Determination of body heat storage in clothing: calorimetry versus thermometry

Two methods of estimating body heat storage were compared under differing conditions of clothing, training, and acclimation to heat. Six male subjects underwent 8 weeks of physical training [60–80% of maximal aerobic power ( ) for 30–45 min · day–, 3–4 days · week^–1 at < 25 °C dry bulb (db)] followed by 6 consecutive days of heat acclimation (45–55% for 60 min · day^–1 at 40°C db, 30% relative humidity)]. Nine other male subjects underwent corresponding periods of control observation followed by heat acclimation. Before and after each treatment, subjects walked continuously on a treadmill (1.34 m · s^–1, 2% grade) in a climatic chamber (40°C db, 30% relative humidity) for an average of 118 min (range 92–120 min) when wearing normal light combat clothing and for an average of 50 min (range 32–68 min) when wearing protective clothing resistant to nuclear, biological, and chemical agents. The heat storage was determined calorimetrically (by the balance of heat gains and losses) and thermometrically [by the conventional equations, using one or two set(s) of relative weightings for the rectal temperature (T _re) to mean skin temperature _sk of 4:1 and 4:1, 2:1 and 4:1, or 2:1 and 9:1 in thermoneutral and hot environments, respectively]. _sk was calculated from 12-site measurements, weighted according to the regional distribution of body surface area and the first eigenvectors of principal component analysis. There were only minor differences (< 5%) between the heat storage values calculated by given weighting factors forT _re and _sk, whether the individual coefficients were derived from estimates of regional surface area or principal component methodologies. When wearing normal clothing, no significant differences were found between the two estimates of heat storage (calorimetry vs thermometry with an invariant relative weighting of 4:1) in any experimental condition, with one specific exception: when wearing protective clothing, thermometry underestimated the heat storage by 24–31%. This underestimation was attenuated by using two sets of relative weightings of 2: 1 and 4: 1 or 2: 1 and 9: 1. The results suggest that when subjects wearing protective clothing are transferred from thermoneutral to hot environments, the accuracy of thermometric estimates of heat storage can be improved by using two sets of weighting factors forT _re and _sk     

Effects of metabolic rate and ambient vapour pressure on heat strain in protective clothing

Studies have shown that variations in ambient water vapour pressure from 1.7 to 3.7 kPa have little effect on heat tolerance time at a metabolic rate above 450 W while wearing protective clothing. With lighter exercise, where tolerance times exceed 60 min, variations in vapour pressure have a significant impact on evaporative heat loss and, therefore, heat tolerance. The present study has examined whether these findings extend to conditions with more extreme variations in vapour pressure. Twelve males performed light (L, 350 W) and heavy (H, 500 W) exercise at 40°C in a dry (D, 1.1 kPa) and humid (H, 4.8 kPa) environment while wearing a semi-permeable nuclear, biological and chemical protective clothing ensemble (0.29 m²×°C⁻¹·W⁻¹ or 1.88 clo; Woodcock vapour permeability coefficient,i _m=0.33). Partitional calorimetry was used to determine the rate of heat storage ( ) with evaporative heat loss from the skin ( ) calculated from changes in dressed mass or the physical properties of the clothing and the vapour pressure gradient between the skin and the environment. Skin vapour pressure was predicted from measurements of water vapour pressure above the skin surface and in the clothing with humidity sensors coupled with thermistors. Final mean skin temperature ( _sk) was higher for the humid trials and averaged 37.4 (0.3)°C, 38.9 (0.4)°C, 37.6 (0.5)°C and 38.5 (0.4)°C for LD, LH, HD and HH, respectively. Final rectal temperature (T _re ) was higher for D with respective values for LD, LH, HD and HH of 39.0 (0.4)°C, 38.7 (0.4)°C, 38.8 (0.4)°C and 38.5 (0.4)°C. Tolerance time was significantly different among the trials and averaged 120.3 (19.3) min, 54.8 (7.3) min, 63.5 (6.9) min and 36.8 (3.1) min for LD, LH, HD and HH, respectively. was overestimated and, therefore, was underestimated when the changes in dressed mass were used to determine evaporative heat loss. When skin vapour pressure determined from the humidity sensor data was used to calculate , heat storage was significantly different among the trials and averaged 15.0 (3.0), 13.0 (1.8), 14.2 (2.6) and 12.2 (1.9) kJ·kg⁻¹ for LD, LH, HD and HH, respectively. It was concluded that while wearing the protective clothing all indices of heat strain, including tolerance time, were significantly affected by the change in ambient water vapour pressure from 1.1 to 4.8 kPa during both light and heavy exercise.     

