Residual analysis in the determination of factors affecting the estimates of body heat storage in clothed subjects

Body heat storage can be estimated by calorimetry (from heat gains and losses) or by thermometry [from changes (Δ) in mean body temperature (T _b) calculated as a weighted combination of rectal (T _re) and mean skin temperatures (T _sk)]. If an invariant weighting factor ofT _re andT _sk were to be used (for instance, ΔT _b = 0.8 · ΔT _re + 0.2 · ΔT _sk under hot conditions), body heat storage could be over- or underestimated substantially relative to calorimetry, depending on whether the subject was wearing light or protective clothing. This study investigated whether discrepancies between calorimetry and thermometry arise from methodological errors in the calorimetric estimate of heat storage, from inappropriate weightings in the thermometric estimate, or from both. Residuals of calorimetry versus thermometric estimates were plotted against individual variables in the standard heat balance equation, applying various weighting factors toT _re andT _sk. Whether light or protective clothing was worn, the calorimetric approach generally gave appropriate estimates of heat exchange components and thus heat storage. One exception was in estimating latent heat loss from sweat evaporation. If sweat evaporation exceeded 650 g·h⁻¹ when wearing normal clothing, evaporative heat loss was overestimated and thus body heat storage was underestimated. Nevertheless, if data beyond this ceiling were excluded from the analyses, the standard 4:1 weighting matched calorimetric heat storage estimates quite well. When wearing protective clothing, the same 4:1 weighting approximated calorimetric heat storage with errors of less than approximately 10%, but only if environmental conditions allowed a subject to exercise for more than 90 min. The best thermometric estimates of heat storage were provided by using two sets of relative weightings, based upon the individual's metabolic heat production ( in kilojoules per metre squared per hour): {4 − [( )· ] 2}:1 for an initial, thermoneutral environment and {4 + [( ) · ] · 5}: 1 for a final, hot environment; the optimal value of lay between 450 and 500 kJ m⁻² · h⁻¹. We concluded that the accuracy of thermometric estimates of heat storage can be improved by modifying weighting factors ofT _re andT _sk according to the environment, type of clothing, and metabolic rate.     

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.     

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.     

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.     

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.     

The effects of two types of clothing on seasonal heat tolerance

The aim of this study was to look at changes in seasonal heat tolerance due to acclimatization produced by different types of clothing. A group of 12 female adults served as subjects in the study which lasted for 3 months from April to June during which the ambient temperature gradually rose. Of the group 6 of them (skirt group) wore knee-length skirts daily, and the others (trouser group) were dressed in full trousers during this acclimatization period. The heat tolerance before and after the acclimatization period was compared between the two groups under conditions in which relative humidity was 30% and ambient temperature was raised to 37°C. Rectal temperature, mean skin temperature and the loss of body mass caused by sweating were measured in the two groups. Before the acclimatization period, no significant differences were found between the two groups. However, observations after the acclimatization period showed higher rectal temperatures in control conditions (ambient air temperature 28°C, relative humidity 60%) in the skirt group. A lower increment of rectal temperature during heat exposure (ambient air temperature 37°C, relative humidity 30%) was also found in this group. Finally, the subjects in the skirt group lost less body mass due to sweating during heat exposure. Consequently, the overall index of physiological strain in the skirt group tended to show a lower value after the period of warm acclimatization. It was concluded that the subjects wearing knee-length skirts improved their heat tolerance with the advance of the seasons.     

An improved estimation of mean body temperature using combined direct calorimetry and thermometry

The conventional method used to estimate the change in mean body temperature (dMBT) is by taking X% of a body core temperature and (1−X)% of weighted mean skin temperature, the value of X being dependent upon ambient temperature. This technique is used widely, despite opposition from calorimetrists. In the present paper we attempt to provide a better method. Minute-by-minute changes in dMBT, as assessed using calorimetry, and 21 (20 if esophageal temperature was unavailable) various regional temperatures (dRBTs), as assessed using thermometry, including 6 subcutaneous measures, were collected from 7 young male adults at 6 calorimeter temperatures. Since a calorimeter measures only changes in heat storage, which can be converted to dMBT, all body temperatures are expressed as changes from the reasonably constant pre-exposure temperatures. The following three aspects were investigated. (1) The prediction of dMBT from the 21 (or 20) dRBTs with multi-linear regression analysis (MLR). This yields two results, model A with rectal temperature (dT _re) alone, and model B with dT _re and esophageal temperature (dT _es). (2) The prediction of dMBT from dT _re with or without dT _es and 13 skin surface temperatures combined to one weighted mean skin temperature (dTˉ _sk), using MLR. This results in models C and D. Six more models (E–J) were added, representing the above two sets in various combinations with four factors. (3) The conventional method calculated with four values for X. Model A predicted better than 0.3 °C in 70% of the cases. Model I was the best amongst the models with 13 weighted skin temperatures (better than 0.3 °C in 60% of the cases). The conventional method was erratic. Accepted: 14 January 2000     

