Rectal temperature, distal sweat rate, and forearm blood flow following mild exercise at two phases of the circadian cycle

Changes in rectal temperature during mild exercise in the middle of the rising (11:00 h) and falling (23:00 h) phases of the circadian rhythm of resting core temperature have been compared. Seven healthy males were studied at rest, while exercising on a cycle ergometer (60 min at 80 W), and during the first 30 min of recovery. Rectal temperature, forearm blood flow, and forearm sweat rate were measured at 1 min intervals throughout. During exercise, there were significant time-of-day differences in the profiles of all three variables, and in the thresholds for increases in forearm blood flow and sweating. Forearm blood flow and sweat rate were recruited more rapidly and to a greater extent with evening exercise, and rectal temperature rose less. Analysis of covariance, with rectal temperature as the covariate, indicated the associations between it and forearm blood flow or sweating were significantly different (p<0.05) between the two times of day. There were also significant (p<0.05) time-of-day effects for forearm blood flow and sweating that were independent of rectal temperature. During recovery, rectal temperature fell more quickly in the late evening than late morning. Forearm blood flow and sweating also showed time-of-day differences, but these did not co-vary with rectal temperature. Control of rectal temperature during exercise and recovery appears to be more effective in the late evening than late morning, and differences in forearm blood flow and sweating, as well as factors independent of these two variables, contribute to this difference. The results support our "heat-gain/heat-loss modes" hypothesis.     

Rectal temperature,distal sweat rate,and forearm blood flow following mild exercise at two phases of the circadian cycle

Changes in rectal temperature during mild exercise in the middle of the rising (11:00 h) and falling (23:00 h) phases of the circadian rhythm of resting core temperature have been compared. Seven healthy males were studied at rest, while exercising on a cycle ergometer (60 min at 80 W), and during the first 30 min of recovery. Rectal temperature, forearm blood flow, and forearm sweat rate were measured at 1 min intervals throughout. During exercise, there were significant time‐of‐day differences in the profiles of all three variables, and in the thresholds for increases in forearm blood flow and sweating. Forearm blood flow and sweat rate were recruited more rapidly and to a greater extent with evening exercise, and rectal temperature rose less. Analysis of covariance, with rectal temperature as the covariate, indicated the associations between it and forearm blood flow or sweating were significantly different (p<0.05) between the two times of day. There were also significant (p<0.05) time‐of‐day effects for forearm blood flow and sweating that were independent of rectal temperature. During recovery, rectal temperature fell more quickly in the late evening than late morning. Forearm blood flow and sweating also showed time‐of‐day differences, but these did not co‐vary with rectal temperature. Control of rectal temperature during exercise and recovery appears to be more effective in the late evening than late morning, and differences in forearm blood flow and sweating, as well as factors independent of these two variables, contribute to this difference. The results support our “heat‐gain/heat‐loss modes” hypothesis.     

Sweating response to abrupt changes in work load

Changes in sweat rate on the palm and on the general body surface in response to stepwise increases and decreases in work load during exercise on a bicycle ergometer were examined in relation to body temperature and heart rate in six male subjects (three trained and three untrained), in an attempt to evaluate thermal and nonthermal factors responsible for those changes. In all the untrained subjects, a transient, marked increase in palmar sweat rate was observed upon an abrupt increase (and occasionally upon an abrupt decrease) in work, while an increase in sweat rate on the general body surface was also rapid and marked. On the other hand, in all the trained subjects, palmar sweat rate was low and hardly showed a substantial increase in response to an abrupt increase in work load, to which sweating on the general body surface responded slowly by a gradual increase. While sweat rate on the general body surface showed a significant correlation with esophageal temperature and with heart rate, palmar sweat rate was not correlated with esophageal temperature but was significantly correlated with heart rate. Moreover, repeated increases and decreases in work load often led to progressive weakening of palmar sweating due apparently to the development of habituation. The present results suggest that responses of sweating to stepwise changes in work load are not solely dependent upon the thermoregulatory mechanism but are affected considerably by increase and decrease in psychic excitement and/or those in discharges of the sympathetic nervous system accompanying changes in work load.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thermoregulatory responses to heat and exercise in Japanese and Caucasians

