Local sweating responses during recovery sleep after sleep deprivation in humans

Changes in the central control of sweating were investigated in five sleep-deprived subjects (kept awake for 40 h) during their recovery sleep under warm ambient conditions [operative temperature (T _o) was either 35 or 38° C]. Oesophageal (T _oes) and mean skin (T _sk) temperatures, chest sweat rate (m _sw,ch), and concomitant electro-encephalographic data were recorded. Throughout the night at 35 or 38° C T _o, m _sw,ch changes were measured at a constant local chest skin temperature (T _ch) of 35.5° C. The results showed that body temperatures (T _oes and T _sk) of sleep-deprived subjects were influenced by thermal and hypnogogic conditions. The m _sw,ch levels correlated positively with T _oes in the subjects studied during sleep stage 1–2 (light sleep: LS), sleep stage 3–4 (slow wave sleep: SWS) and rapid eye movement (REM) sleep. Contrary to what has been reported in normal sleep, firstly, the T _oes threshold for sweating onset differed between REM sleep and both LS and SWS, and, secondly, the slopes of the m _sw,ch versus T _oes relationships were unchanged between REM and non-REM (i.e. LS or SWS) sleep. The changes observed after sleep deprivation were hypothesized to be due to alterations in the functioning of the central nervous system controller.     

Thermoregulation during heat exposure of young children compared to their mothers

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

Control of heat-induced cutaneous vasodilatation in relation to age

Summary Well matched unacclimatised older (age 55–68, 4 women, 2 men) and younger (age 19–30, 4 women, 2 men) subjects performed 75 min cycle exercise (40% ) in a hot environment (37°C, 60% rh). Rectal temperature (T _re), mean skin temperature (¯T _sk), arm blood flow (ABF, strain gauge plethysmography), and cardiac output (Q, CO₂ rebreathing) were measured to examine age-related differences in heat-induced vasodilatation.T _re and¯T _sk rose to the same extent in each group during the exposure. There was no significant intergroup difference in sweat rate (older: 332±43 ml · m^–2 · h^–1, younger: 435±49 ml · m^–2 · h^–1; mean±SEM). However, the older subjects responded to exercise in the heat with a lower ABF response which could be attributed to a lower for the same exercise intensity. The slope of the ABF-T _re relationship was attenuated in the older subjects (9.3±1.3 vs 17.9±3.3 ml · 100 ml^–1 · min^–1 · °C^–1,p <0.05), but theT _re threshold for vasodilatation was about 37.0°C for both groups. These results suggest an altered control of skin vasodilatation during exercise in the heat in older individuals. This attenuated ABF response appears to be unrelated to , and may reflect an age-related change in thermoregulatory cardiovascular function.     

Skin AVA and capillary dilatation and constriction induced by local skin heating

In conscious sheep, total femoral blood flow and flow through arteriovenous anastomoses (AVAs) and capillaries (CAP) in skin of the hindleg were measured employing electromagnetic and radioactive microsphere techniques. Core temperature (T _c) was manipulated using intravascular heat exchangers and hindleg skin temperature (T _sk) was manipulated by immersion in temperature controlled water. WithT _c set 1°C above normal, AVA flow was highest at the lowestT _sk tested (34°C); AVAs progressively constricted asT _sk was increased from 34 to 40–41°C, then dilated again asT _sk reached the highest levels tested (42–44°C). Skin CAP flow was not altered byT _sk of 34 to 42°C but was increased at aT _sk of 44°C. Therefore total skin blood flow followed essentially the same pattern as AVA flow; total femoral flow also followed this pattern. WhenT _c was set 0.5°C below normal, AVA flow was low at all levels ofT _sk. It is concluded thatT _c plays a dominant role in control of skin blood flow, however, onceT _c is at a level requiring increased heat loss,T _sk exerts an extremely potent influence on the nature and magnitude of changes in skin blood flow. The pattern of flow changes appears to reflect principally a negative feedback mechanism aimed at maintainingT _sk at approximately 40°C; this may contrast with mechanisms associated with sweating and/or active vasodilation in other species.     