Solar heat load: heat balance during exercise in clothed subjects

Summary Six subjects exercised for 60 min on a cycle ergometer. Their backs were exposed to an artificial sun with a spectral distribution similar to sunlight and an intensity of 724 W m^–2. Each subject took part in four experiments in random order: wearing suits of polyester (insulation value = 0.5 clo), white (WP) or black (BP), or cotton (0.6 clo), white (WC) or black (BC). Measured by partitional calorimetry, the calculated heat losses and gains for the four conditions balanced within less than 10%. The differences between the short-wave radiation gains of subjects in white or black garments were small. This is due to the transparency of the white materials, which allows a larger percentage of the radiation to penetrate the clothing. The surface temperatures of the sun-exposed areas were very high, especially in the black suits. This promotes dry heat loss. Therefore the sweat loss in the black suits and the differences between the black and white clothes became relatively small. The physiological strain in steady-state exercise, as expressed by average heart rates, was 142 (WP), 154 (BP), 151 (WC), and 160 (BC) beats min^–1; the sweat losses were 649 (WP), 666 (BP), 704 (WC), and 808 (BC) g. For both of these measures values for white polyester were significantly less than those for black cotton.     

Body temperature and heart rate relationships during submaximal bicycle ergometer exercises

Summary Twenty-three male subjects performed submaximal exercise at approximately 80% max on a bicycle ergometer. Rectal temperature, skin temperatures, and heart rate measurements were taken during the exercise and during the corresponding recovery periods. The lag in body temperature (rectal temperature and mean body temperature) responses at the onset of exercise or recovery from exercise was shown in comparison with heart rate. Certain differences existed in the relationship between body temperature and heart rate during the exercise and the recovery period. The correlation coefficient of body temperature with heart rate was high from the 24th min of exercise until the 21st min of the recovery period. The regression equation of post-work rectal temperature on heart rate at moderate work loads (approximately 80% max) in the present study was similar to the equation at light work loads (25∼35% max) for data reported from different laboratories. However, at the termination of exercise the regression equation of rectal temperature on heart rate at light work loads from different laboratories does not agree with the equation of rectal temperature on heart rate at moderate exercise, in the present study. There is good agreement with the equation from the maximum state at a moderate exercise (times of maximum heart rate against maximum body temperature). The correlation coefficients for rectal temperature on heart rate at the maximum state were high. The relationship between body temperature and heart rate at the termination of light exercise is similar to that between these two parameters at the maximum state for moderate exercise.     

How should body heat storage be determined in humans: by thermometry or calorimetry?

Summary The aim of this study was to determine whether in humans there are differences in the heat storage calculated by partitional calorimetry (S, the balance of heat gains and heat losses) compared to the heat storage obtained by conventional methods (thermometry) via either core temperature or mean body temperatures ( , whereT _c is core temperature and is mean skin temperature) when two different sites are used as an index ofT _c [rectal (T _re) and auditory canal (T _ac) temperatures]. Since women respond to the heat differently than men, both sexes were studied. After a stabilisation period at thermal neutrality, six men and seven women were exposed to a globe temperature of 50°C, relative humidity of 17% and wind speed of 0.8–1.0 m·s^–1 for 90 min semi-nude at rest, whereT _re,T _ac, , metabolic rate, dry (radiant+convective heat exchange) and evaporative heat losses,S, heat storage byT _c ( ) and heat storage by were assessed every minute. In the men,S was equal to 350.8(SEM 49.6) kJ whereas amounted to only 114.6(SEM 16.2) and 196.7(SEM 32.3) kJ forT _re andT _ac, respectively (P<0.05). Final underestimatedS by 49% [177.7(SEM 23.0) kJ;P<0.05] whereas was not significantly different than S [255.7(SEM 37.9) kJ]. In the women,S corresponded to a total of 294.3(SEM 23.2) kJ, a value that was very similar to the 262.6(SEM 31.0) kJ], whereas underpredicted by 35% [190.4(SEM 26.3) kJ;P<0.05]. As in the men,S _{T c} was much lower thanS [116.6(SEM 19.9) and 190.3(SEM 24.2) kJ forT _re andT _ac, respectively;P<0.05]. Using seven other well-known weighting coefficients, could under- and overestimateS by up to 55% and 11%, respectively. In all subjects, a large portion of the variance (68% and 75%) in the difference betweenS and , could be explained primarily by the T _ac. The results demonstrated that although some estimates of thermometric heat storage matched the calorimetricS, other predictions underestimated it by up to 67% during passive heating. It is suggested that these differences can be explained in part by he site chosen to representT _c, the use of eitherT _c or in the heat storage calculation, and the thermoneutral/hot weighting coefficient(s) chosen to determine . Until more representative measurements of body temperatures at different depths (core, shell and intermediate) are possible, the use of and -derived heat storage is difficult to justify.     