Responses to heat and exercise were studied in 9 male Japanese subjects who walked on a treadmill at a speed of 4.4 – 4.8 km/h at 0 grade for 2 hours in a climatic chamber in July 1973, in Nagoya Japan. The results were compared with those obtained in a similar study made in July 1966 in Cincinnati, Ohio. The following results were obtained: (1) Japanese showed a 1.8 times higher rate of sweating than Caucasians. Total sweat from the whole body during 2 hours walk was also higher in Japanese. (2) Japanese exhibited lower chloride concentration in local sweat than Caucasians in spite of their higher dietary salt intake, higher serum chloride concentration and higher rate of sweating. While in Caucasians the sweat chloride concentration showed a tendency to continue to rise during the later period of the walk in spite of decreasing sweat rate after sweat suppression occurred, in Japanese it tended to fall in parallel with the sweat rate. No difference was observed in the length of the latent time of sweat suppression. (3) There were no differences in rectal temperature or heart rate, both at the period of equilibrium rectal temperature and at the end of the walk. (4) Mean skin temperature during the walk was significantly higher in Japanese than in Caucasians. It was concluded that the Japanese group was better heat acclimatized than Caucasians, though the two groups were considered to have been naturally heat exposed by season to the same extent.     

Influence of training on sweating responses during submaximal exercise in horses.

Sweating responses were examined in five horses during a standardized exercise test (SET) in hot conditions (32-34 degrees C, 45-55% relative humidity) during 8 wk of exercise training (5 days/wk) in moderate conditions (19-21 degrees C, 45-55% relative humidity). SETs consisting of 7 km at 50% maximal O(2) consumption, determined 1 wk before training day (TD) 0, were completed on a treadmill set at a 6 degrees incline on TD0, 14, 28, 42, and 56. Mean maximal O(2) consumption, measured 2 days before each SET, increased 19% [TD0 to 42: 135 +/- 5 (SE) to 161 +/- 4 ml. kg(-1). min(-1)]. Peak sweating rate (SR) during exercise increased on TD14, 28, 42, and 56 compared with TD0, whereas SRs and sweat losses in recovery decreased by TD28. By TD56, end-exercise rectal and pulmonary artery temperature decreased by 0.9 +/- 0.1 and 1.2 +/- 0.1 degrees C, respectively, and mean change in body mass during the SET decreased by 23% (TD0: 10.1 +/- 0.9; TD56: 7.7 +/- 0.3 kg). Sweat Na(+) concentration during exercise decreased, whereas sweat K(+) concentration increased, and values for Cl(-) concentration in sweat were unchanged. Moderate-intensity training in cool conditions resulted in a 1.6-fold increase in sweating sensitivity evident by 4 wk and a 0.7 +/- 0.1 degrees C decrease in sweating threshold after 8 wk during exercise in hot, dry conditions. Altered sweating responses contributed to improved heat dissipation during exercise and a lower end-exercise core temperature. Despite higher SRs for a given core temperature during exercise, decreases in recovery SRs result in an overall reduction in sweat fluid losses but no change in total sweat ion losses after training.     

Heat regulation during exercise with controlled cooling

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.     

Functional differentiation of thermoregulation, with particular references to seasonal variation of whole body sweat rate of exercising man

We investigated the seasonal variation of sweating response during exercise. Four adult healthy men repeated a moderate bicycle exercise (60 watts) in a climatic chamber of an ambient temperature of 30 degrees C (relative humidity, 45%) in winter, spring, summer, and fall. In summer, sweat rate immediately increased as soon as the exercise started, whereas in winter in a few minutes. The mean sweat rate during exercise was significantly different between winter and summer. The transient reduction of the Tsk was observed at the beginning of the exercise in winter. The Tsk decreased in proportion to increasing of sweat rate in each season. Significantly negative correlations were found between sweat rate and the rate of change of Tsk during exercise in each season. The slope and intercept of regression line were significantly different between winter and summer. The index of sweating was made available for the relative value, changing rate against annual mean value of total sweat loss (delta SR, %). The relative value rather than the absolute value (i.e., expressed as g.m-2.h-1) corrected well with skin temperature. It is suggested that the present results may reflect adapted changes in the thermoregulatory mechanisms to seasonal acclimatization. Moreover, the fall in skin temperature during exercise may be not due to increased evaporative cooling, but may be the result of vasoconstriction probably caused by non-thermal factors.     