Enhancement of finger blood flow response of postprandial human subjects to the increase in body temperature during exercise

Summary The present study was performed to investigate the effect of food intake on thermoregulatory vasodilatation in seven healthy male volunteers. The changes in oesophageal (T _oes) and mean skin temperatures, finger and forearm blood flows (BF), oxygen consumption (VO₂) and heart rate (f _c) with and without food intake were measured before and during a 40-min exercise at an intensity of 35% maximal O₂ consumption at an ambient temperature of 25°C. Exercise commenced 60 min after food intake. Ingestion of food equivalent to 50.2 kJ · kg body mass^–1 elevated mean body temperature, BF,VO₂ andf _c in 60 min. Four subjects responded to exercise with a marked increase in finger BF and with no sweating (non-sweating group), while the other three responded with perspiration over almost the whole skin area and with little change in finger BF. Further analyses were made mainly in the non-sweating group. The postprandial increases inT _oes, BF,VO₂ andf _c were persistent during exercise. The rate of increase in finger BF with the increase inT _oes and mean body temperature was significantly greater with food intake than without. However, there was no difference in the response of forearm BF to exercise between the two conditions. These results suggested that food intake enhanced finger BF response to the increase in deep body temperature during exercise. It was also concluded that there was a regional difference in cutaneous vasomotor response to thermal load in the post-prandial subjects.     

Thermal tolerance following artificially induced polycythaemia

Polycythaemia has been shown to improve physical performance, possibly due to increased arterial oxygen transport. Enhanced thermoregulatory function may also accompany this manipulation, since a greater proportion of the cardiac output becomes available for heat dissipation. We further examined this possibility in five trained men, who participated in three-phase heat stress trials (20 min rest, 20 min cycling at 30% peak power W_peak and 20 min at 45% W_peak at 38.3 (SEM 0.7)°C [relative humidity 41.4 (SEM 2.9)%]. Trials were performed during normocythaemia (control) and polycythaemia, obtained by reinfusion of autologous red blood cells and resulting in significant elevation of arterial oxygen transport. During the polycythaemic trials, the subjects demonstrated diminished thermal strain, as evidenced by a significant reduction in cardiac frequency (f _c: 12 beats · min^–1 lower throughout the test;P < 0.05), and reduced auditory canal temperatures (T _ae) during the latter 20-min phase (P < 0.05). Forearm sweat onset was more rapid (363.0 compared to 1083.0 s;P < 0.05), and forearm sweat rate (. _msw) sensitivity was elevated from 1.80 to 2.91 · mg · cm^–2 · min^–1 · °C^–1 (P < 0.05). Foreheadm _sw was depressed during the final 20 min, while forearmm _sw was greater during all test phases, averaging 0.94 and 1.20 mg · cm^–2 · min^–1, respectively, over the 60 min. Skin blood flows for the upper back, upper arm and forearm were reduced (P < 0.05). Polycythaemia enhanced thermoregulation, through an elevation in forearm sweat sensitivity and.m _sw, but not via increased cutaneous blood flow. These modifications occurred simultaneously with decreases inf _c andT _ae, resulting in greater thermal tolerance.     

Effects of training and acclimation on heat tolerance in exercising men wearing protective clothing