Metabolic and thermal responses of men wearing cold-protective clothing to various degrees of cold stress

Summary Five subjects were exposed in a climatic chamber for 1 h to air temperatures of 0, –10, and –15 C wearing cold-protective clothing. Heat production, and mean skin and rectal temperatures were studied.During the first 5 min of cold exposure, heat production attained high values and then decreased. The peak levels of this initial metabolic rise were higher in lower air temperatures, while the corresponding mean skin temperatures showed no significant differences in any air temperature. The relationship between changes of heat production and mean skin temperature values during the first 5 min differs from those later in the experiments, which showed a good linear relationship (r=0.89).As the experimental condition required that the body was covered with thick clothes, immediate stimulation to part of the face and the respiratory tract must be greater than stimulation of whole body skin. It is suggested that the metabolic rise during the first 5 min is related to abrupt cold stimulation to the face and mucous membrane of the respiratory tract, and to the subsequent appearence of thermal muscular tone or tension. In contrast, mean skin temperature became lower during the later period even with the cold-protective clothing, and heat production increased again at the onset of frank shivering. It is suggested that the heat production changes occurring during the later period showed that the stimulus to the shivering center from cold receptors in the skin is powerful enough to produce an increase in metabolism.     

Heat storage and body temperature during cooling and rewarming

Summary During calorimetric experiments with forced cooling and rewarming, changes in rectal temperature (T _re) and mean skin temperature ( _sk) allowed calculations of Burton's (1935) weighting coefficient a, which relates body temperature change to change in mean body temperature ( _b). Calculating _b from change in body heat content (H _b), which was determined from direct and indirect calorimetry, included individualized values for body specific heat based on body fat content. In five different cooling procedures there were two with cooling by exposure to cold water and three with cooling in a tubing suit; two of the procedures included mild exercise. The H _b ranged from –335 to –1600 kJ; rewarming restored body heat content. The mean (SEM) value of a in 119 determinations was 0.75 (0.01). This small variability in the coefficient probably came from the large values of H _b and from the use of maximal changes in _sk andT _re, including afterdrop. Change inT _re by itself correlated with _b, but with much variability. In forced body cooling and rewarming, 0.75(T _re) + 0.25 ( _sk) gives an accurate estimate of _b, hence change in body heat storage.     

Melatonin has no effect on tolerance to uncompensable heat stress in man

This study examined whether a 5 mg dose of melatonin induced a lower rectal temperature (T _re) response at rest in both a cool and hot environment while wearing normal military combat clothing, and then examined the influence of this response on tolerance to exercise in the heat while wearing protective clothing. Nine men performed four randomly ordered trials involving 2 h of rest at ambient temperatures of either 23 °C or 40 °C followed by exercise at an ambient temperature of 40 °C. The double-blind ingestion of placebo or melatonin occurred after 30 min of rest. The mean T _re during rest at 23 °C had decreased significantly from 36.8 (SD 0.1) °C to 36.7 (SD 0.2) °C at 90 min following the ingestion of the drug, whereas values during the placebo trial did not change. The lower T _re response during the melatonin trial remained during the first 50 min of exercise in the heat while wearing the protective clothing. Since the final mean T _re at the end of exercise also was significantly reduced for the melatonin [39.0 (SD 0.4) °C] compared with the placebo [mean 39.1 (SD 0.3) °C] trial, tolerance times approximated 95 min in both conditions. During rest at 40 °C, melatonin did not affect the mean T _re response which increased significantly during the last 90 min from 36.9 (SD 0.1) °C to 37.3 (SD 0.1) °C. This increase in T _re during the rest period prior to donning the protective clothing decreased tolerance time approximately 30 min compared with the trials that had involved rest at 23 °C. Total heat storage summated over the rest and exercise periods was not different among the trials at 15 kJ · kg⁻¹. It was concluded that the small decrease in T _re following the ingestion of 5 mg of melatonin at rest in a cool environment had no influence on subsequent tolerance during uncompensable heat stress. Accepted: 26 June 2000     