Effects of 6 versus 12 days of heat acclimation on heat tolerance in lightly exercising men wearing protective clothing

This study investigated the influence of 6 versus 12 days of heat acclimation on the tolerance of low-intensity exercise in the heat while wearing protective clothing. Sixteen young men were acclimated by treadmill walking (50% of each subject's maximal aerobic power for 60 min -day-') in a climatic chamber [40°C dry bulb (db), 30% relative humidity] for either 6 consecutive days or two 6-day periods, separated by a 1-day rest. Before and after heat acclimation, the subjects performed a heat-exercise test (1.34m·s^–1, 0% grade; 40°C db, 30% relative humidity), either under control conditions [wearing normal light combat clothing (continuous exercise;n = 5)] or when wearing protective clothing resistant against nuclear, biological, and chemical (NBC) agents (repeated bouts of 15-min walk + 15-min rest;n = 8). Criteria for halting the test exercise were a rectal temperature (T _re) of 39.3°C, a heart rate (f _c) 95% of the subject's observed maximum, unwillingness of the subject to continue, or the elapse of 150 min. Heat acclimation decreased overall test values ofT _re,f _c, and mean skin temperature for both control and protective clothing conditions. When wearing normal combat clothing, acclimation responses were about twice as large after 12 than after 6 days, but the response was not increased by longer acclimation when wearing NBC protective clothing. Both 6 and 12 days of acclimation increased tolerance times in NBC protective clothing by about 15 min [from 97 (4) to 112 (6) min and from 108 (10) to 120 (10) min for 6 and 12 days, respectively]. We conclude that the physiological strain and limitation of heat-exercise tolerance imposed by wearing NBC protective clothing are not reduced if heat acclimation is prolonged from 6 to 12 days.     

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.     

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.     

Responses of young and older men during prolonged exercise in dry and humid heat

Summary Eight young, sedentary men (aged 34 years, SD 3) and six older moderately active, unacclimated men (aged 57 years, SD 2) walked on a treadmill at 30% of their maximum oxygen consumption up to 3.5 h in a thermoneutral [dry bulb temperature (T _db) 21°C, relative humidity (r.h.) 43%)], a warm humid (T _db 30°C, r.h. 80%) and a hot dry (T _db 40°C, r.h. 20%) environment while wearing ordinary working clothes (0.7 c/o). Their oxgen consumption, heart rate (f _c), rectal (T _re) and mean skin temperature (T_sk), sweat rate (SR), and evaporative rate (ER) were measured during the tests. The ratings of thermal sensation (TS) and perceived exertion (RPE) were assessed using standard scales. In the heat stress tests, the number of experiments discontinued did not significantly differ between the two groups. The mean levels and end-exercise values of T _re, T_sk, f _c, TS and RPE were not significantly different between the young and older subjects in any of the environments. In the warm humid environment, however, the T _re and RPE of the older subjects increased continuously (P<0.05) during the test compared to the young subjects. No significant difference between the groups was observed in SR or in ER. In the hot dry environment, however, the ER of older men increased more slowly compared to the young men. In spite of some time-related differences observed in T _re, RPE, and ER, the older subjects did not exhibit higher f _c during exercise in the heat, they were not more hyperthermic and their performance times were similar to the young subjects. Therefore, it was concluded that older calendar age is not necessarily associated with a reduced ability to exercise in a hot environment and other factors, such as physical activity habits and aerobic capacity, may be equally important in determining heat tolerance in the elderly.     

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.     

A sedentary day: effects on subsequent sleep and body temperatures in trained athletes

Exercise effects on sleep in fit healthy people have been difficult to determine because their sleep is close to optimal, leaving little room for improvement. Another method for assessing exercise effects on sleep is to significantly reduce the degree of activity in highly active people. Fifteen trained athletes who exercised daily at a moderate to high intensity were employed. By requesting that subjects remain sedentary in the laboratory for an entire day, the effect of reduced exercise on subsequent sleep parameters was assessed. Sleep and temperature were recorded after a sedentary day and after a normal day of moderate to high activity (control condition) in a counterbalanced design. In the sedentary condition, slow-wave sleep (SWS) decreased by a mean of 15.5+/-7.0 min and slow-wave activity (SWA) differed significantly (P<.05) between conditions in the first hour of sleep only. Rapid eye movement (REM) sleep increased by a mean of 17.9+/-5.7 min in the sedentary condition, while sleep onset latency (SOL) to Stages 1 and 2 increased by 10.2 and 10.7 min, respectively, and REM sleep latency decreased by 24.0+/-6.8 min (all P<.05). Between conditions, there was no overall effect on total sleep time (TST), sleep efficiency, wake after sleep onset or core or foot temperatures (P>.05). With reduced exercise load, SWS pressure may have been reduced, resulting in lower levels of SWS and increased REM sleep. Thus, the data indicate that reducing exercise has significant effects on sleep that may have implications for athletes tapering for competition.     