This study examined the effectiveness of endurance training and heat acclimation in reducing the physiological strain imposed by exercising in the heat while wearing protective clothing. Seven young men underwent 8 weeks of physical training [60–80% maximal aerobic power (VO_2max) for 30–45 min · day^–1, 3–4 days · week^–1 at < 25° C] followed by 6 days of heat acclimation (45–55% VO_2max for 60 min · day^–1 at 40° C, 30% relative humidity). Nine other young men underwent corresponding periods of control observation and heat acclimation. Before and after each treatment, subjects completed a treadmill walk (4.8 km · h^–1, 2% grade) in a climatic chamber (40° C, 30% relative humidity), wearing in turn normal combat clothing or clothing protecting against nuclear, biological, and chemical (NBC) agents. Criteria for halting this test were: (1) a rectal temperature (T _re) of 39.3° C; (2) a heart rate (f _c) 95% of the subject's observed maximum, maintained for 3 min; (3) unwillingness of the subject to continue; (4) the elapse of 120 min. The training regimen increased mean VO_2max by 16% and mean plasma volume by 8%. When tested in normal combat clothing, the rates of increase in T _re and f _c were slower after training. However, when wearing NBC protective clothing, the only significant change induced by training was a higher mean skin temperature (T _sk) in the early part of the test. Heat acclimation increased the mean plasma volume of untrained subjects by 8%, but their VO_2max remained unchanged. When tested in normal combat clothing, acclimation decreased their mean values of T _re, T _sk, f _c, and metabolic rate. When wearing NBC protective clothing, the only significant decrease after acclimation was in overall T _re. In trained subjects, heat acclimation induced no further improvement in any physiological variable when wearing normal combat clothing, but reduced overall T _re and T _sk when wearing NBC protective clothing. Training- or acclimation-induced increases of sweat secretion (an average increment of 0.14–0.23 kg · h^–1) were not accompanied by any statistically significant increase in sweat evaporation when wearing NBC protective clothing. Moreover, tolerance times were unchanged in either normal combat (116–120 min) or NBC protective clothing (47–52 min). We conclude that neither endurance training nor heat acclimation do much to improve exercise tolerance when wearing NBC protective clothing in hot environments, because any added sweat secretion decreases blood volume and increases discomfort without augmenting body cooling.     

Effect of pyridostigmine on the exercise-heat response of man

Summary The effect of pyridostigmine on thermoregulatory responses was evaluated during exercise and heat stress. Eight heat acclimated, young adult male subjects received four doses of pyridostigmine (30 mg) or identical placebo tablets every 8 h, in a double blind, randomized, cross-over trial. A 30.3%, SD 4.6% inhibition of the circulating cholinesterase (ChE) activity was induced in the pyridostigmine-treated group. The subjects were exposed to 170-min exercise and heat-stress (dry bulb temperature, 33° C; relative humidity 60%) consisting of 60 min in a sitting position and two bouts of 50-min walking (1.39 m · s^–1, 5% gradient) which were separated by 10-min rest periods. No differences were found between treatments in the physiological responses and heat balance parameters at the end of exposure: heart rate (f _c) was 141 beats · min^–1, SD 16 and 150 beats · min^–1, SD 12, rectal temperature (T _re) was 38.5°C, SD 0.4° and 38.6°C, SD 0.3°, heat storage was 60 W · m^–2, SD 16 and 59 W · m^–2, SD 15 and sweat rate was 678 g · h^–1, SD 184 and 661 g · h^–1, SD 133, in the pyridostigmine and placebo treatments, respectively. The changes in T _re and f _c over the heat-exercise period were parallel in both study and control groups. Pyridostigmine caused a slight slowing of f _c (5 beats·min^–1) which was consistent throughout the entire exposure (P<0.001) but was of no clinical significance. The overall change in f_c was similar for both groups. We have concluded that pyridostigmine administration, in a dose sufficient to induce a moderate degree of ChE inhibition, does not significantly affect performance of exercise in the heat.     

Thermoregulatory responses to intermittent exercise are influenced by knit structure of underwear

Summary The purpose of this study was to evaluate the role of knit structure in underwear on thermoregulatory responses. Underwear manufactured from 100% polypropylene fibres in five different knit structures (1-by-1 rib, fleece, fishnet, interlock, double-layer rib) was evaluated. All five underwear prototypes were tested as part of a prototype clothing system. Measured on a thermal manikin these clothing systems had total thermal resistances of 0.243, 0.268, 0.256, 0.248 and 0.250 m² · K · W^–1, respectively (including a value for the thermal resistance of the ambient environment of 0.104 m² · K · W^–1). Human testing was done on eight male subjects and took place at ambient temperature (T _a)=5°C, dew point temperature (T _dp)=–3.5° C and air velocity (V _a)=0.32 m · s^–1. The test comprised a repeated bout of 40-min cycle exercise (315 W · m^–2; 52%, SD 4.9% maximal oxygen uptake) followed by 20 min of rest (62 W · m^–2). The oxygen uptake, heart rate, oesophageal temperature, skin temperature,T _a,T _dp at the skin and in the ambient air, onset of sweating, evaporation rate, non-evaporated sweat accumulated in the clothing and total evaporative loss of mass were measured. Skin wettedness was calculated. The differences in knit structure of the underwear in the clothing systems resulted in significant differences in mean skin temperature, local and average skin wettedness, non-evaporated and evaporated sweat during the course of the intermittent exercise test. No differences were observed over this period in the core temperature measurements.The views, opinions and/or findings in this report are those of the authors and should not be construed as an official Department of the Army position, policy, or decision, unless so designated by other official documentation     

