tested on their marksmanship skills using a small arms simulator. Subjects conducted three trials in an environmentally controlled chamber, 1 wk apart in a counter-balanced order. Trial conditions were neutral (N; 22 degrees C), hot (H; 35 degrees C), and cold (C; 5 degrees C), and all were preceded with an immersion in water to chest level to establish thermal strain (for H and C) or to maintain thermal neutrality prior to shooting. Subjects fired a C7 rifle at various targets during sixteen 5-min engagements over 2 h. Targets were presented either as moving in an urban setting or as pop-ups in a lane setting. Body temperatures, heart rate, saliva cortisol concentration, subjective ratings, and measures of marksmanship accuracy and precision were recorded and analyzed using a repeated measures design. RESULTS: Mean +/- SD of the rectal temperature during the shooting were 36.9 +/- 0.2 degrees C, 37.7 +/- 0.3 degrees C, and 36.8 +/- 0.6 degrees C for N, H, and C, respectively (H > N, C). Corresponding hand temperatures were 31.5 +/- 1.2 degrees C, 35.2 +/- 0.7 degrees C, and 19.4 +/- 3.0 degrees C (H > N > C). These body temperatures were consistent with the subjective indices of heat illness, cold discomfort, and muscle discomfort. Cortisol concentration and perceived exertion were higher during H. Surprisingly, thermal strain did not degrade marksmanship. Instead, some aspects of precision (horizontal displacement) and accuracy (shooting error) were poorer during N compared with H and C. CONCLUSIONS: Deep body temperatures not higher than approximately 37.9 degrees C and not less than 36.4 degrees C (accompanied by hand temperatures of not less than 19 degrees C) do not negatively affect rifle marksmanship performance, even if significant respective sensations of heat and cold are present.     

Sexual variations in thermoregulation during heat stress

Four male and three female physically fit, but untrained subjects performed a treadmill walking task in neutral (25 degrees C), warm (32 degrees C) and hot (40 degrees C) environments. The treadmill grade for each subject was based on 50% VO2 max as determined in a neutral environment. Environmental exposures were 2 h in duration divided into 40 min of rest, work, and recovery. No distinct sexual differences in rectal and skin temperature responses were observed in the three environments. The male subjects had higher heart rates and greater evaporative weight losses during exercise in all environments. The female subjects experienced less severe increases in metabolic requirements during work in the warm and hot environments than the male subjects. The greater percentage of increase in heart rates relative to changes in the metabolic cost of work in the females suggested a greater cardiovascular component of thermal regulation in the female than in the male subjects. The results of this study suggest that physically fit females are capable of working in the heat about as well as males when work load is relative to individual maximal aerobic capacity.     

Efficacy of wrist/palm warming as an EVA countermeasure to maintain finger comfort in cold conditions

INTRODUCTION: This study explored the effectiveness of local wrist/palm warming as a potential countermeasure for providing finger comfort during extended duration EVA. METHODS: There were six subjects (five males and one female) who were evaluated in a liquid cooling/warming garment (LCWG) wearing modified liquid cooling/warming (LCW) gloves in three different experimental conditions: Condition 1: Stage 1--no LCWG, LCW glove inlet water temperature 33 degrees C; Stage 2--no LCWG, LCW glove inlet water temperature cooled to 8 degrees C; Stage 3--no LCWG, LCW glove inlet water temperature warmed to 45 degrees C; Condition 2: Stage 1--LCWG and LCW glove inlet water temperature 33 degrees C; Stage 2--LCWG inlet temperature cooled to 31 degrees C, LCW gloves, 8 degrees C; Stage 3--LCWG inlet water temperature remains at 31 degrees C, LCW glove inlet water temperature warmed to 45 degrees C; Condition 3: Stage 1--LCWG and LCW gloves 33 degrees C; Stage 2--LCWG inlet water temperature cooled to 28 degrees C, LCW gloves, 8 degrees C; Stage 3--LCWG remains at 28 degrees C, LCW glove water temperature warmed to 45 degrees C. RESULTS: Wrist/palm area warming showed a statistically significant increase in finger temperature (Tfing) in Stage 3 compared with Stage 2. Blood perfusion showed a trend toward a significantly greater value in Stage 3 compared with Stage 2. The LCW gloves were significantly more effective in increasing Stage 3 Tfing in Condition 1 (33 degrees C) compared with Condition 3 (28 degrees C). Across conditions, subjective perception of heat in the hands was significantly greater at Stage 3 than Stage 2; perception of overall body heat showed a trend for higher heat ratings in Stage 3 than Stage 2. CONCLUSIONS: Local wrist/palm warming was effective in increasing blood circulation to the distal upper extremities, suggesting the potential usefulness of this technique for enhancing astronaut comfort during EVA while decreasing power requirements.     

Thermal regulation in the heat during exercise after caffeine and ephedrine ingestion.