Thermoregulatory responses to upper body exercise

Summary The purpose of this study was to compare thermoregulatory responses between upper body and lower body exercise. Nine male subjects performed 60 min of arm crank (AC) and cycle (CY) exercise at the same absolute intensity (oxygen uptake=1.61·min^–1) and at the same relative intensity (60% of ergometer specific peak oxygen uptake) in a temperate (24 C, 20% rh) environment. During the absolute intensity experiments, rectal temperature and sweating rate responses were essentially the same for both modes of exercise. In addition, no differences were found for chest, back, arm, or thigh skin temperatures, but calf skin temperature was significantly (P<0.05) lower during arm crank than cycle exercise. During the relative intensity experiments, thermoregulatory responses were lower during arm crank than cycle exercise. In addition, we found no difference between esophageal and rectal temperature values elicited by arm crank exercise. These results indicate that the examined thermoregulatory responses are independent of the skeletal muscle mass employed and dependent upon the absolute metabolic intensity.     

Different thermal conditions of the extremities affect thermoregulation in clothed man

The effects of different types of clothing on human deep body temperature were studied with six healthy male subjects in a supine posture. Two clothing ensembles were employed for the present study: A covered the whole body area with garments except the face (1.97 clo) and B covered only the trunk and the upper half of the extremities with garments (1.53 clo). The experiment was carried out in a climatic chamber at 55% ± 5% relative humidity under cooling and warming temperatures: the temperature was changed from 22°C to 10°C (cooling) and returned to 22°C again (warming). The major findings were: rectal temperature (T _re) continued to decrease gradually in A throughout the experiment, whereas in B it increased during cooling, and returned to previous levels during warming. As a result, Tre and chest skin temperature were maintained at a higher level in B than in A. Internal tissue conductances were greater in A than in B both during cooling and during warming. Thermal comfort appeared to have been influenced more by the rate of skin temperature change than by the level of skin temperature per se. It was concluded that peripheral vasoconstriction in B induced less heat flow from core to shell, and, thus, the core temperature was maintained at a higher level in B than in A.     

Changes in rectal and mean skin temperature in response to suggested heat during hypnosis in man

Rectal temperature, mean skin temperature and heart rate were recorded in 7 subjects during hypnosis, induced either alone or while sensations of heat were suggested. During hypnosis alone, a fall in the heart rate of about 10 beat·min^?1 was the only autonomic response observed; body temperatures were unaltered. In contrast, during hypnosis with suggestion of heat, the following changes occurred: (1) Mean rectal temperature decreased 0.20°C (p<0.05) within 50 min. Its mean time course differed significantly from that for hypnosis alone (p<0.001). (2) Comparison of individual rectal temperature time sequences showed that in fact this temperature only declined in 4 subjects out of 7, and tended to form a plateau located 0.35°C below the value of the preceding waking state. Despite reinforcement of heat suggestion, the plateau continued until the end of the hypnotic trance. (3) Mean skin temperature tended to rise. (4) When hypnosis with suggestion ceased, both rectal and skin temperatures very slowly returned to their levels during the preceding waking state.     

Hand immersion in cold water alleviating physiological strain and increasing tolerance to uncompensable heat stress

The current study examines the use of hand immersion in cold water to alleviate physiological strain caused by exercising in a hot climate while wearing NBC protective garments. Seventeen heat acclimated subjects wearing a semi-permeable NBC protective garment and a light bulletproof vest were exposed to a 125 min exercise-heat stress (35 degrees C, 50% RH; 5 km/h, 5% incline). The heat stress exposure routine included 5 min rest in the chamber followed by two 50:10 min work-rest cycles. During the control trial (CO), there was no intervention, whilst in the intervention condition the subjects immersed their hands and forearms in a 10 degrees C water bath (HI). The results demonstrated that hand immersion in cold water significantly reduced physiological strain. In the CO exposure during the first and second resting periods, the average rectal temperature (T (re)) practically did not decrease. With hand immersion, the mean (SD) T (re) decreased by 0.45 (0.05 degrees C) and 0.48 degrees C (0.06 degrees C) during the first and second rest periods respectively (P < 0.005). Significant decreases in skin temperature, sweat rate, heart rate, and heat storage was also noted in the HI vs. the CO trials. Tolerance time in the HI exposure were longer than in the CO exposure (only 12 subjects in the CO trial endured the entire heat exposure session, as opposed to all 17 subjects in the HI group). It is concluded that hand immersion in cold water for 10 min is an effective method for decreasing the physiological strain caused by exercising under heat stress while wearing NBC protective garments. The method is convenient, simple, and allows longer working periods in hot or contaminated areas with shorter resting periods.     