Perception of effort during high-intensity exercise at low,moderate and high wet bulb globe temperatures

The purpose of this study was to determine the effect of low, moderate and high wet bulb globe temperatures (T _wbg) on cardiovascular variables and ratings of perceived exertion (RPE) during moderately prolonged, high-intensity exercise. Six subjects [four men and two women; mean (SD) age, 22.0 (1.2) years; maximum oxygen consumption ({ie519-1}), 51.0 (8.4) ml · kg^–1 · min^–1] completed 30 min of exercise (80% {ie519-2}) on a cycle ergometer at low [14.7 (2.1)°C], moderate [21.0 (1.5)° C], and high [27.4 (2.3)° C]T _wbg. Two additional subjects completed 20 min of exercise in the high temperature condition, but completed 30 min in the moderate and lowT _wbg. Heart rate (f _c), blood pressure, blood lactate (La), mean skin temperature ( _sk), , and RPE were measured at 10, 20 and 30 min. Results showed thatf _c, rate pressure product, RPE, pulmonary ventilation and ventilatory equivalent for oxygen increased (P < 0.05) across time for all conditions, while decreased across time. _sk andf _c were significantly greater across time in the high condition [35.9 (0.65)° C; 176 (12.6) beats · min^–1] compared to the moderate [34.6 (1.5)° C; 170 (17.2) beats · min^–1] and the low condition [31.7 (1.5)° C; 164 (17.1) beats-min^–1]. However, there were no differences throughout exercise in RPE [high,.16.2 (2.0); moderate, 16.4 (2.2); low, 16.3 (1.9)] and across the conditions. These data suggest that RPE is closely related to metabolic intensity but is not a valid indicator of cardiovascular strain during exercise in highT _wbg conditions.     

Gender differences in physiological reactions to thermal stress

Following an extensive anthropometric evaluation, thermoregulatory responses were studied in nine men and nine women who performed immersed exercise with post-exercise rest in 28°C water. During the post-exercise period esophageal temperature (T _es), oxygen consumption, heat flux and skin blood perfusion were monitored at 10s intervals, with average minute values used for calculations. The T _es (relative to restingT _es) at which sweating abated and shivering commenced were defined as the T _es thresholds for the cessation of sweating and onset of shivering, respectively. No significant gender differences were evident in the sweating and shivering threshold T _es values, or the magnitude of the null-zone. Usingz-tests for parallelism the rates of core cooling across the null-zone were not found to differ significantly between genders, nor were the slopes of the perfusion: T _es responses across the null-zone or the post-threshold shivering responses (ml·kg^–1·min^–1·°C^–1). The slope of the sweating response (measured from immersion until sweat cessation; g·m^–2·min^–1°C^–1) was, however, significantly lower in the female than in the male samples (z = 3.93;P < 0.01). Despite the gender-related dimorphic distribution of adipose tissue, both men and women lost equal proportions of their total heat flux from central and peripheral measurement sites. Performing a standardized regression using the rate of core cooling across the null-zone as the dependent variable and gender as a dummy variable, gender and adipose tissue mass were not found to be significant factors in determining the rate of core cooling, while mass ( = 1.73;P < 0.05) and muscle mass ( = 1.86;P < 0.05) did contribute significantly to the rate of core cooling. It was concluded that, except for the quantitative differences in the sweating response, men and women respond to deviations in core temperature in a similar manner, with mass and muscle mass modifying this response.     

Is exhaustive training adequate preparation for endurance performance?

