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¹ Zinman College, Wingate Institute, Israel² Griffith University, Gold Coast, Australia ³ Heler Institute, Tel. Hashomer, Sackler School of Medicine, Tel. Aviv University, Israel

	Abstract

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The purpose of this investigation was to compare the thermoregulatory responses during exercise in a hot climate among three age categories. Eight prepubertal (PP), eight young adult (Y) and eight elderly (O) male subjects cycled at an intensity of 50 ± 1% of their maximum oxygen uptake for 85 min (three 20 min bouts with three 7 min rest periods) in hot and dry conditions (41 ± 0.67°C, 21 ± 1% relative humidity). During the exercise-in-heat protocol, rectal temperature (T_re) skin temperatures (T_sk), heart rate (HR), , RER, sweat rate, and the number of heat activated sweat glands (HASG) were determined. Despite highest and lowest end-exposure T_re in the Y and O groups, respectively, the rise in rectal temperature (accounting for differences in baseline T_re) was similar in all age groups. Changes in body heat storage (S), both absolute and relative to body mass, were highest in the Y and O groups and lowest in the PP group. While end-session as well as changes in mean skin temperature were similar in all three age groups, HR (absolute and percentage of maximum) was significantly lower for the O compared with the PP and Y groups. Total body as well as per body surface sweating rate was significantly lower for the PP group, while body mass-related net metabolic heat production ((M – W) kg^–1) and heat gained from the environment were highest in the PP and lowest in the O group. Since mass-related evaporative cooling (E_sk kg^–1) and sweating efficiency (E_sk/M_sw kg^–1) were highest in the PP and lowest in the O group, the mass-dependent heat stored in the body (S kg^–1) was lowest in the PP (1.87 ± 0.03 W kg^–1) and highest in Y and O groups (2.19 ± 0.08 and 1.97 ± 0.11 W kg^–1, respectively). Furthermore, it was calculated that while the O group required only 4.1 ± 0.5 W of heat energy to raise their body core temperature by 1°C, and the Y group needed 6.9 ± 0.9 W (1°C)^–1, the PP group required as much as 12.3 ± 0.7 W to heat up their body core temperature by 1°C. These results suggest that in conditions similar to those imposed during this study, age and age-related characteristics affect the overall rate of heat gain as well as the mechanisms through which this heat is being dissipated. While prepubertal boys seem to be the most efficient thermoregulators, the elderly subjects appear to be the least efficient thermoregulators.

(Received 9 May 2004; accepted after revision 12 August 2004; first published online 24 August 2004)
Corresponding author O. Inbar: Zinman College, Wingate Institute, Israel. Email: inbar{at}macam.ac.

	Introduction

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Several factors affect the responses of an individual to the combined stresses of exercise and heat, including age, anthropometric characteristics, maximal aerobic capacity and the level of acclimatization. Often these factors create methodological constraints when comparing the effects of age on the thermoregulatory response to exercise. Typically, young children and older people (as groups) have low maximal aerobic power, high adiposity and small body stature and body mass compared with young adults. Such morphological and physiological characteristics imply relatively large surface area-to-mass ratio (especially in children), lower sweat rate, lower cardiac output and poor control of peripheral blood flow (especially in older individuals) compared with young healthy adults (Astrand, 1952; Bar-Or, 1989; Kenney, 1997; Pandolf, 1991).

Although similar absolute intensity imposes a comparable metabolic heat production, many heat loss effectors responses, notably sweating rate, are related to relative intensity (% ). Therefore, an experiment where both young and old subjects are matched for and thereby being able to exercise at similar absolute and relative exercise intensity, would be fundamental to our understanding of the changes in temperature regulation with age. While this experimental approach is theoretically ideal, the practicalities of matching young and old subjects for are not only difficult, but also entail additional confounding elements into the age-span comparisons. In selecting older subjects whose fitness level matches that of young adults and/or children, individuals are included who are highly trained and therefore more heat acclimatized than their older counterparts, and their study-matched younger subjects (Drinkwater & Horvath, 1979; Haymes et al. 1975).

Although most research on human thermoregulation has focused on young adult males, several studies have compared thermoregulatory responses between children and adults or between young adults and older individuals. For more systematic reviews see Bar-Or (1989) and Kenney (1997). Of those studies investigating age-related differences in thermoregulation during exercise, it seems that there is only one that included all three lifespan stages (childhood, adulthood and old age; Drinkwater & Horvath, 1979). Furthermore, the above study was the only one in which thermoregulation during exercise was compared between young children (prepuberal girls) and elderly individuals (women).