Prediction of mean skin temperature in warm environments

Summary The data collected by the authors in four experimental series have been analysed together with data from the literature, to study the relationship between mean skin temperature and climatic parameters, subject metabolic rate and clothing insulation. The subjects involved in the various studies were young male subjects, unacclimatized to heat. The range of conditions examined involved mean skin temperatures between 33‡ C and 38‡ C, air temperatures (T_a) between 23‡ C and 50‡ C, ambient water vapour pressures (P_a) between 1 and 4.8 kPa, air velocities (V_a) between 0.2 and 0.9 m · s⁻¹, metabolic rates (M) between 50 and 270 W · m⁻², and Clo values between 0.1 and 0.6. In 95% of the data, mean radiant temperature was within ±3‡ C of air temperature. Based on 190 data averaged over individual values, the following equation was derived by a multiple linear regression technique: ˉT_sk=30.0+0.138T_a+0.254P_a−0.57V_a+1.28 · 10⁻³ M−0.553 Clo. This equation was used to predict mean skin temperature from 629 individual data. The difference between observed and predicted values was within ±0.6‡ C in 70% of the cases and within ±1‡ C in 90% of the cases. It is concluded that the proposed formula may be used to predict mean skin temperature with satisfactory accuracy in nude to lightly clad subjects exposed to warm ambient conditions with no significant radiant heat load.     

Evaluation of a temperate-environment test of heat tolerance in prior heatstroke patients and controls

Summary It has been reported that scores from a temperate-environment step test describe the heat-tolerance status of prior heatstroke patients (HP). This investigation evaluated the ability of this temperate-environment heat-tolerance test (HTT) to indicate altered heart rate (HR) and rectal temperature (T _re )responses of HP, after 7 days of heat acclimation. On day 1, ten male HP (61 ± 7 days post-heatstroke) and five control subjects (C) bench-stepped (0.30 m high, 27 steps · min^–1) for 15 min (25.8° C dry bulb, 16.2° C wet bulb). On days 2–8, subjects underwent heat acclimation (40.1° C dry bulb, 23.8°C wet bulb; treadmill, 90 min · day^–1). Heat acclimation resulted in significant decreases in final HR (152±5 vs 130±3 beats·min^–1 P<0.025) and finalT _re (38.62±0.11 vs 38.13±0.07°C,p < 0.01) in HP. One HP but no C was defined heat intolerant, exhibiting inability to adapt to daily exercise in the heat. On day 9, HP repeated HTT, exactly as performed on day 1; mean group HTT scores did not change (day 1=39±6; day 9=48±6,P>0.05). All physical characteristics and physiological responses of HP (days 1, 2, 7, 9) were statistically similar (P>0.05) to those of C. In contrast to heat-acclimation data, HTT scores (score 30) indicated that four HP were heat intolerant on day 1 and two HP were heat intolerant on day 9. It was concluded that HTT was not a substitute for lengthier tests of heat tolerance conducted in hot environments, because HTT scores (at 25.8°C did not reflect HR andT _re responses (at 40.1° C) in 33% of heat-acclimated (e.g., heat-tolerant) HP. In addition, HTT scores did not validly discriminate between heat tolerant and heat-intolerant HP.     

Comparative physiological responses of normotensive and essentially hypertensive men to exercise in the heat

Summary Six essentially hypertensive men (average resting arterial pressure of 150/97 mm Hg) and eight normotensive controls (average resting arterial pressure of 115/73 mm Hg) were tested during 1 h of dynamic leg exercise in a warm environment. The groups were well matched for age, max, body surface area, weight, and body fat. Environmental conditions were 38 C dry-bulb, 28 C wet-bulb; exercise intensity was approximately 40% max (85–90 W). There were no significant intergroup differences in core or mean skin temperatures, calculated heat exchange variables, heart, or sweat rates. Blood pressure differences between the groups were maintained (P<0.01). The hypertensive group responded with a significantly lower stroke index (P<0.01) and cardiac index (P<0.01), and a decreased slope of the rise in forearm blood flow (P<0.01) due to an higher vascular resistance (P<0.01). The combined heat load (M + R + C) presented was not sufficient to override the hypertensives' higher cutaneous vasoconstrictor tone. However, on a practical basis, the hypertensives were able to tolerate exercise in the heat as well as their normotensive counterparts.     