Summary Our purpose was to test the significance of exhaustive training in aerobic or endurance capacity. The extent of adaptations to endurance training was evaluated by assessing the increase in physical performance capability and oxidative markers in the organs of rats trained by various exercise programs. Rats were trained by treadmill running 5 days · week^–1 at 30 m · min^–1 for 8 weeks by one of three protocols:T ₁ — 60 min · day^–1;T ₂ — 120 min · day^–1; andT ₃ — 120 min · day^–1 (3 days · week^–1) and to exhaustion (2 days · week^–1). GroupsT ₂ andT ₃ ran for longer thanT ₁ in an endurance exercise test (P<0.05), in which the animals ran at 30 m · min^–1 to exhaustion; no difference was observed between groupsT ₂ andT ₃. All 3 trained groups showed a similar increase (20–27%) in the fast-twitch oxidative-glycolytic (FOG) fibers with a concomitant decrease in the fast-twitch glycolytic (FG) fiber population in gastrocnemius (p<0.05). The capillary supply in gastrocnemius increased with the duration of exercise (p<0.05): no difference was found between groupsT ₂ andT ₃. Likewise, no distinction was seen between groupsT ₂ andT ₃ in the increase in succinate dehydrogenase activity in gastrocnemius and the heart. These results suggest that the maximal adaptive response to endurance training does not require daily exhaustive exercise.     

The effect of diurnal variation on the regional differences in sweating and skin blood flow during exercise

The aim of the present study was to examine changes in the control of heat-dissipation responses to exercise associated with the diurnal variation in core temperature from the viewpoint of the regional response patterns. We studied seven men during exercise on a cycle ergometer at 100 W for 40 min at 25°C at 0630 (morning) 1630 (evening) hours on 2 separate days. Oesophageal temperature (T _oes), local skin temperature, local sweating rate ( ) on the forehead, back, forearm and thigh, and skin blood flow by laser Doppler flowmeter (LDF) on the back and forearm were measured continuously. TheT _oes at rest was significantly higher in the evening than in the morning, the difference averaging approximately 0.4°C (P < 0.05). TheT _oes thresholds for each site in and that for back in LDF were significantly different between the two times of day (P < 0.05). The change inT _oes thresholds for sweating and vasodilatation for morning and evening were similar toT _oes at rest. Although on the forehead was significantly higher in the morning than in the evening, on the back was significantly higher in the evening than in the morning (P < 0.05). Total local sweating rate ( ) for each site during exercise was significantly higher on the forehead than on the forearm in the morning, and on the back than on the forearm in the evening, respectively (P < 0.05). The results would suggest that the diurnal variation of heat-dissipation responses to exercise is influenced not only by a central controlling mechanism but also by changes in the regional differences.     

Exercise thermoregulation after 6 h of chair rest, 6° head-down bed-rest,and water immersion deconditioning in men

The purpose was to investigate the mechanism for the excessive exercise hyperthermia following deconditioning (reduction of physical fitness). Rectal (T _re) and mean skin ( ) temperatures and thermoregulatory responses were measured in six men [mean (SD) age, 32 (6) years; mass, 78.26 (5.80) kg; surface area, 1.95 (0.11)m²; maximum oxygen uptake ( ), 48 (6) ml·min^–1·kg^–1; whilst supine in air at dry bulb temperature 23.2 (0.6)°C, relative humidity 31.1 (11.1)% and air speed 5.6 (0.1) m·min^–1] during 70 min of leg cycle exercise [51 (4)% ] in ambulatory control (AC), or following 6 h of chair rest (CR), 6° head-down bed rest (BR), and 20° (WI20) and 80° (WI80) foot-down water immersion [water temperature, 35.0 (0.1)°C]. Compared with the AC exercise T _re [mean (SD) 0.77 (0.13)°C], T _re after CR was 0.83 (0.08)°C (NS), after BR 0.92 (0.13)°C (^*P<0.05), after WI80 0.96 (0.13)°C^*, and after WI20 1.03 (0.09)°C^*. All responded similarly to exercise: they decreased (NS) by 0.5–0.7°C in minutes 4–8 and equilibrated at +0.1 to +0.5°C at 60–70. Skin heat conductance was not different among the five conditions (range = 147–159 kJ·m^–2·h^–1·°C^–1). Results from an intercorrelation matrix suggested that total body sweat rate was more closely related toT _re at 70 min (T _re70) than limb sweat rate or blood flow. Only 36% of the variability inT _re70 could be accounted for by total sweating, and less than 10% from total body dehydration. It would appear that multiple factors are involved which may include change in sensitivity of thermo- and osmoreceptors.     