Although a few studies suggest that children's temperature regulation is as effective as that of adults (Gullestad, 1975; Davies, 1981; Docherty et al. 1986), most others indicate that children are less effective thermoregulators (Sohar & Shapira, 1965; Van Beamont, 1965; Wagner et al. 1972; Sloan & Keatinge, 1973; Drinkwater et al. 1977; Delamarche et al. 1990; Havenith, 2001). The major reasons for the above discrepancy are: (1) that the authors have not qualified their conclusions to the specific ambient conditions that prevailed during their experiments; and (2) that the authors, in most cases, have not applied thermodynamic computations to their data analyses.

While the most apparent thermoregulatory-related difference between children and young adults is the lower overall sweating rate by children (Anderson & Kenney, 1987; Bar-Or, 1989; Delamarche et al. 1990), reduced peripheral blood flow (PBF) seems to be the most apparent difference between young adults and older individuals (Kenney, 1997; Inoue et al. 1999). Both may be accompanied by a greater rate of increase in body temperature and reduced heat tolerance in both children and the elderly.

The increasing number of children involved in high-level athletic competition and of elderly persons who participate in regular exercise to maintain their health, and the susceptibility of children and the elderly to heat disorders (Applegate et al. 1981; Smoyer, 1998) may raise the risk of heat-related disorders in these two age groups. The present study is thus timely and represents, to the best of our knowledge, the first attempt to aptly compare the thermoregulatory responses of males across three lifespan stages (i.e. childhood, adulthood and old age) during exercise in a hot and dry climate.

The purpose of the present study was to compare the thermoregulatory responses to cycle exercise at 50% in a hot and dry environment (40–42°C dry bulb, 20–22% relative humidity) in males with life stages ranging from prepubescence through adulthood to old age.

Based on the reported age-related functional changes of both sensors and effectors, as well as of central neural structures, we hypothesized that lower sweating rates and higher core temperature would be found during exercise in hot dry heat in prepubertal boys and old males compared with young males.

	Methods

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Subjects

Twenty-four healthy, prepubertal (PP, N = 8), young adult (Y, N = 8) and older (O, N = 8) male subjects volunteered to participate in the study. All subjects were physically active but were not participating in any regular, organized competitive sport. The physical characteristics of the subjects are summarized in Table 1. All subjects had been healthy non-smokers for at least 10 years (the Y and the O subjects), had resting blood pressure of lower than 140/90 mmHg, had normal resting and exercise 12-lead ECG, and had normal physical examination, which revealed no evidence of clinically significant cardiovascular or lung disease.

Protocol and procedures

Experiments were performed at both the Wingate Institute (Israel; PP and Y groups) and at Griffith University (Gold Coast, Australia; O group). Experimental procedures and conditions (in and outside the laboratory) were similar at each location. Although some equipment was different, instrument calibration and the presence of the primary author (O. Inbar) at both sites assured consistency of results. All measurements were performed during the late winter months to reduce the possibility that subjects would become heat acclimatized. Data collection took place between 13.00 and 16.00 h.

During the first visit to the laboratory, subjects were familiarized with all intended procedures and then written consent was obtained from each subject in the Y and O groups and from the parent(s) or guardian of subjects in the PP group. During the second visit, anthropometric measurements were taken and peak oxygen uptake determined on an electronically braked cycle ergometer (Lode, Groningen, The Netherlands). Peak oxygen uptake was determined using open-circuit spirometry in a thermoneutral environment, using a progressive (1 min stages) and a continuous cycle ergometer test. Each group of subjects completed a 3 min warm up of unloaded cycling. For the PP group, subjects then commenced cycling at 15 W followed by a 10 W min^–1 load increment. For the Y group, subjects commenced cycling at 50 W followed by a 25 W min^–1 load increment to exhaustion. For the O group, subjects commenced cycling at 15 W, followed by a 15 W min^–1 load increase to exhaustion. Expired gases were analysed for percentages of oxygen using paramagnetic (Beckman E-2 at Wingate) or zirconium cell analysers (at Griffith University) and for carbon dioxide using infrared analysers (Beckman LB-1; in both centres). Analysers were calibrated with standard gases immediately prior to and following each exercise test. Expired gas was collected in Douglas bags and volumes were determined using a calibrated spirometer (Collins at Wingate Institute and Hans Rudolph at Griffith University). For all subjects, heart rate (HR) was monitored using a 3-lead ECG (Elma 3-channel recorder and scope (Seattle, WA, USA) in the CM5 configuration. The maximal exercise test was terminated upon self-determined exhaustion or when the subject could no longer maintain a 40 r.p.m. cadence, despite verbal encouragement by the investigators. Peak oxygen uptake was considered to be the highest value obtained in a given 60 s period. Secondary criteria for achieving included respiratory exchange ratio (RER) > 1.15, HR > 95% of the subject's age-predicted maximal heart rate, or visible signs of exhaustion, such as breathlessness and/or inability to maintain the required power output.