Dynamics of sweating in men and women during passive heating

The dynamics of sweating was investigated at rest in 8 men and 8 women. Electrical skin resistance (ESR), rectal temperature (Tre) and mean skin temperature (Tsk) were measured in subjects exposed to 40 degrees C environmental temperature, 30% relative air humidity, and 1 m X s-1 air flow. Sweat rate was computed from continuous measurement of the whole body weight loss. It was found that increases in Tre, Tsk and mean body temperature (Tb) were higher in women than in men by 0.16, 0.38 and 0.21 degrees C, but only the difference in delta Tb was significant (p less than 0.05). The dynamics of sweating in men and women respectively, was as follows: delay (td) 7.8 and 18.1 min (p less than 0.01), time constant (tau) 7.5 and 8.8 min (N.S.), inertia time (ti) 15.3 and 26.9 min (p less than 0.002), and total body weight loss 153 and 111 g X m-2 X h-1 (p less than 0.001). Dynamic parameters of ESR did not differ significantly between men and women. Inertia times of ESR and sweat rate correlated in men (r = 0.93, p less than 0.001), and in women (r = 0.76, p less than 0.02). In men, delta Tre correlated with inertia time of sweat rate (r = 0.81, p less than 0.01) as well as with the inertia time of ESR (r = 0.83, p less than 0.001). No relation was found between delta Tre and the dynamics of sweating in women. It is concluded that the dynamics of sweating plays a decisive role in limiting delta Tre in men under dry heat exposure. The later onset of sweating in women does not influence the rectal temperature increase significantly. In women, delta Tre is probably limited by a complex interaction of sweating, skin blood flow increase, and metabolic rate decrease.     

Thermoregulation in hyperhydrated men during physical exercise

The influence of hyperhydration on thermoregulatory function was tested in 8 male volunteers. The subjects performed cycle exercise in the upright position at 52% Vo2max for 45 min in a thermoneutral (Ta = 23 degrees C) environment. The day after the control exercise the subjects were hyperhydrated with tap water (35 ml X kg-1 of body weight) and then performed the same physical exercise as before. Total body weight loss was lower after hyperhydration (329 +/- 85 g) than during the control exercise (442 +/- 132 g), p less than 0.05. The decrease in weight loss after hyperhydration was probably due to a decrease in dripped sweat (58 +/- 64 and 157 +/- 101 g, p less than 0.05). With hyperhydration delay in onset of sweating was reduced from 5.8 +/- 3.2 to 3.7 +/- 2.0 min (p less than 0.05), and rectal temperature increased less (0.80 +/- 0.20 and 0.60 +/- 0.10 degrees C, p less than 0.01). The efficiency of sweating was higher in hyperhydrated (81.4%) than in euhydrated subjects (57.1%), p less than 0.01. It is concluded that hyperhydration influences thermoregulatory function in exercising men by shortening the delay in onset of sweating and by decreasing the quantity of dripped sweat. As a result, the increases in body temperature in hyperhydrated exercising men are lower than in normally hydrated individuals.     

Body heat balance in man subjected to endogenous and exogenous heat load

The body heat balance, measured by a thermometric method, was investigated in humans subjected to endogenous and exogenous heat load. The purpose of the present study was to test the concept of heat exchange by a servomechanism in human thermoregulation. Two series of experiments were performed on male volunteers. In series I 15 subjects performed physical exercise (50% VO2 max) for 60 min at a constant ambient temperature of 25 degrees C. In series II 16 subjects rested in a climatic chamber where the ambient temperature was elevated over 30 min from 22 to 42 degrees C and kept stable at this level during the subsequent 60 min. It was found that in both series of experiments the sweating rate followed an exponential curve exhibiting an inertial course. Heat was stored in the body mainly at the beginning of experiment. In series I the net body heat load of 125 W/m2 was equalized by sweat evaporation, beginning after 40 min of the exercise. In series II the net body heat load of 80 W/m2 was equalized in the same way, starting after 35 min of the constant high ambient temperature. In both series of experiments the amount of heat stored in the body calculated from the body heat balance was quite close to the amount of heat calculated from the calorimetric equation. It is concluded, that under the present experimental conditions, heat loss from the body by sweat evaporation seems to be a regulated variable in the human thermoregulatory system. The observed increase in rectal temperature may result from an inertial course of the sweating reaction.     

