The effects of two types of clothing on seasonal cold tolerance

This study investigated the effects of two types of clothing, leaving legs covered or uncovered, on seasonal cold tolerance in women. Experiments were carried out to compare cold tolerance at an ambient temperature (T _a) of 10° C in December between two groups of subjects, who wore either knee-length skirts (skirt group) or full-length trousers (trouser group) for 3 months from September to November. The main results are summarized as follows: rectal temperatures continued to fall for 40 min in the trouser group when the subjects were covered by a blanket, while it became stable in 30 min in the skirt group; rectal temperatures showed greater increases in the skirt group when the blanket was removed after 40 min exposure to T _a of 10° C; metabolic heat production was kept significantly lower in the skirt group when uncovered or covered by a blanket at T _a of 10° C; metabolic heat production was negatively correlated with mean skin temperature and was always higher in the trouser group when measured at the same mean skin temperature; in the uncovered condition diastolic blood pressure increased significantly in the trouser group but not in the skirt group. These results would suggest that the subjects who wore skirts for 3 months from September to November had improved their ability to tolerate the cold.     

Effects of moisture absorption by clothing on thermal responses during intermittent exercise at 24°C

The effects of two kinds of clothing with different properties with respect to moisture absorption on thermophysiological responses and pulse rate were studied during intermittent exercise at an ambient temperature (T _a) of 24°C. The two kinds of clothing ensemble tested were cotton T-shirt with short sleeves and cotton long-sleeved working dress with full-length trousers (C), and polyester T-shirt with short sleeves and polyester long-sleeved working dress with full-length trousers (P), the thermal resistances of which were nearly equal. Five women aged 21–32 years, served as subjects. The environmental conditions were 24°CT _a, 50% relative humidity and 0.14m·s^–1 air velocity. The subjects, wearing either C or P, exercised for 10-min on a cycle ergometer at an intensity of 30% maximal oxygen uptake and then 10-min rest. This sequence was repeated four times. Rectal and skin temperatures at several sites, local sweating rate, pulse rate and clothing microclimates were continuously compared between C and P throughout the experiment. The major findings were firstly, rectal temperature rose significantly higher in P; secondly, pulse rate was higher in P both during exercise and rest; thirdly, clothing surface temperature on the back rose highly significantly during the fourth exercise period and then fell significantly during the fourth rest period in C; and fourthly, four out of five subjects felt wetter in P during the latter half of the experiment. These results are discussed from the viewpoint that the reduced thermal insulation due to the absorption of moisture in C accelerated dry heat loss, resulting in an inhibition of the increases in core temperature and pulse rate.     

Thermoregulatory responses of firemen to exercise in the heat

Summary Twelve volunteer (VF) and 12 professional firemen (PF) wearing only brief trunks exercised on an electrically-braked cycle ergometer at three-five exercise intensities. After 45 min of exercise at 75 W, the exercise intensity was elevated in steps of 25 W every 15 min until the subject was exhausted. Air temperature was regulated to equal skin temperature (36°–38°C) and relative humidity was regulated at 52%. The two groups of firemen were comparable in terms of body mass, age and maximum oxygen consumption. Their oxygen consumption, rectal and skin temperatures, sweating and heart rate were measured during the tests. Blood lactate concentration was measured before, during and after the test. The physiological strain was higher in VF as indicated by higher heat storage, heart rate, skin and rectal temperatures. Sweat rate tended to be lower in VF than PF. The results indicated a better adaptation of the professional compared to the volunteer firemen to work in the heat, although the degree of heat acclimatization was considered to be equally minimal in both groups.     

Heat of sorption induced by sweating affects thermoregulatory responses during heat load