Three to four days following the peak oxygen uptake test, subjects came to the laboratory to exercise in the climatic chamber. Climatic chamber conditions were set at 41 ± 0.67°C dry bulb, and 21 ± 1% relative humidity (RH) with circulating air velocity of <0.3 m s^–1. To ensure full hydration and stable conditions prior to entering the climatic chamber, all subjects were required to drink 1% of their body mass in water, while resting in a thermoneutral environment for 1 h. Following the 60 min rest period, resting neutral measurements of weight (nude), body temperature and HR were taken. Subjects were dressed in athletic shorts and shoes only (insulating values of clothing (clo) = 0.45 W; Shapiro et al. 1982).

The climatic chamber protocol consisted of three 20 min bouts of exercise, followed by 7 min rest periods. Subjects exercised on an electronically braked cycle ergometer (Lode) at a work rate requiring 50% . The climatic chamber session lasted a total time of 85 min or until termination criteria were reached. Termination criteria included rectal temperature (T_re) > 39.1°C, arrhythmias, nausea, dizziness, chills, exhaustion or headache.

During the exercise-in-heat protocol, rectal temperature (T_re, measured using a Yellow Springs Instruments series 400 thermistor inserted 7–8 (PP) or 10 cm (Y and O) beyond the anal sphincter), skin temperatures (T_sk, weighted mean of back, forearm and thigh measurements, using silver constantan thermistors; Yellow Springs Instruments series 400 thermistor) and heart rate (3-channel recorder and scope; Elma, Seattle, WA, USA) were monitored continuously. Metabolic variables (Oxygen uptake , CO₂ production , minute ventilation and RER) were determined at the mid-point of the second bout of exercise for 10 min, employing the instruments used for the tests. Body mass was determined during each rest period using a scale (Shekel/Wedderburn, Israel/Australia) accurate to 10 g. Subjects were encouraged to drink measured tap water throughout the climatic chamber protocol. Changes in body mass were subsequently corrected for fluid intake (given ad libitum and measured in a graduated container). The sweating pattern (number of heat-activated sweat glands (HASG) and sweating rate per gland (SGO)) was determined by the starch–iodine technique (Bar-Or et al. 1968), obtained from five skin sites during the three rest periods following each exercise bout.

Calculations

Adiposity was estimated from the sum of four skin folds measured over the triceps, biceps, subscapular and suprailiac regions using age-appropriate equations (Durnin & Womsley, 1974; Slaughter et al. 1988). Each skin fold measurement was taken in triplicate and recorded as the mean of each of the three measurements. Subject height was measured using a stadiometer (Holtain, Crymych, Wales). Body surface area (A_D) was calculated from height and body mass, according to DuBois & DuBois (1916). Whole-body sweating rate (M_sw) was calculated from the change in body mass, corrected for fluid intake, and the change in mass of clothes and electrodes (due to sweating). Metabolic heat production (M) was calculated from the and the related respiratory exchange ratio values measured during exercise, corrected for external work done (M – W).

Change in body heat storage (S) was calculated according to Gagge (1972), from the individual's specific body heat and the change in mean body temperature (T_b; T_b = 0.8T_re + 0.2T_sk), as: S (W) = specific body heat x T_b.

The specific heat of each subject was computed by assigning specific heat values of 0.406, 0.464 and 1.157 W kg^–1 °C^–1 for lean tissue, fat and water, respectively (Bar-Or et al. 1969).

Dry heat exchange through radiation, convection and conduction (R + C) was calculated from the difference between ambient temperature (T_a) and mean T_sk, considering body surface area (A_D) or body mass (kg) as follows (Gagge, 1972):

(1)

Actual evaporative heat loss of sweat (E_sk) was calculated from the heat balance equation as:

(2)

Respiratory heat loss was considered negligible (Shapiro et al. 1982).