Importance of dynamics of sweating in men during exercise

Influence of dynamics of sweating on rectal temperature increase was tested in 3 groups of men performing cycle exercise with intensity of 65, 90 and 120 W, respectively, in 22 degrees C chamber temperature and 30% of relative air humidity. During exercise at 65 and 90 W the subjects wore suits while exercising with intensity of 120 W they wore only shorts. The dynamics of sweating was described by delay in onset of sweating and time constant of the reaction. Wearing caused significant increase in skin humidity and decreased evaporative rate of sweating. Sweat rate during steady state was related to the metabolic rate in naked (r = 0.89, p less than 0.002) as well as in wearing subjects (r = 0.93, p less than 0.01). Delay in onset of sweating was, in average, 5 min with a time constant of 7 min. Both factors showed a tendency to be shorter with increasing work intensity. Mean increase in rectal temperature was proportional to the intensity of exercise although the individual delta Tre correlated well with the dynamics of sweating in naked (r = 0.83, p less than 0.01) and wearing subjects (r = 0.84, p less than 0.01). Since delta Tre was smaller in subjects with shorter inertia time of sweating in response to beginning of exercise at the same intensity it is concluded that the dynamics of sweating can play an important role in limiting body temperature increase in working men.     

Thermoregulatory adjustments during continuous heat exposure

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 degrees C, dew-point temperature = 7 degrees C; air velocity = 0.2 m.s-1). Data were obtained at rest and during exercise (relative mechanical workload = 35% VO2max). 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.     

Correlation between heat tolerance during exercise and maximum aerobic work capacity

Observation of the physiological responses during exercise in a hot environment and measurement of maximal work capacity were made on eight young male subjects, ages 20--22. Exercise was performed on a bicycle ergometer at a constant work load of 450 kg . m/min at a cycling rate of 50 rpm for 30 min in a climatic chamber at 30 degree C with 70% relative humidity. The maximum work capacity was measured by bicycle ergometer exercise. Heat tolerance during exercise was assessed by the magnitude of physiological strain expressed by the combination of relative rise in rectal temperature, relative water loss and relative salt loss. Heat load during exercise was calculated using metabolic rates at rest and during exercise, assuming heat loss through the respiratory tract to be 10 percent of metabolic rate. Fairly good correlations were found between the ratio of work done to maximum work capacity and rise in rectal temperature, ratio of body weight loss to body weight and heat tolerance during exercise. Close correlations were found among relative heat load during exercise and rise in rectal temperature, relative body weight loss and heat tolerance. Heat tolerance during exercise in a hot environment correlated well to capacity of heat dissipation and maximum work capacity.     

Physiological reaction of women during exercise and recovery, in a comfortable environment and a hot environment

Physiological reaction and oxygen intake during exercise and recovery were measured in fourteen young female Japanese during the follicular phase of their menstrual cycle at 25 degree C with 50% relative humidity and at 35 degree C with 50% relative humidity. Subjects, clad in bathing suits only, performed a bicycle ergometer exercise at a constant work load of 600 kg . m/min at a cycling rate of 50 rpm for 20 min and recovered while remaining on the bicycle ergometer for 40 min. The mean values of sweat volume and skin temperature were significantly greater at 35 degree C than at 25 degree C. It has been shown that heart rate and rectal temperature during exercise were slightly higher at 35 degree C than at 25 degree C, while those during recovery were significantly higher at 35 degree C than at 25 degree C. Oxygen intake, oxygen debt, and the fall in diastolic blood pressure after exercise were considerably greater at 35 degree C than at 25 degree C. The increase in oxygen intake in a hot environment might result from an increased metabolism due to higher body temperature and increased energy requirement for heat dissipation such as profuse sweating, higher heart rate, and increased ventilatory volume. The increase in oxygen debt in a hot environment might reflect the increased metabolism caused by higher body temperature and the increased production of lactic acid in the working muscle as a result of an insufficient blood supply to the muscle. The increases in sweat volume, oxygen intake during exercise, and oxygen debt in women in a hot environment were considerably smaller than corresponding values for men. The smaller increase in sweat volume in women in a hot environment could reflect a smaller oxygen intake and a more marked dilation of skin vessels in women than in men.     