To investigate the effects of heat of sorption per se induced by sweating on thermoregulatory responses in clothed subjects, two experiments were carried out. In experiment 1 [ambient temperature (T _a) 27.2 °C, 50% relative humidity (r.h.)], seven female subjects immersed their lower-legs in a water bath at a temperature raised between 35–41 °C during 70 min wearing garments made for the experiment of either 100% cotton (C) or 100% polyester (P). Skin blood flow at the forearm was significantly greater in C than in P (P < 0.05). The increase of mean skin temperature (Tˉ _sk) and clothing surface and clothing microclimate temperatures were significantly higher in C than that of P after the onset of sweating (P < 0.05). Furthermore, these responses were accompanied by warmer and more uncomfortable sensations in C than in P despite a lower rectal temperature in C compared with P and identical mean body temperature in both sets of garments. In experiment 2, to simulate the clothing microclimate after the onset of sweating, C and P garments were exposed in the climate chamber to r.h. raised from 50% to 95% at a constant T _a of 27.2 °C. The clothing surface temperature rose by 2.2 °C for C and by 0.5 °C for P after the increase of r.h. These results clearly showed a more marked increase in heat of sorption in C than in P. These results indicated that the heat of sorption per se after the onset of an increasing clothing microclimate vapour pressure, mimicking the onset of sweating, enhanced thermoregulatory responses such as skin blood flow, (Tˉ _sk) and subjective voting in C. Accepted: 22 February 2000     

Acclimatization of women to heat after training

Summary During March and April three Caucasian women (ages 38, 22, 20) participated in a 3-week training program at 26 °C DB and 17 °C WB followed by 7 days of walking on a treadmill (3.5 mph, 2.2 to 4.0 per cent grade) for 1 h each day in an environmental chamber (42.2 °C DB, 26.4 °C WB). 17 days later the subjects were tested again under the same hot conditions. For comparison, on the day after the 7-day acclimatization period and again after the terminal test, the subjects worked at equivalent work loads outside the chamber (26 °C DB, 17 °C WB). They wore shoes, socks, and cotton bathing suits. No subjects were able to complete the task on the first day, experiencing various degrees of discomfort, hypotension, and marked elevations of heart rate, rectal temperature, and skin temperature. Some degree of acclimatization was evident by the second day. After the seventh exposure they felt better, were able to work longer and had increased evaporative loss, with smaller elevations in heart rate and skin and rectal temperatures compared to the third day in the heat. The subjects exhibited an almost complete loss of heat acclimatization at the end of the subsequent 17-day period of no thermal stress.Supported in part by the Medical Research and Development Division, Office of the Surgeon General, Department of the Army, Contract No. DA-49-193-MD-2202 and U.S. Public Health Service Training Grant No. 5 T01 HD00235.Postdoctoral Trainee, USPHS Training Grant No. 5 T01 HD00235.     

Physiological and subjective responses to thermal transients of exercising subjects dressed in cold-protective clothing

In cold conditions variations in the physical activity of clothed individuals and rest periods in a moderate temperature may result in a disturbance of heat balance and thermal comfort of the individual, in particular when sweating occurs. The purpose of the study was to examine thermal responses in persons dressed in winter clothing during changes of exercise intensity (high to low) and ambient temperature, and to investigate whether there were any effects on these responses due to fibre material (wool and synthetic). Two types of transient condition were studied, an exercise level transient (E) and a temperature transient (T). Ten healthy male subjects dressed in multi-layer winter clothing ensembles with different levels of total insulation walked on a treadmill at an ambient temperature of – 10°C. The garments were manufactured from wool, giving insulations of 2.6 clo, in T only and otherwise of 3.2 clo, or synthetic fibres, giving insulations of 2.4 clo in T only and otherwise of 3.1 clo. In E the subjects exercised at a high intensity for 50 min followed by 60 min walking at low intensity. In T they walked at a moderate speed for 90 min in ambient temperature of – 10°C, rested in temperatures of + 22°C for 30 min and walked in the cold climatic chamber for another 45 min. The skin temperature, sweating responses and thermal sensations were higher/warmer with increasing insulation during exercise. The wool fibre material resulted in a slightly higher mean skin temperature (about 0.3°C) during exercise, but no differences in subjective responses were found. The rest period had only a small influence on the subsequent thermal responses. The interindividual variations in thermal responses were large.     