Comparison of two cool vests on heat-strain reduction while wearing a firefighting ensemble

This study evaluated the effectiveness of a six-pack versus a four-pack cool vest in reducing heat strain in men dressed in firefighting ensemble, while resting and exercising in a warm/humid environment [34.4°C (day bulb), 28.9°C (wet bulb)]. Male volunteers (n = 12) were monitored for rectal temperature (T _re), mean skin temperature (T _sk), heart rate, and energy expenditure during three test trials: control (no cool vest), four-pack vest, and six-pack vest. The cool vests were worn under the firefighting ensemble and over Navy dungarees. The protocol consisted of two cycles of 30 min seated rest and 30 min walking on a motorized treadmill (1.12 m · s^–1, 0% grade). Tolerance time for the control trial (93 min) was significantly less than both vest trials (120 min). Throughout heat exposure, energy expenditure varied during rest and exercise, but no differences existed among all trials (P > 0.05). During the first 60 min of heat exposure, physiological responses were similar for the four-pack and six-pack vests. However, during the second 60 min of heat exposure the six-pack vest had a greater impact on reducing heat strain than the four-pack vest. PeakT _e andT _sk at the end of heat exposure for 6-pack vest [mean (SD) 38.0(0.3)°C and 36.8(0.7)°C] were significantly lower compared to four-pack [38.6 (0.4)°C and 38.1(0.5)°C] and controls [38.9(0.5)°C and 38.4(0.5)°C]. Our findings suggest that the six-pack vest is more effective than the four-pack vest at reducing heat strain and improves performance of personnel wearing a firefighting ensemble.     

Passive hyperthermia reduces voluntary activation and isometric force production

It has been suggested that a critically high body core temperature may impair central neuromuscular activation and cause fatigue. We investigated the effects of passive hyperthermia on maximal isometric force production (MVC) and voluntary activation (VA) to determine the relative roles of skin (T_sk) and body core temperature (T_c) on these factors. Twenty-two males [O_2max=64.2 (8.9) ml kg^–1 min^–1, body fat=8.2 (3.9)%] were seated in a knee-extension myograph, then passively heated from 37.4 to 39.4°C rectal temperature (T_re) and then cooled back to 37.4^oC using a liquid conditioning garment. Voluntary strength and VA (interpolated twitch) were examined during an isometric 10-s MVC at 0.5°C intervals during both heating and cooling. Passive heating to a T_c of 39.4^oC reduced VA by 11 (11)% and MVC by 13 (18)% (P<0.05), but rapid skin cooling, with a concomitant reduction in cardiovascular strain [percentage heart rate reserve decreased from 64 (11)% to 29 (11)%] and psychophysical strain did not restore either of these measures to baseline. Only when cooling lowered T_c back to normal did VA and MVC return to baseline (P<0.05). We conclude that an elevated T_c reduces VA during isometric MVC, and neither T_sk nor cardiovascular or psychophysical strain modulates this response. Results are given as mean (SD) unless otherwise stated.     

Influence of the menstrual cycle and oral contraceptives on thermoregulatory responses to exercise in young women