Temperature and sweating responses in one-legged and two-legged exercise.

In looking at the thermoregulatory responses resulting from symmetrical or asymmetrical exercise, this paper has focused on the effect of local skin temperature (Tsk,local) on local sweat rates (msw,local) during one-legged (W1) and two-legged (W2) exercise on an ergocycle. Five subjects underwent four 3-h tests at 36 degrees C, each consisting of six 25-min exercise periods alternating with 5-min rest periods. The subjects performed W1 and W2 at 45 and 90 W, respectively, either dehydrated or rehydrated. Body temperatures and total sweat rate were measured as well as four msw,local (on chest and thighs), assessed from sweat capsules under which Tsk,local was maintained at predetermined levels (37.0 degrees C and 35.5 degrees C). The combinations of Tsk,local levels, capsule locations, exercise intensity and hydration level chosen in our protocol led to the following results. The hydration level affected rectal temperature but not total or msw,local. No specific effect of muscle activity was found; msw,local on thighs of resting and working legs were similar. The msw,local were only influenced by exercise intensity, msw,local being more elevated during the higher intensity. No significant effect of Tsk,local on msw,local was found, whatever the experimental condition and/or the location. It was concluded that local thermal effects on msw,local could have been masked by the strong central drive for sweating which has been found to exist in subjects exercising in a warm environment.     

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

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 (Ta) 21 degrees C, relative humidity (r.h. 50%)], a warm, humid (Ta 34 degrees C, r.h. 80%) and a hot, dry (Ta 45 degrees C, r.h. 20%) climate at rest and at two exercise intensities [25%, and 45% maximal O2 intake (VO2max)], 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 VO2max and the sweat rate versus increase in core temperature (total r2 = 92%). For the skin temperature variation, the relative influence of individual parameters (sweat gain, VO2max) 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, VO2max (total r2 = 54%-70%). For the variance in heart rate the VO2max was the most relevant parameter, followed by the setpoint of the sweat rate:rectal temperature relationship (total r2 = 88%). Blood pressure and skin blood flow predictions were also shown to improve by the addition of individual characteristics to the model.(ABSTRACT TRUNCATED AT 250 WORDS)     

Primate models to study eccrine sweating

The histochemistry and histology of the eccrine sweat gland in the rhesus monkey (Macaca mulatta) are described. The histochemical distribution and localization of enzymes and substrates are very similar to those found in the human; innervation is cholinergic. Active eccrine glands on the general body surface average 136 glands/cm². Above the thermal neutral zone (TNZ), sweating is the major avenue for heat loss and the role of panting in dissipating heat is relatively insignificant. The intrahypothalamic administration of prostaglandin E₁ (PGE₁) suppresses sweating and leads to an increase in core temperature. A linear relation is found between local sweat rates on the general body surface and clamped hypothalamic temperature. Studies also provide direct support for the concept that brain temperature and skin temperature interact additively in the control of sweating in higher primates. The functional characteristics of eccrine sweating in the patas monkey (Erythocebus) are qualitatively similar to those in the rhesus monkey. The patas monkey maintains a relatively constant rectal temperature (37.6–38.4°C) when equilibrated to a wide range of ambient temperaures of 15–40°C. Eccrine sweating is the main effector system for heat dissipation above the TNZ. We emphasize here that evaporative heat loss that is due to sweating is related to both mean skin and mean body temperature and at 40°C is 40% higher than that recorded from the rhesus monkey. These results indicate that the patas monkey, because of its high sweating capacity and other similarities with the human eccrine system, is a most appropriate animal model for comparative studies of eccrine sweat gland function in primates in general.     