BACKGROUND: Ingesting a combination of caffeine and ephedrine (C+E) has been shown to raise metabolic heat production and body temperature. This side effect of C+E ingestion may be positive during a cold stress scenario, however, during heat stress it could prove to be detrimental. Thus, the purpose of this study was to clarify the effect of C+E ingestion on body temperature regulation during moderate exercise in a hot dry environment. METHODS: Ten, healthy, non heat acclimated, males exercised at 50% VO2peak in a 40 degrees C and 30% RH environment until rectal temperature reached 39.3 degrees C; heart rate (HR) remained at 95% of peak value or greater for 3 min, dizziness or nausea precluded further exercise, or 3 h had elapsed. They did this four times at weekly intervals: familiarization (Fam), control (Cont), placebo, and C+E (5 mg . kg(-1) caffeine + 1 mg . kg(-1) ephedrine) trials. The Fam and Cont treatments were done first and sequentially while the placebo and C+E treatments were balanced and double-blind. Tolerance times, mean skin temperature (Tsk), rectal temperature (Tre), Vo2, Vco2, VE, sweat rate (SR), HR, and sensation of thermal comfort were measured. RESULTS: Tolerance times (mean+/-SD in minutes) were similar for the placebo (120.0+/-28.4) and C+E (121.3+/-33.9) trials and both times were significantly longer than Cont (106.6+/-24.0) trial. C+E did not affect Tsk, initial TrC, delta Tre, SR or the sensation of thermal comfort. VO2 and VF, were significantly increased by C+E. HR was elevated by C+E compared with the other trials, but only during the initial 20 min of exercise. CONCLUSION: Although the metabolic rate was slightly increased with C+E treatment, it was sufficiently offset by increased heat loss mechanisms so that internal body temperature was not increased during moderate exercise in a hot, dry environment.     

Toe temperatures during arctic training

The surface toe temperature of 10 subjects was monitored in the field in Arctic Norway (minimum air temperature -27 degrees C). The lowest skin temperature recorded was 1.9 degrees C. The mean estimated time for the toe temperature to cool from 25 degrees C to 5 degrees C was 109 minutes (SD, 10.2) at an ambient temperature of -21 degrees C. One subject experienced a toe temperature below 5 degrees C for 2.9 hours during a 27-hour period. Surprisingly none of the subjects demonstrated clinical signs of cold injury, but this does not mean that this exposure was without risk. Cold sensitization could have occurred.     

Effectiveness of an air-cooled vest using selected air temperature and humidity combinations

We evaluated the effectiveness of an air-cooled vest in reducing thermal strain of subjects exercising in the heat (49 degrees C dry bulb (db), 20 degrees C dew point (dp] in chemical protective clothing. Four male subjects attempted 300-min heat exposures at two metabolic rates (175 and 315 W) with six cooling combinations--control (no vest) and five different db and dp combinations. Air supplied to the vest at 15 scfm ranged from 20-27 degrees C db, 7-18 degrees C dp; theoretical cooling capacities were 498-687 W. Without the vest, endurance times were 118 min (175 W) and 73 min (315 W). Endurance times with the vest were 300 min (175 W) and 242-300 min (315 W). The five cooling combinations were similarly effective in reducing thermal strain and extending endurance time, although there was a trend for the vest to be more effective when supplied with air at the lower dry bulb temperature. At 175 W, subjects maintained a constant body temperature; at 315 W, the vest's ability to extend endurance is limited to about 5 hours.     

Effects of precooling on thermoregulation during subsequent exercise

PURPOSE: The purpose of this study was to examine the effect of a decreased body core temperature before a simulated portion of a triathlon (swim,15 min; bike, 45 min) and examine whether precooling could attenuate thermal strain and increase subjective exercise tolerance in a warm environment (26.6 degrees C/60% relative humidity (rh)). METHODS: Six endurance trained triathletes (28+/-2 yr, 8.2+/-1.7% body fat) completed two randomly assigned trials 1 wk apart. The precooling trial (PC) involved lowering body core temperature (-0.5 degrees C rectal temperature, Tre) in water before swimming. The control trial (CON) was identical except no precooling was performed. Water temperature and environmental conditions were maintained at 25.6 degrees C and 26.6 degrees C/60% rh, respectively, throughout all testing. RESULTS: Mean time to precool was 31+/-8 min and average time to reach baseline Tre during cycling was 9+/-7 min. Oxygen uptake (VO2), HR, skin temperature (Tsk), Tre, RPE, and thermal sensation (TS) were recorded following the swim segment and throughout cycling. No significant differences in mean body (Tb) or Tsk were noted between PC and CON, but a significant difference (P < 0.05) in Tre between treatments was noted through the early phases of cycling. No significant differences were reported in HR, VO2, RPE, TS, or sweat rate (SR) between treatments. Body heat storage (S) was negative following swimming in both PC (-92+/-6 W x m2) and CON (-66+/-9 W x m2). A greater S occurred in PC (109+/-6 W x m2) vs CON (79+/-4 W x m2) during cycling (P < 0.05). CONCLUSIONS: Precooling attenuated the rise in Tre, but this effect was transient. Therefore, precooling is not recommended before a triathlon under similar environmental conditions.     

Prediction of body temperatures during exercise in flying clothing.

A mathematical model has been used to describe experimental results for core and skin temperatures in subjects undergoing a rest/activity cycle in two aircrew clothing assemblies at two environmental temperatures (wet bulb, globe temperature (WBGT) indices of 25.9 and 28.9 degrees C). The model presented compares well with published data for subjects in standard aircrew equipment assemblies. Aircrew flying at a WBGT of 28.9 degrees C in chemical defence clothing may reach an unacceptable level of mean body temperature within 40 min and deep body temperature will rise at 1 degree C.hr-1. To prevent deterioration in flying performance during repeated sorties, an alteration in the work/rest activity pattern or the introduction of effective cabin or personal conditioning systems may be required,