Thermoregulatory changes induced during heat acclimatization by controlled hyperthermia in man

1. Heat acclimatization has been carried out by a controlled hyperthermia procedure, and induced thermoregulatory changes have been investigated for nine fit young men.2. During the experiments two types of thermal responses became apparent. The subjects tolerating the rise in the central temperature during hyperthermia displayed a marked improvement of their sweating capacity and their sweating performances, and a parallel shift towards lower body temperatures of the curves relating sweat rate to tympanic, rectal and mean skin temperatures.3. This shift occurred without any change in the slope, that is without any change in the gain of the central control system. It was accompanied by a concomitant shift of the threshold of sweating onset, so that for a given central temperature there was a higher sweat rate following treatment.4. The sweat decline appeared late in tolerant subjects. This decline was early in the intolerant subjects and appeared as early as the first day of heat treatment. Moreover, these subjects displayed no increase of sweat output with repeated sessions of heat treatment.5. The core-periphery temperature gradient measured during the cooling stage did not change in the tolerant subjects when acclimatization had developed. This gradient increased in the intolerant subjects because deep temperature decreased slowly whilst skin temperature decreased rapidly. The core of intolerant subjects retained the stored heat and dissipated it slowly. The tolerant subjects seemed on the contrary to display in the same conditions increased tissue conductance enabling rapid dissipation of the stored heat.     

Effect of skin temperature on the ion reabsorption capacity of sweat glands during exercise in humans

The effect of skin temperature on the ion reabsorption capacity of sweat glands during exercise in humans is unknown. In this study, eight healthy subjects performed a 60-min cycling exercise at a constant intensity (60% VO_2max) under moderate (25°C) and cool (15°C) ambient temperatures at a constant relative humidity of 40%. The sweating rate (SR), index of sweat ion concentration (ISIC) by using sweat conductivity, esophageal temperature (Tes), mean skin temperature, and heart rate (HR) were measured continuously under both ambient temperatures. The SR and ISIC were significantly lower at the cool ambient temperature versus the moderate temperature. There were no significant differences in the changes in HR and esophageal temperature between these ambient temperature conditions, while the mean skin temperature was significantly lower at the cool ambient temperature by almost 3°C (P<0.05). The slopes of the relationships between Tes and the SR and ISIC were significantly lower and the thresholds of these relationships were significantly higher at the cool ambient temperature (P<0.05). The ion reabsorption capacity of the sweat glands was significantly lower (P<0.05) in a cool environment (0.21±0.04 vs. 0.52±0.06 mg/cm²/min at 15 and 25°C, respectively) as evaluated using the relationships for SR and ISIC. The results suggest that the ion reabsorption capacity of the sweat glands is influenced by skin temperature during exercise in humans.     

Thermoregulatory stress during rest and exercise in heat in patients with a spinal cord injury

Summary Twelve subjects with spinal cord injuries and four controls (all male) were exposed to heat while sitting at rest or working at each of three environmental temperatures, 30, 35 and 40°C, with a relative humidity of 50%. Exercise was accomplished at a load of 50 W on a friction-braked cycle ergometer which was armcranked or pedalled. Functional electrical stimulation of the legs was provided to the subjects with quadriplegia and paraplegia to allow them to pedal a cycle ergometer. The data showed that individuals with quadriplegia had the poorest tolerance for heat. As an example, in this group, accomplishing armcrank ergometry while working at an environmental temperature of 40°C resulted in an increase in aural temperature of 2°C in 30 min. The aural temperature of individuals with paraplegia working for the same length of time under the same conditions rose approximately 1°C. There was virtually no change in the aural temperature in the control subjects.     

The relative influence of physical fitness,acclimatization state,anthropometric measures and gender on individual reactions to heat stress

Summary An experiment was set up to quantify the relative influence of fitness, acclimatization, gender and anthropometric measures on physiological responses to heat stress. For this purpose, 12 male and 12 female subjects were exposed to a neutral [ambient temperature (T _a) 21°C, relative humidity (r.h. 50%)], a warm, humid (T _a 34°C, r.h. 80%) and a hot, dry (T _a 45°C, r.h. 20%) climate at rest and at two exercise intensities [25%, and 45% maximal O₂ intake (VO_2max)], seated seminude in a net chair behind a cycle ergometer. Their physiological responses were recorded and the data submitted to a multiple regression analysis. It was shown that for the variance in heat storage, the percentage of body fat and the surface to mass ratio had relatively the largest influence of all the individual parameters, followed by VO_2max and the sweat rate versus increase in core temperature (total r ²=92%). For the skin temperature variation, the relative influence of individual parameters (sweat gain, VO_2max) was small. For body core temperatures, individual parameters had a large influence. The largest effect was due to the percentage of fat and the surface to mass ratio, followed by the sweating setpoint and, finally, VO_2max (total r ² = 54%–70%). For the variance in heart rate the VO_2max was the most relevant parameter, followed by the setpoint of the sweat rate:rectal temperature relationship (total r ²=88%). Blood pressure and skin blood flow predictions were also shown to improve by the addition of individual characteristics to the model. Body surface area, VO_2max and the sweating setpoint were shown to have a large influence but the proportion of the variance explained by these variables was too small (r ² < 70%) to use them as strain predictors, however. For all the predicted variables, it was shown that gender lost its influence, once VO_2max or anthropometric data were introduced into the prediction equation.     