Rates of fluid ingestion alter pacing but not thermoregulatory responses during prolonged exercise in hot and humid conditions with appropriate convective cooling

The aim of the study was to examine the effects of fluid replacement on thermoregulation and cycling performance in hot, humid conditions. Six male cyclists (PPO = 426 ± 39 W) performed six 80 km time trials. Subjects replaced 0% (0); 33% (33); 66% (66); or 100% (100) of the weight lost during an “ad libitum” trial (Ad Lib). In another condition (WET), subjects rinsed their mouths at 10 km intervals. There was no trial effect on any thermoregulatory variables or on performance. When WET, 0, 33 (“LO”) were compared to Ad Lib; 66, 100 (“HI”), power output was higher in HI (209 ± 22 vs. 193 ± 22 W, p < 0.05). Restricting fluid below ad libitum rates impaired performance (LO group). Rates greater than ad libitum did not result in further improvements. Ad libitum fluid ingestion is optimal as it prevents athletes from ingesting too little or too much fluid.     

Central thermosensitivity in conscious goats: Hypothalamus and spinal cord versus residual inner body

Experiments were performed on conscious goats to confirm the suggestion that in this species the inner body contains more thermosensitive structures than those residing in the hypothalamus and spinal cord. For this purpose goats were chronically implanted with local thermodes and intravascular heat exchangers to allow independent temperature control of the hypothalamus, spinal cord and residual inner body. With the hypothalamus and spinal cord clamped simultaneously at different levels between 32°C and 40°C, residual internal temperature was lowered by subtracting heat via the intravascular heat exchanger. The residual internal temperature at which shivering and increased heat production occured due to heat extraction, was directly related to the value of the combined hypothalamic and spinal cord clamp temperature. The higher hypothalamic and spinal cord clamp temperatures were, the lower residual internal temperature fell before shivering occurred and heat production rose. Plosts relating residual internal temperature to hypothalamic and spinal cord temperature at different levels of heat production showed the signal input generated within the residual inner body to be of nearly the same order of magnitude as that from the hypothalamus and spinal cord.This work was supported by DFG Je 57/3.     

Comparison of rectal and aural core body temperature thermometry in hyperthermic, exercising individuals: a meta-analysis

Objective:

To compare mean differences in core body temperature (T_core) as assessed via rectal thermometry (T_re) and aural thermometry (T_au) in hyperthermic exercising individuals.

Data Sources:

PubMed, Ovid MEDLINE, SPORTDiscus, CINAHL, and Cochrane Library in English from the earliest entry points to August 2009 using the search terms aural, core body temperature, core temperature, exercise, rectal, temperature, thermistor, thermometer, thermometry, and tympanic.

Study Selection:

Original research articles that met these criteria were included: (1) concurrent measurement of T_re and T_au in participants during exercise, (2) minimum mean temperature that reached 38°C by at least 1 technique during or after exercise, and (3) report of means, standard deviations, and sample sizes.

Data Extraction:

Nine articles were included, and 3 independent reviewers scored these articles using the Physiotherapy Evidence Database (PEDro) scale (mean  =  5.1 ± 0.4). Data were divided into time periods pre-exercise, during exercise (30 to 180 minutes), and postexercise, as well as T_re ranges <37.99°C, 38.00°C to 38.99°C, and >39.00°C. Means and standard deviations for both measurement techniques were provided at all time intervals reported. Meta-analysis was performed to determine pooled and weighted mean differences between T_re and T_au.

Data Synthesis:

The T_re was conclusively higher than the T_au pre-exercise (mean difference [MD]  =  0.27°C, 95% confidence interval [CI]  =  0.15°C, 0.39°C), during exercise (MD  =  0.96°C, 95% CI  =  0.84°C, 1.08°C), and postexercise (MD  =  0.71°C, 95% CI  =  0.65°C, 0.78°C). As T_re measures increased, the magnitude of difference between the techniques also increased with an MD of 0.59°C (95% CI  =  0.53°C, 0.65°C) when T_re was <38°C; 0.79°C (95% CI  =  0.72°C, 0.86°C) when T_re was between 38.0°C and 38.99°C; and 1.72°C (95% CI  =  1.54°, 1.91°C) when T_re was >39.0°C.

Conclusions:

The T_re was consistently greater than T_au when T_core was measured in hyperthermic individuals before, during, and postexercise. As T_core increased, T_au appeared to underestimate T_core as determined by T_re. Clinicians should be aware of this critical difference in temperature magnitude between these measurement techniques when assessing T_core in hyperthermic individuals during or postexercise.