Thermoregulation during heat exposure of young children compared to their mothers

The study was conducted to investigate the thermoregulation of young children compared to that of adults. A group of 19 children (ages 9 months-4.5 years), with only 3 children aged 3 years or above, and 16 adults first rested in a thermoneutral room (air temperature 25°C relative humidity 50%, air velocity 0.2 m·s^–1). They were then exposed to a hot room (air temperature 35°C, relative humidity 70%, air velocity 0.3 m·s^–) next door for 30 min, and then returned to the thermoneutral room where they stayed for a further 30 min. The rectal temperature (T _re), skin temperatures (T _sk) at seven sites, heart rate (HR), total sweat rate ( ), local sweat rate ( ) and the Na⁺ concentration of the sweat were measured. There was no significant difference inT _re between the children and their mothers in the rest phase. However, theT _re of the children increased as soon as they entered the hot room and was significantly higher than during the control period, and than that of the mothers during heat exposure. MeanT _sk, forehead, abdomen and instepT _sk were significantly higher in the children during both the thermoneutral and heat exposure. The was significantly higher and Na⁺ concentrations in the sweat on the back and upperarm were significantly lower for the children during the heat exposure. They had a greater body surface area-to-mass ratio than the mothers by 64%, which indicated that they had advantages for thermal regulation. However, the sweating andT _sk responses of the children were not enough to prevent a rise in body temperature. These results would suggest that the young children had the disadvantage of heating up easily due to their smaller body sizes and there may be maturation-related differences in thermoregulation during the heat exposure between young children and mothers.     

The effect of the rate of heat storage on serum heat shock protein 72 in humans

Hsp72 concentration has been shown to be higher in the serum (eHsp72) of runners with symptoms of heat illness than in non-ill runners. Recently, it has been suggested that the rate of heat storage during exercise in the heat may be an important factor in the development of heat stroke. Therefore, we compared the effect of two rates of heat storage on eHsp72 concentration during exercise in which subjects reached the same final core temperature. We hypothesized that with a lower rate of heat storage the increase in eHsp72 would be attenuated compared to a higher rate of heat storage. Nine heat acclimated subjects performed two exercise trials in a counterbalanced order in the heat (42°C, 30% relative humidity). The trials consisted of walking on a treadmill (~50% VO ₂ peak) dressed in military summer fatigues until rectal temperature reached 38.5°C. A high rate of heat storage (HS, 1.04 ± 0.10 W m⁻² min⁻¹, mean ± SE) occurred when subjects walked without cooling. To produce a lower rate of heat storage (LS, 0.54 ± 0.09 W m⁻² min⁻¹) subjects walked while wearing a water-perfused cooling vest underneath clothing. eHsp72 increased pre- to post-exercise (P < 0.05) but there was no difference (P > 0.05) in eHSP between the two rates of heat storage (LS 1.25 ± 0.73 to 2.23 ± 0.70 ng ml⁻¹, HS 1.04 ± 0.57 to 2.02 ± 0.60 ng ml⁻¹). This result suggests that eHsp72 is a function of the core temperature attained rather than the rate of heat storage.     

Convective heat transfer area of the human body

In order to clarify the heat transfer area involved in convective heat exchange for the human body, the total body surface area of six healthy subjects was measured, and the non-convective heat transfer area and floor and chair contact areas for the following nine common body positions were measured: standing, sitting on a chair, sitting in the seiza position, sitting cross-legged, sitting sideways, sitting with both knees erect, sitting with a leg out, and the lateral and supine positions. The main non-convective heat transfer areas were: the armpits (contact between the upper arm and trunk regions), contact between the two legs, contacts between the fingers and toes, and contact between the hands and the body surface. Also, when sitting on the floor with some degree of leg contact (sitting in the seiza position, cross-legged, or sideways), there was a large non-convective heat transfer area on the thighs and legs. Even when standing or sitting in a chair, about 6–8% of the body surface did not transfer heat by convection. The results showed that the effective thermal convective area factor for the naked whole body in the standing position was 0.942. While sitting in a chair this factor was 0.860, while sitting in a chair but excluding the chair contact area it was 0.918, when sitting in the seiza position 0.818, when sitting cross-legged 0.843, in the sideways sitting position 0.855, when sitting with both knees erect 0.887, in the leg-out sitting position 0.906, while in the lateral position it was 0.877 and the supine position 0.844. For all body positions, the effective thermal convective area factor was greater than the effective thermal radiation area factor, but smaller than the total body surface area.     