The local training effect of secretory activity on the response of eccrine sweat glands

1. The influence of repeatedly raising the body temperature by radiant heat to a level at which acclimatization to heat is normally acquired was investigated in two series of experiments, the first without the subjects sweating, the second with sweating.2. In a second investigation local sweat-gland activity was induced by drug injections on successive days without raising the body temperature.3. These experiments show that the increased sweating capacity characteristic of acclimatization to heat is a result of sweat-gland activity and does not appear to be induced by or to depend on an elevated body temperature.4. Secretory activity results in a loss of glycogen from sweat-gland cells on the first day of heat exposure but not after the glands have been ;trained' by acclimatization to heat.5. The state of acclimatization has no influence on the threshold concentration of acetylcholine required to elicit sweating when injected intradermally.     

Sweating response in physically trained men to sustained handgrip exercise in mildly hyperthermic conditions

To investigate the effects of physical training on heat loss response to sustained handgrip exercise (non-thermal factors), we compared the sweating response during isometric handgrip exercise to mild hyperthermia in physically trained and untrained subjects. Seven trained and untrained male subjects (maximal oxygen uptake 62.7 +/- 2.4 and 42.7 +/- 1.6 mL kg-1 min-1, respectively, P < 0.05) performed isometric handgrip exercises at 20, 35 and 50% maximal voluntary contraction (MVC) for 60 s. The study was conducted in a climatic chamber with a regulated ambient temperature of 35 degrees C and relative humidity of 50% to induce sweating response at rest by rising skin temperature without a marked change in internal temperature. Sublingual and mean skin temperatures (thermal factors) in both trained and untrained groups were essentially constant throughout all exercise intensities. Changes in heart rate, mean arterial blood pressure, and rating of perceived exertion with increased exercise intensity were similar in both groups. Sweating rate (SR) on the limbs (mean value of forearm and thigh) was significantly greater in the trained group than in the untrained group at 50% MVC (P < 0.05). In addition, the slopes of the relationship between increased SR and exercise intensity (% MVC) on the trunk (chest) and limbs were significantly greater in the trained group than in the untrained group (P < 0.05). Our results suggest that the sweating response caused by non-thermal factors against a background of changing thermal factors was enhanced by physical training. It is also thought that the enhanced sweating response may be especially evident on the limbs than on the trunk, such as improvement of sweating response associated with thermal factors.     

Ambient temperature and the pituitary hormone responses to exercise in humans

Pituitary hormones have an important role during exercise yet relatively little is known about the stimulus for their release. Body temperature progressively increases during prolonged steady-state exercise in the heat and we have investigated the role that this may play in the release of prolactin, growth hormone and cortisol (as an indicator of adrenocorticotropic hormone) into the circulation. Fit young male subjects exercised at 73% V(O2,max) until volitional fatigue at 20 degrees C and at 35 degrees C (30% relative humidity at both temperatures). Rectal temperature and mean skin temperature were monitored and blood samples analysed for lactate, glucose, cortisol, growth hormone and prolactin concentrations. During the first 20 min, core temperature rose continuously and to a similar extent at both temperatures, while mean skin temperature was approximately 4 degrees C lower during exercise in the cool. Blood glucose concentration was essentially constant throughout the period of exercise while lactate concentration increased in the first 10 min and then remained constant with very similar changes in the two exercise conditions. Prolactin and growth hormone concentrations both increased during the exercise period while the concentration of cortisol declined slightly before rising slightly over the 40 min period. Prolactin release was significantly greater when exercise was carried out in the heat while there was no difference in the release of growth hormone or cortisol in the two conditions. When plotted as a function of rectal temperature, growth hormone concentration showed a linear relationship which was the same at ambient temperatures of 35 degrees C and 20 degrees C. Prolactin concentration had a curvilinear relationship with rectal temperature and this differed markedly at the two ambient temperatures. Cortisol concentration showed no dependence on any measure of body temperature. Our results are consistent with some aspect of body temperature being a stimulus for growth hormone and prolactin secretion; however, the precise mechanism clearly differs between the two hormones and we suggest that skin temperature modulates prolactin release, but does not affect the release of growth hormone.     