Summary Thermoregulatory responses to exercise in relation to the phase of the menstrual cycle were studied in ten women taking oral contraceptives (P) and in ten women not taking oral contraceptives (NP). Each subject was tested for maximal aerobic capacity ( ) and for 50% exercise in the follicular (F) and luteal (L) phases of the menstrual cycle. Since the oral contraceptives would have prevented ovulation a quasi-follicular phase (q-F) and a quasi-luteal phase (q-L) of the menstrual cycle were assumed for P subjects. Exercise was performed on a cycle ergometer at an ambient temperature of 24° C and relative air humidity of 50%. Rectal (T _re), mean skin ( ), mean body ( ) temperatures and heart rate (f _c) were measured. Sweat rate was estimated by the continuous measurement of relative humidity of air in a ventilated capsule placed on the chest, converted to absolute pressure (PH₂O_chest). Gain for sweating was calculated as a ratio of increase inPH₂O_chest to the appropriate increase inT _re for the whole period of sweating (G) and for unsteady-state (G_u) separately. The did not differ either between the groups of subjects or between the phases of the menstrual cycle. In P, rectal temperature threshold for sweating (T _{re, td}) was 37.85° C in q-L and 37.60° C in q-F (P < 0.01) and corresponded to a significant difference fromT _re at rest. TheT _re, andf _c increased similarly during exercise in q-F and q-L. No menstrual phase-related differences were observed either in the dynamics of sweating or in G. In NP,T _{re, td} was shorter in L than in F (37.70 vs 37.47° C,P<0.02) with a significantly greater value fromT _re at rest. The dynamics and G for sweating were also greater in L than in F. The G_u was 36.8 versus 16.6 kPa · ° C^–1 (P<0.01) while G was 6.4 versus 3.8 kPa · ° C^–1 (P<0.05), respectively. TheT _re, andf _c increased significantly more in phase F than in phase L. It was concluded that in these women performing moderate exercise, there was a greater temperature threshold and larger gains for sweating in phase L than in phase F. Intake of oral contraceptives reduced the differences in the gains for sweating making the thermoregulatory responses to exercise more uniform.     

Comparison in men of physiological responses to exercise of increasing intensity at low and moderate ambient temperatures

Summary In six male subjects the sweating thresholds, heart rate (f _c, as well as the metabolic responses to exercise of different intensities [40%, 60% and 80% maximal oxygen uptake (VO_2max)], were compared at ambient temperatures (T _a) of 5° C (LT) and 24° C (MT). Each period of exercise was preceded by a rest period at the same temperature. In LT experiments, the subjects rested until shivering occurred and in MT experiments the rest period was made to be of exactly equivalent length. Oxygen uptake (VO₂) at the end of each rest period was higher in LT than MT (P< 0.05). During 20-min exercise at 40%VO_2max performed in the cold no sweating was recorded, while at higher exercise intensities sweating occurred at similar rectal temperatures (T _re) but at lower mean skin (T _sk) and mean body temperatures (T _b) in LT than MT experiments (P<0.001). The exercise inducedVO₂ increase was greater only at the end of the light (40%VO_2max) exercise in the cold in comparison with MT (P<0.001). Bothf _c and blood lactate concentration [la]_b were lower at the end of LT than MT for moderate (60%VO_2max) and heavy (80%VO_2max) exercises. It was concluded that the sweating threshold during exercise in the cold environment had shifted towards lower (T _b) andT _sk. It was also found that subjects exposed to cold possessed a potentially greater ability to exercise at moderate and high intensities than those at 24° C since the increases inT _re,f _c and [la]_b were lower at the lowerT _a.     

The relative influence of body characteristics on humid heat stress response

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

Mechanisms of potentiation in sweating induced by long-term physical training

To evaluate the mechanism of potentiation of sweating after long-term physical training, we compared sweating function in trained and untrained subjects using the frequency of sweat expulsion (f _sw) as an indicator of central sudomotor activity. Nine trained male subjects (trained group) and eight untrained male subjects (untrained group) performed 30-min cycle exercise at 35% maximal oxygen uptake at 25°C ambient temperature and 35% relative humidity. Oesophageal temperature (T _oes), mean body temperature _b, chest sweating rate ( _sw,chest), forearm sweating rate ( _forearm), andf _sw were measured. The slopes of the _sw,chest versus body temperature (T _oes and _b) and versusf _sw relationships in the trained group were significantly greater than those in the untrained group (both,P < 0.05), while there was no difference between the groups in the slopes of the _sw,chest versus body temperature or versusf _sw relationships. Neither the body temperature threshold for initiation of chest or forearm sweating nor the slope of thef _sw- _b relationship differed between groups. We concluded that, during light exercise at moderate ambient temperature, the _sw,chest in the subjects who had undergone long-term physical training was greater than that in the untrained subjects while the _sw,forearm was not changed. The greater _sw,chest in the trained subjects was concluded to be due to an increase of sensitivity of peripheral mechanisms.