Skin and core temperatures as determinants of heat production and heat loss in the goat

In 82 experiments on 10 goats body core temperature (T _core) was altered between 35° and 42°C by external heat exchangers acting on blood temperature while skin temperature (T_skin) was maintained constant, by a circulating shower bath, at different levels between 32° and 44°C. At all skin temperatures at least fourfold increases of heat production (M) and respiratory evaporative heat loss (REHL) occurred whenT _core was lowered or raised, respectively. The lower T_skin was, the higher were the thresholds ofT _core, at which M or REHL exceeded resting levels. The lower T_skin was, the higher were the slopes, at which M or REHL changed per unit ofT _core. At a given T_skin, the slopes decreased with increasing M or REHL, and were dependent on the range ofT _core. The higher the range ofT _core, the steeper changed M and REHL with changingT _core, if all other variables were held constant. The results support the concept that an exponential relationship betweenT _core and the rate of core temperature signals is the primary cause of the effects exerted by T_skin on the slopes, at which M or REHL change per unit ofT _core.     

The relative influence of body characteristics on humid heat stress response

The present study was designed to determine the relative importance of individual characteristics such as maximal oxygen uptake ( O_2max), adiposity, DuBois body surface area (A _D), surface to mass ratio (A _D: mass) and body mass, for the individual's reaction to humid heat stress. For this purpose 27 subjects (19 men, 8 women), with heterogeneous characteristics ( O_2max 1.86–5.28 1 · min^–1; fat% 8.0%–31.9%; mass 49.8–102.1 kg; A _D 1.52–2.33 m²) first rested (30 min) and then exercised (60 W for 1 h) on a cycle ergometer in a warm humid climate (35°C, 80% relative humidity). Their physiological responses at the end of exercise were analysed to assess their relationship with individual characteristics using a stepwise multiple regression technique. Dependent variables (with ranges) included final values of rectal temperature (T _re 37.5–39.0°C), mean skin temperature (T _sk 35.7–37.5°C), body heat storage (S 3.2–8.1 J · g^–1), heart rate (HR 100–172 beat · min^–1), sweat loss (397–1403g), mean arterial blood pressure (BP_a, 68–96 mmHg), forearm blood flow (FBF, 10.1–33.9 ml · 100ml^–1 · min^–1) and forearm vascular conductance (FVC = FBF/BP_a, 0.11–0.49 ml · 100 ml^–1 · min^–1 · mmHg^–1). The T _re, T _sk and S were (34%–65%) determined in the: main by ( O_2max), or by exercise intensity expressed as a percent age of O_2max (% O_2max). For T _re, A _D: mass ratio also contributed to the variance explained, with about half the effect of ( O_2max), For T _sk, fat% contributed to the variance explained with about two-third the effect of O_2max. Total body sweat loss was highly dependent (50%) on body size (A _D or mass) with regular activity level having a quarter of the effect of body size on sweat loss. The HR, similar to T _re, was determined by O_2max (48%–51%), with less than half the effect of A _D or A _D :mass (20%). Other circulatory parameters (FBF, BP_a, FVC) showed little relationship with individual characteristics ( < 36% of variance explained). In general, the higher the ( O_2max), and/or the bigger the subject, the lower the heat strain observed. The widely accepted concept, that body core temperature is determined by exercise intensity expressed as % O_2max and sweat loss by absolute heat load, was only partially supported by the results. For both variables, other individual characteristics were also shown to contribute.     