The evaporative cooling power needed to maintain thermal equilibriubm (E_req) was calculated according to the basic equations of Givoni & Goldman (1972):

(3)

As an indication of sweating sensitivity, whole-body sweating rate (M_sw) was divided by the elevation of rectal temperature (T_re) during the heat exposure. Sweating efficiency was established as the ratio between evaporative heat loss and total sweating rate (E_sk:M_sw).

Statistical analysis

A one-way analysis of variance was performed to assess group differences in subject characteristics, baseline values and variables measured once during the experimental session (e.g. ). Where significant differences were found, post hoc analyses were performed using Tukey's confidence interval. In order to compare the sequential changes from pre-exposure values of the physiological variables measured repeatedly during the session (e.g. T_re, T_sk and HR), a repeated measures analysis of variance, RM ANOVA) was carried out and subsequently a 95% confidence interval (CI) was calculated for every time interval for each group. A significant difference (P < 0.05) between two groups at any time interval was established when a given mean value of one group was outside the CI of another group. In order to account for age-related differences in presession values (Anderson & Kenney, 1987), differences among the three age groups in T_re, T_sk and HR at the end of the heat exposure were established by using analysis of covariance (ANACOVA), where the respective presession values served as covariates. Statistical significance was set at = 0.05 for all statistical tests.

	Results

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The physical characteristics of the three subject groups are presented in Table 1. The children (PP) were significantly lighter and shorter, and had a smaller body surface area (A_D) and lean body mass (LBM), higher peak heart rate (HR_peak), and greater body surface to mass ratio (A_D/mass) than both the young adults and the elderly subjects. The oldest group (O) demonstrated significantly lower and peak heart rate, but higher adiposity compared with the two younger groups.

The cardiopulmonary responses obtained during exercise in the hot environment at an intensity 50% , are presented in Table 2. Importantly, the relative metabolic rate (% ) did not differ between groups (52, 51 and 51% in the PP, Y and O groups, respectively). Moreover, the RER was similar among the three groups. However, the Y group had a significantly higher absolute power output (W) and (l min^–1) than both the PP and the O groups. Mass-related power output (W kg^–1) was also highest in the Y group followed by the PP and O groups. Relative (ml kg^–1 min^–1) was highest and similar in the Y and P groups and appreciably lower in the O group (Table 2).

Figure 1 demonstrates the sequential changes in T_re, T_sk and HR during exercise in the climatic chamber for the three groups. Mean T_re, T_sk and HR were highest throughout the session in the Y and the PP groups and lowest in the O group (Fig. 1). The thermoregulatory responses during the exercise with heat exposure are presented in Table 3. At the end of the heat exposure, the elderly subjects (O) demonstrated significantly lower T_re and HR (both in absolute and relative terms) than both the PP and the Y groups, with no such differences in T_sk. However, when accounting for the age-related differences in the prechamber values in these three variables, thereby comparing the estimated end-session values in these parameters (using ANACOVA), no age-related differences were found in the rise in both T_re and T_sk. In contrast, the end-session and changes in HR remained significantly smaller in the O group.

View larger version (24K): [in this window] [in a new window]   Figure 1.  Rectal (Tre) and mean skin (Tsk) temperatures, and heart rate (HR) responses during three exercise bouts in dry heat in the prepubertal boys (PP, ), young men (Y, ) and the elderly (O, •) Data points are means ± S.E.M.

Table 5 presents the sweating responses during exercise in the heat, by age group. As expected, the overall sweat rate (M_sw) was lowest in the PP group and highest in the Y group, with the O group demonstrating intermediary values. Even when normalized to body surface area (M_sw/A_D) it was still lowest and highest in the PP and the Y groups, respectively, with the O group being similar to the PP group (Table 5).

The density of the heat-activated sweat glands (HASG cm^–2) was also highest in the PP and lower and similar in the Y and O groups. In contrast, sweat produced by each sweat gland (SGO) was lowest in the PP and highest in the Y and the O groups (Table 5).

	Discussion

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The results of the present study imply that age and its related physical and physiological characteristics do have apparent consequences on both the end-exposure T_re and on overall thermal strain, when exercising at a moderate and equally intense load in hot and dry environmental conditions.