Thermoregulatory responses of lower limb amputees during exercise in a hot environment

Lower limb amputees (LLAs) have less skin surface required for sweating; thus, the ability to dissipate heat from the body may decrease and the risk of heat illness may increase during exercise in a hot environment. However, no study has compared the thermoregulatory responses during exercise between LLAs and able-body (AB) individuals with different body surface areas. This study aimed to compare the thermoregulatory responses of LLAs with those of AB individuals during exercise in a hot environment. Seven LLAs (LLA group) and 7 able-body individuals (AB group) participated in the study. A 60% peak power output of arm crank upper-body exercise was performed for 60 min in a hot environment (32 °C, 50% relative humidity). There was no difference in the increase in rectal temperature (LLA: 0.8 ± 0.2 °C, AB: 0.8  ± 0.2 °C) and mean skin temperature between the groups during the 60-min exercise. In the LLA group, the accumulated local sweat rate of the thigh during exercise was significantly higher on the non-cut side than on the cut side (64.6 ± 43.0 mg/h vs. 37.0 ± 27.2 mg/h, p < 0.05). The total sweat rate was significantly higher in the LLA group than in the AB group (1.18 ± 0.37 kg/h vs. 0.84 ± 0.10 kg/h, p < 0.05). Thermal sensation and comfort were lower in the LLA group than in the AB group. Different heat loss responses were observed in the AB and LLA groups during exercise in the heat. The LLA group compensates for sweating on the cut side due to an increase in sweat loss on the intact limb, thereby preserving appropriate thermoregulation during exercise.     

Equine sweating responses to submaximal exercise during 21 days of heat acclimation.

This study examined sweating responses in six exercise-trained horses during 21 consecutive days (4 h/day) of exposure to, and daily exercise in, hot humid conditions (32-34 degrees C, 80-85% relative humidity). On days 0, 3, 7, 14, and 21, horses completed a standardized exercise test on a treadmill (6 degrees incline) at a speed eliciting 50% of maximal O(2) uptake until a pulmonary artery temperature of 41.5 degrees C was attained. Sweat was collected at rest, every 5 min during exercise, and during 1 h of standing recovery for measurement of ion composition (Na(+), K(+), and Cl(-)) and sweating rate (SR). There was no change in the mean time to reach a pulmonary artery temperature of 41.5 degrees C (range 19.09 +/- 1.41 min on day 0 to 20.92 +/- 1.98 min on day 3). Peak SR during exercise (ml. m(-2). min(-1)) increased on day 7 (57.5 +/- 5. 0) but was not different on day 21 (48.0 +/- 4.7) compared with day 0 (52.0 +/- 3.4). Heat acclimation resulted in a 17% decline in SR during recovery and decreases in body mass and sweat fluid losses during the standardized exercise test of 25 and 22%, respectively, by day 21. By day 21, there was also a 10% decrease in mean sweat Na(+) concentration for a given SR during exercise and recovery; this contributed to an approximately 26% decrease in calculated total sweat ion losses (3,112 +/- 114 mmol on day 0 vs. 2,295 +/- 107 mmol on day 21). By day 21, there was a decrease in sweating threshold ( approximately 1 degrees C) but no change in sweat sensitivity. It is concluded that horses responded to 21 days of acclimation to, and exercise in, hot humid conditions with a reduction in sweat ion losses attributed to decreases in sweat Na(+) concentration and SR during recovery.     

Effect of exercise intensity on the postexercise sweating threshold.

The hypothesis that the magnitude of the postexercise onset threshold for sweating is increased by the intensity of exercise was tested in eight subjects. Esophageal temperature was monitored as an index of core temperature while sweat rate was measured by using a ventilated capsule placed on the upper back. Subjects remained seated resting for 15 min (no exercise) or performed 15 min of treadmill running at either 55, 70, or 85% of peak oxygen consumption (V(o2 peak)) followed by a 20-min seated recovery. Subjects then donned a liquid-conditioned suit used to regulate mean skin temperature. The suit was first perfused with 20 degrees C water to control and stabilize skin and core temperature before whole body heating. Subsequently, the skin was heated ( approximately 4.0 degrees C/h) until sweating occurred. Exercise resulted in an increase in the onset threshold for sweating of 0.11 +/- 0.02, 0.23 +/- 0.01, and 0.33 +/- 0.02 degrees C above that measured for the no-exercise resting values (P < 0.05) for the 55, 70, and 85% of V(o2 peak) exercise conditions, respectively. We did note that there was a greater postexercise hypotension as a function of exercise intensity as measured at the end of the 20-min exercise recovery. Thus it is plausible that the increase in postexercise threshold may be related to postexercise hypotension. It is concluded that the sweating response during upright recovery is significantly modified by exercise intensity and may likely be influenced by the nonthermal baroreceptor reflex adjustments postexercise.