Sweating responses to a sustained static exercise is dependent on thermal load in humans

The purpose of this project was to test the hypothesis that internal temperature modulates the sweating response to sustained handgrip exercise. Ten healthy male subjects immersed their legs in 43 degrees C water for 30-40 min at an ambient temperatures of 30 degrees C and a relative humidity of 50%. Sweating responses to 50% maximal voluntary contraction isometric handgrip exercise (IH) were measured following the onset of sweating (i.e. following slight increases in internal temperature), and after more pronounced increases in internal temperature. Oesophageal temperature (Tes) was significantly lower during the first bout of exercise (37.54 +/- 0.07 degrees C) relative to the second bout (37.84 +/- 0.12 degrees C; P < 0.05). However, the increase in mean sweating rate (SR) from both the chest and forearm (non-glabrous skin) was significantly greater during the first IH bout relative to the second bout (P < 0.05). Increases in mean arterial blood pressure and palm SR (glabrous skin) did not differ significantly between exercise bouts, while heart rate and rating of perceived effort were significantly greater during the second bout of IH. As Tes and mean skin temperature did not change during either bout of exercise, the changes in SR from non-glabrous skin between the bouts of IH were likely because of non-thermal factors. These data suggest that sweating responses from non-glabrous skin during IH vary depending on the magnitude of thermal input as indicated by differing internal temperatures between bouts of IH. Moreover, these data suggest that the contribution of non-thermal factors in governing sweating from non-glabrous skin may be greatest when internal temperature is moderate (37.54 degrees C), but has less of an effect after greater elevations in internal temperature (i.e. 37.84 degrees C).     

Thermoregulatory adjustments during continuous heat exposure

Summary Body temperature regulation was studied in 6 male subjects during an acclimation procedure involving uninterrupted heat exposure for 5 successive days and nights in a hot dry environment (ambient temperature =35° C, dew-point temperature =7° C; air velocity = 0.2 m·s^–1). Data were obtained at rest and during exercise (relative mechanical workload =35% ). At rest, hourly measurements were made of oesophageal and 4 local skin temperatures, to allow the calculation of mean skin temperature, and of body motility and heart rate. During the working periods these measurements were made at 5 min intervals. Hourly whole-body weight loss was measured at rest on a sensitive platform scale while in the working condition just before starting and immediately after completing the bicycle exercise. The results show that, in both exercise and at rest, the successive heat exposures increased the sweat gland output during the first 3 days. Afterwards, sweat rate decreased without any corresponding change, in body temperature. For the fixed workload, the sweat rate decline was associated with a decrease in circulatory strain. Adjustments in both sweating and circulatory mechanisms occur in the first 3 days of continuous heat exposure. The overall sweat rate decline could involve a redistribution of the regional sweating rates which enhances the sweat gland activities of skin areas with maximal evaporative efficiencies.     

Identification of sudomotor activity in cutaneous sympathetic nerves using sweat expulsion as the effector response

Summary In a warm environment at ambient temperatures between 25° and 38°C (relative humidity 50%–60%) the relationship between sympathetic activity in cutaneous nerves (SSA) and pulses of sweat expulsion was investigated in five young male subjects. The SSA was recorded from the peroneal nerve using a microelectrode. Sweat expulsion was identified on the sweat rate records obtained from skin areas on the dorsal side of the foot, for spontaneous sweating and drug-induced sweating, using capacitance hygrometry. Sweat expulsion was always preceded by bursts of SSA with latencies of 2.4–3.0 s. This temporal relationship between bursts of SSA and sweat expulsion was noted not only in various degrees of thermal sweating but also in the sweating evoked by arousal stimuli, or by painful electric stimulation. The amplitude of the sudomotor burst was linearly related to the maximal rate of increase of the corresponding sweat expulsion, the amplitude of the expulsion and the integrated amount of sweat produced for the duration of the expulsion. The results provide direct evidence that sweat expulsion reflects directly centrally-derived sudomotor activity.     