不同环境温度对精氨酸加压素引起的大鼠低温的影响及其与尾部散热变化的关系

 目的：探讨不同环境温度对精氨酸加压素（AVP）引起的大鼠低温及其与尾部散热变化的关系,以确定是否外周给予AVP能提高大鼠尾部散热反应。方法：实验用成年雄性SD大鼠,在3种不同环境温度（12 ℃、22 ℃和32 ℃）下,用无线遥测技术连续记录体核温度和尾部皮肤温度。上午10:00给大鼠腹腔注射AVP(10 μg/kg)或V1a受体阻断剂(30 μg/kg)。同时观察AVP或V1a受体阻断剂对大鼠背斜方肌微血管直径和理毛行为的影响。结果：(1)在3种不同环境温度中,AVP引起大鼠低温均伴有尾部皮肤温度降低反应。(2) V1a受体阻断剂能够阻断AVP引起低温和尾部皮肤温度降低效应。(3)AVP能明显引起背斜方肌微血管收缩反应。(4)AVP能提高大鼠的理毛行为(唾液理毛),而且这种作用也能被外周给予V1a受体阻断剂所阻断。(5)内源性AVP不参与正常大鼠尾部散热过程。结论：外周给予AVP引起大鼠低温,不是由于其降低了体温调定点,可能是由于其抑制了体温调节性产热和提高唾液理毛活动以增加体表蒸发散热所致。     

The effects of pre-warming on the metabolic and thermoregulatory responses to prolonged submaximal exercise in moderate ambient temperatures

To determine the effects of pre-warming on the human metabolic and thermoregulatory responses to prolonged steady-rate exercise in moderate ambient temperatures and relative humidities [means (SD) 21.7 (2.1)° C and 36.7 (5.4)%, respectively], six healthy men each ran at a steady-rate (70% maximal oxygen uptake) on a treadmill until exhausted after being actively pre-warmed (AH), passively pre-warmed (PH), and rested (Cont). Exercise time to exhaustion was significantly reduced following both AH and PH compared to Cont [AH 47.8 (14.0) min, PH 39.6 (16.0) min, Cont 62.0 (8.8) min; P<0.05]. During exercise there were no significant differences in oxygen uptake, total sweat loss, mean skin temperature (T_sk) and the thermal gradient (T _re–T_sk, where T _re is rectal temperature) following the three conditions. Serum prolactin, plasma catecholamine and plasma free fatty acid concentrations were also similar between all three trials. In contrast, T _re, mean body temperature, heart rate and ratings of perceived exertion were significantly greater during the initial 25 min of exercise following both AH and PH, compared with Cont (P<0.05). At exhaustion, there were no significant differences in the metabolic and thermoregulatory responses to exercise between the trials. The current findings demonstrate that AH and PH promote a reduction in prolonged submaximal endurance performance under moderate environmental temperatures compared with pre-exercise rest. Such observations appear likely to have been mediated through mechanisms associated with the earlier development of high internal body temperature which resulted in changes in the capacity for heat storage. Electronic Publication     

Influence of skin temperature on heat pain threshold in humans

We compared the effect of skin temperature on the critical threshold temperature eliciting heat pain with the effect of skin temperature on the response latency to the first heat pain sensation in healthy human subjects. Also, we determined the effect of the duration of a heat stimulus ramp on pain threshold. Furthermore, we determined the effect of skin temperature on mechanically induced pain. We found that the latency to the first pain sensation induced by a radiant heat stimulus was significantly decreased with an increase in the skin temperature (25–35 °C). However, independent of the rate of the stimulus rise (3–10 °C/s) and independent of the stimulus location (hairy vs glabrous skin), the threshold temperature for eliciting the heat pain sensation, determined with a contact thermostimulator, was not changed by a change in the skin temperature in the same subjects. With a fast rate of stimulus rise, a higher pain threshold was obtained than with a slow rise of stimulus temperature. However, this difference was found only with subject-controlled ascending stimuli (method of limits) but not with experimenter-controlled, predetermined stimulus ramps (method of levels). The heat pain threshold was higher in the glabrous skin of the hand than in the hairy skin of the forearm. With increasing stimulus duration (2.5–10s), the threshold temperature eliciting the heat pain sensation was significantly decreased. The mechanically induced pain threshold was not influenced by the skin temperature. The results indicate that the critical temperature for eliciting heat pain is independent of the skin temperature in humans. However, a change in skin temperature is an important source of an artefactual change in heat pain sensitivity when the radiant heat method (latency or energy) is used as an index of pain sensitivity. With a method dependent on reaction time (the method of limits), the heat pain threshold was artefactually increased, with fast rates of stimulus rise due to the long delay of slowly conducting heat pain signals in reaching the brain. With an increase in the duration of the heat stimulus, the critical temperature for eliciting pain sensation was significantly decreased, which may be explained by central neuronal mechanisms (temporal summation).