The amount of heat required to be dissipated to achieve thermal balance (E_req) was the highest in the young adult group (Y), followed by the oldest (O) and the youngest (PP) groups respectively. These differences mirror age-related variations in absolute mechanical power load, differences in mechanical and/or metabolic efficiency, and diversity in body dimensions and composition (mass, A_D, A_D: mass, water, and adiposity) (Astrand, 1952; Smolander et al. 1990). Accordingly, evaporative heat loss (E_sk) as well as overall sweat rate (M_sw) was also higher in the Y than in the two other age groups. These findings are in line with previous studies reporting relatively low sweating rate in young children and the elderly (Bar-Or, 1989; Inoue et al. 1999; Kenney, 1997).

In the Y group, high sweating sensitivity (M_sw/T_re) and relatively high sweating efficiency (E_sk/M_sw) were evident (see Table 5). In the PP group, the need for overall body cooling (E_req) was relatively low and thus could be achieved with high sweating efficiency and despite low sweat rate and sweat sensitivity. These findings are similar to those reported by Araki et al. (1979), and by Inbar et al. (1985).

In the O group sweat gland productivity (SGO) was high, while the relatively inefficient cooling rate in this group was due mainly to their low overall sweating rate, resulting, at least partially, from their low sweating sensitivity. These findings are in accordance with earlier studies of Wagner et al. (1975) and of Hellon and Lind (1956), who reported that older individuals have a reduced rate of sweat secretion and a delayed onset of sweating during periods of increasing core temperature and heat stress. It has been suggested that inefficient cooling capacity in older people is caused by an age related decrease in the sensitivity to cholinergic stimulation and a reduced androgenic stimulation of the secretory coil, that ultimately result in a reduced capacity of the sweating mechanisms in this population (Kenney, 1997).

Additionally, age-related atrophy of the skin's structures, altering the size and/or secretory capacity of the sweat glands, also attenuate sweat rate (Anderson et al. 1987; Wagner et al. 1972).

Older individuals respond to an exercise-in-heat stress with a lower peripheral blood flow (PBF) at a given T_re compared with a younger population, even when physical and physiological characteristics are matched (Inoue et al. 1999; Kenney, 1997; Slaughter et al. 1988).

The low final HR (in absolute and relative (to measured peak HR) terms), as well as the slow and low increase in HR, are suggestive of a low peak cardiac output and possibly poor PBF. Moreover, this group had the slowest sequential rise in T_sk, which could also imply a relatively slow PBF. If the elderly indeed had a lower PBF, this may have increased the rate of convective heat gains from the environment (increased T_sk – T_a gradient) and slowed down the transfer of heat from the core to the periphery.

It is suggested, therefore, that when exercising in hot and dry environments, when body dimensions are not accounted for and relative exercise intensity and duration are similar, the overall thermal strain (S and end-session T_re) is higher in the Y group than in the O and the PP groups.

Metabolic heat production, normalized to body weight, was the highest in the PP group and the lowest in the O group. This was the case despite the fact that the Y group exercised at a higher relative load (to body mass) than the PP group. It seems, therefore, that in normalizing the exercise intensity or the work rate to the individual's , one may standardize the exogenous power but certainly not the absolute or the relative (to body mass) metabolic heat production.

Dry heat gain, when normalized to body mass (R+C kg^–1), was higher in the PP group than in the two other age groups. This is most likely consequent to the boys' higher A_D: mass ratio. Akin to the required heat to be dissipated per kg body weight, mass-related evaporative heat loss (E_sk kg^–1) was also highest in the PP group and lowest in the O group. This is more likely due to their higher sweating efficiency and relatively high sweating sensitivity, and despite their lower mass- and surface area-related sweat rate and lower sweat rate/gland (SGO) (see Tables 4 and 5).

The high relative sweating efficiency demonstrated by the youngest group is in contrast to the findings of others (Bar-Or, 1989; Falk et al. 1991b). While those studies claim that the sweating mechanisms are the children's major disadvantage in thermoregulation, our results suggest that the sweating efficiency is the children's major advantage. One possible explanation for such a discrepancy could be related to the way sweating efficiency was defined. Using the proposed definition of sweating efficiency (E_sk/M_sw) implies that factors other than just M_sw affect the overall and the mass- or surface area-related cooling rate. It is possible that in children the smaller, less widespread sweat drops may have resulted in a higher evaporative cooling than the larger, more dispersed sweat drops in the young adults and the elderly. Additionally, the larger drops may be more likely to coalesce and drip, thus providing less cooling. The lower electrolyte concentration in children's sweat (Bar-Or, 1989; Dill et al. 1966) might be another explanation for their higher cooling (evaporative) efficiency (lowered latent heat).