Heat regulation during exercise with controlled cooling

Summary During heavy sustained exercise, when sweating is usually needed to dissipate the extra metabolic heat, controlled cooling caused heat loss to match total heat production with little sweating. The total heat produced and metabolic rate were varied independently by having subjects walk uphill and down. Heat loss was measured directly with a suit calorimeter; other measurements included metabolic energy from respiratory gas exchange and body temperatures. Thermoregulatory sweating was minimized by adjusting cooling in the calorimeter suit. Heat loss rose to match total heat, not metabolic rate, and there was a slow rise in rectal temperature. In the absence of major thermoregulatory response rectal temperature correlated most closely with total heat; it also correlated with the relative oxygen cost of exercise. Heat flow or heat content appeared to be the controlled variable and body temperature rise a secondary event resulting from thermal transport lag.     

Determination of the cooling capacity for body ventilation system

Body ventilation systems (BVS) are effective in reducing heat strain, but the amount of heat that a BVS removes from a human body is unclear. The purpose of this study was to propose a method for quantifying BVS cooling capacity using manikin evaluation and modeling. Cooling capacity was calculated as the product of maximum cooling potential and cooling efficiency. The maximum cooling potential is calculated as the difference in enthalpy between the air entering and exiting the BVS where the outlet air temperature is equal to skin temperature with a relative humidity of 100%. The cooling efficiency, defined as a ratio of the cooling capacity to the maximum cooling potential, can be determined through measurements on sweating thermal manikins. A BVS system was evaluated on a manikin with the ventilation fan ON (flow rate 4.7 L/s) or OFF under eleven ambient conditions. The measured cooling efficiencies were 0.31 ± 0.02 and almost constant. Using this cooling efficiency, the BVS cooling capacities at various skin temperature and ambient conditions were estimated. This two-step approach can be used to quantify BVS cooling effectiveness during physiology studies. First, the cooling efficiency is determined on sweating thermal manikins. Second, the cooling capacity is calculated from the skin temperature, ambient temperature and relative humidity. However, various factors may reduce the actual cooling provided by the BVS, and the calculated cooling capacity should be considered the upper limit for cooling.     

Comparison of thermoregulatory responses to exercise in dry heat among prepubertal boys, young adults and older males

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 (V(O2peak)) for 85 min (three 20 min bouts with three 7 min rest periods) in hot and dry conditions (41 +/- 0.67 degrees C, 21 +/- 1% relative humidity). During the exercise-in-heat protocol, rectal temperature (T(re)) skin temperatures (T(sk)), heart rate (HR), V(O2), V(CO2) V(E), 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 (DeltaS), 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 (DeltaS 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 degrees C, and the Y group needed 6.9 +/- 0.9 W (1 degrees C)(-1), the PP group required as much as 12.3 +/- 0.7 W to heat up their body core temperature by 1 degrees 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.     

Physiological significance of hydrophilic and hydrophobic textile materials during intermittent exercise in humans under the influence of warm ambient temperature with and without wind

The purpose of this present study was to compare the physiological effects of the hydrophilic and hydrophobic properties of the fabrics investigated in exercising and resting subjects at an ambient temperature of 30°C and a relative humidity of 50% with and without wind. Three kinds of clothing ensemble were tested: wool and cotton blend with high moisture regain (A), 100% cotton with intermediate moisture regain (B), 100% polyester clothing with low moisture regain (C). The experiments were performed using seven young adult women as subjects. They comprised six repeated periods of 10-min exercise on a cycle ergometer at an intensity of 40% maximal oxygen uptake followed by 5?min of rest (20?min for the last rest). The experiments comprised two sessions. During session I (first three repetitions of exercise and rest) the subjects were exposed to an indifferent wind velocity and during session II (last three repetitions of exercise and rest) they were exposed to a wind velocity of 1.5?m?·?s^?1. Rectal temperature and skin temperatures at eight sites, pulse rate and clothing microclimate were recorded throughout the whole period. The main findings can be summarized as follows: rectal temperature during session II was kept at a significantly lower level in A than in B and C. Clothing microclimate humidity at the chest was significantly lower in A than in B and C during session II. Skin and clothing microclimate temperatures at the chest were significantly lower in A than in B and C during session II. Pulse rate was significantly higher in C than in A and B during sessions I and II. It was concluded that the hydrophilic properties of the fabrics studied were of physiological significance for reducing heat strain during exercise and rest especially when influenced by wind.