Sweating sensitivity was highest in the young adults (Y), pointing to the likelihood of over sweating or less efficient evaporative cooling on their part. Considering sweat efficiency also as an indirect index of unevaporated sweat and assuming that 1 g of evaporated sweat is equivalent to 0.673 W of heat, it seems that sweat in the youngest subjects (PP) dripped rather lightly (10^%). The young adults and particularly the elderly subjects, on the contrary, revealed higher percentage of sweat that went unevaporated.

Highest and lowest heat storage was noted in the young adults and the prepubertal boys, respectively, even when normalized to body weight (see Table 4). Hence, the boys, it seems, possessed the most efficient thermoregulatory apparatus of the studied age groups. This finding is further authenticated when calculating the ratio of the combined heat gained from the surrounding environment (R + C/kg) and that produced internally (M – W/kg) to the rise in body core temperature (T_re/kg). From Figure 2, one can observe that prepubertal boys can accumulate more heat before the temperature of 1 kg of their body mass is elevated by 1°C (12.3 ± 0.7 vs. 6.9 ± 0.9, and 4.1 ± 0.5 W kg^–1 1 °C^–1, in the PP, Y, and the O groups, respectively).

View larger version (35K): [in this window] [in a new window]   Figure 2.  Mass-related heat-gain ([M – W]/mass) + ([R + C]/mass) needed to raise body core temperature by 1°C in the different age groups Values are means + 1S.E.M.

It should also be indicated that the higher cooling efficiency in the PP group, in spite of their lowest sweat rate, kept their mean skin temperature from being elevated above that shown in the other two age groups.

A few studies have suggested that under both neutral and warm environments (T_a equal or lower than T_sk), children are as effective thermoregulators as young adults (Bar-Or, 1989). Smolander et al. (1990) and others (Havenith, 2001; Shibasaki et al. 1997)) also found that prepubertal boys maintained their core temperature at least as effectively as young adult males, despite their larger surface-area-to-mass ratio and lower sweat rate. These results corroborate our suggestion that differences in the surface-area-to-mass ratio are, at least under the present study's conditions, less significant in thermoregulation than previously assumed. Furthermore, factors associated with growth, such as increased surface area, decreased sweat gland density, increased sweat rate per gland (Landing et al. 1968), and enhanced hormonal response (Falk et al. 1991a) as well as increased sweat drop area (Falk et al. 1992), may also be less important than previously implied.

Our findings of least efficient thermoregulatory responses in the older subjects are in agreement with previous reports (Anderson & Kenney, 1987; Kenney, 1997; Pandolf, 1991). These researchers suggested that the lower PBF in elderly subjects during heat exposure and exercise is associated with lower cardiac output and a slower decrease in the splanchnic and renal flows, together with age-related alterations in the peripheral mechanisms and cutaneous vasculature. The significantly lower HR (in both absolute and relative terms), and the slower and smaller rise in T_sk shown by our elderly subjects relative to the two younger groups, could imply sluggish blood flow to the periphery which, when combined with their lowest evaporative cooling efficiency, may well explain their least efficient thermoregulatory apparatus.

In conclusion, under the conditions of the present study, age and age-related characteristics appear to affect the overall rate of heat gain as well as the mechanisms through which this heat is being dissipated. It was shown that when exposed to equal relative physiologic and climatic stresses, pre-pubertal boys are the most efficient thermo-regulators. The elderly subjects, as expected and as previously reported, seem to be the least efficient thermoregulators, while young males are intermediate thermoregulators. The major advantage of the children seems to be related to their highly efficient sweat evaporation, possibly consequent to their sweating characteristics (glands' density, size, shape, and distribution). The major disadvantage of the elderly seem to lie in their relatively reduced blood flow to the periphery, as demonstrated in this study by their slow increase in T_sk, and their least efficient sweat evaporation.

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	Acknowledgements

 
We are grateful for the cooperation of all the subjects who participated in the study, especially the children and their parents. We also wish to thank Dr J. Browning for his invaluable technical assistance in counting the activated sweat glands, Dr B. Falk for her editorial comments, and Mrs A. Zeev and M. Arnon for their valuable role in the data analysis.

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