Effects of estrus cycle on thermoregulatory responses during exercise in rats

Summary In female rats, rectal temperature (T _re), tail vasomotor response, oxygen uptake , and carbon dioxide production were measured in proestrus and estrus stages during treadmill running at two different speeds at an ambient temperature (T _a) of 24° C. Experiments were performed at 2.00–6.00 a.m., when the difference inT _re was greatest between the two stages;T _re at rest in the estrus stage was 0.54° C higher than in the proestrus stage. In a mild warm environment, thresholdT _re for a rise in tail skin temperature (T _tail) was also higher in the estrus stage than in the proestrus stage. In contrast, no difference was seen in the thresholdT _re and steady stateT _re at the end of exercise between proestrus and estrus stages. These values were higher at the higher work intensity. was also similar between the two stages, except in the second 5 min after the beginning of exercise, when was greater andT _re rose more steeply in the proestrus stage. These data indicate that deep body temperature during exercise is regulated at a certain level depending on the work intensity and is not influenced by the estrus cycle.This study was supported in part by a Grant-in Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan (Grant No. 62480114)     

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.     

Thermoregulation at rest and during exercise in prepubertal boys

Summary Thermal balance was studied in 11 boys, aged 10–12 years, with various values for maximal oxygen uptake ( ), during two standardized sweating tests performed in a climatic chamber in randomized order. One of the tests consisted in a 90-min passive heat exposure [dry bulb temperature (T _db) 45° C] at rest. The second test was represented by a 60-min ergocycle exercise at 60% of individual (T _db 20° C). At rest, rectal temperature increased during heat exposure similar to observations made in adults, but the combined heat transfer coefficient reached higher values, reflecting greater radiative and convective heat gains in the children. Children also exhibited a greater increase in mean skin temperature, and a greater heat dissipation through sweating. Conversely, during the exercise sweating-test, although the increase in rectal temperature did not differ from that of adults for similar levels of exercise, evaporative heat loss was much lower in children, suggesting a greater radiative and convective heat loss due to the relatively greater body surface area. Thermophysiological reactions were not related to in children, in contrast to adults.     

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.     

Safe cooling limits from exercise-induced hyperthermia

We evaluated the cooling rate of hyperthermic subjects, as measured by three estimates of deep core temperatures (esophageal, rectal and aural canal temperatures), during immersion in a range of water temperatures. The objective of the study was to compare the three indices of core temperature and define safe cooling limits when using rectal temperature to avoid the development of hypothermia. On 4 separate days, seven subjects (four males, three females) exercised for 45.4±4.1 min at 65% at an ambient temperature of 39°C, RH: 36.5%, until rectal temperature (T _re) increased to 40.0°C (39.5°C for two subjects). Following exercise, the subjects were immersed in a circulated water bath controlled at 2, 8, 14 and 20°C until T _re returned to 37.5°C. When T _re reached normothermia during the cooling period (37.5±0.05°C), both esophageal (T _es) (35.6±1.3°C) and aural canal (T _ac) (35.9±0.9°C) temperatures were approaching or reaching hypothermia, particularly during immersion in 2°C water (T _es=34.5±1.2°C). On the basis of the heat loss data, the heat gained during the exercise was fully eliminated after 5.4±1.5, 7.9±2.9, 10.4±3.8 and 13.1±2.8 min of immersion in 2, 8, 14 and 20°C water, respectively, with the coldest water showing a significantly faster cooling rate. During the immersion in 2°C water, a decrease of only 1.5°C in T _re resulted in the elimination of 100% of the heat gained during exercise without causing hypothermia. This study would therefore support cooling the core temperature of hyperthermic subjects to a rectal temperature between 37.8°C (during immersion in water >10°C) and 38.6°C (during immersion in water <10°C) to eliminate the heat gained during exercise without causing hypothermia.     

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.     

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.     

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

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

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.     

One night of sleep deprivation decreases treadmill endurance performance

The aim was to test the hypothesis that one night of sleep deprivation will impair pre-loaded 30 min endurance performance and alter the cardio-respiratory, thermoregulatory and perceptual responses to exercise. Eleven males completed two randomised trials separated by 7 days: once after normal sleep (496 (18) min: CON) and once following 30 h without sleep (SDEP). After 30 h participants performed a 30 min pre-load at 60% $ \dot{V}{\text{O}}_{2\max } $ followed by a 30 min self-paced treadmill distance test. Speed, RPE, core temperature (T _re), mean skin temperature (T _sk), heart rate (HR) and respiratory parameters ( $ \dot{V}{\text{O}}_{2} $ , $ \dot{V}{\text{CO}}_{2} $ , $ \dot{V}{\text{E}} $ , RER pre-load only) were measured. Less distance (P = 0.016, d = 0.23) was covered in the distance test after SDEP (6037 (759) 95%CI 5527 to 6547 m) compared with CON (6224 (818) 95%CI 5674 to 6773 m). SDEP did not significantly alter T _re at rest or thermoregulatory responses during the pre-load including heat storage (0.8°C) and T _sk. With the exception of raised $ \dot{V}{\text{O}}_{2} $ at 30 min on the pre-load, cardio-respiratory parameters, RPE and speed were not different between trials during the pre-load or distance test (distance test mean HR, CON 174 (12), SDEP 170 (13) beats min^?1: mean RPE, CON 14.8 (2.7), SDEP 14.9 (2.6)). In conclusion, one night of sleep deprivation decreased endurance performance with limited effect on pacing, cardio-respiratory or thermoregulatory function. Despite running less distance after sleep deprivation compared with control, participants’ perception of effort was similar indicating that altered perception of effort may account for decreased endurance performance after a night without sleep.     

Control of sweating during the human menstrual cycle

Summary Thermoregulatory responses were studied in seven women during two separate experimental protocols in the follicular (F, days 4–7) phase and during the luteal (L, days 19–22) phase of the menstrual cycle. Continuous measurements of esophageal temperature (T _es), mean skin temperature ( ), oxygen uptake and forearm sweating ( ) were made during all experiments. Protocol I involved both passive heat exposure (3 h) and cycle exercise at ∼80% peak during which the environmental chamber was controlled atT _a=50.0° C, rh=14% (P _w=1.7 kPa). In protocol II subjects were tested during thirty-five minutes of exercise at ∼85% peak atT _a=35° C and rh=25% (P _w=1.4 kPa). The normal L increase in restingT _es (≈0.3° C) occurred in all seven subjects. was higher during L than F in all experiments conducted at 50° C. During exercise and passive heat exposure, theT _es threshold for sweating was higher in L, with no change in the thermosensitivity (slope) of toT _es between menstrual cycle phases. This rightward or upward shift inT _es threshold for initiation of sweating averaged 0.5° C for all experiments. The data indicate the luteal phase modulation in the control of sweating in healthy women is also apparent during severe exercise and/or heat stress.     

The influence of menthol on thermoregulation and perception during exercise in warm, humid conditions

Menthol has recently been added to various cooling products that claim to enhance athletic performance. This study assessed the effect of two such solutions during exercise in warm, humid conditions. Twelve participants (22 ± 2.9 years; [(V)\dot]\textO_2\textpeak \dot{V}{\text{O}}_{{2{\text{peak}}}} 47.4 ± 6.2 mL kg⁻¹ min⁻¹) completed a peak power (PO_peak) test and three separate exercise bouts in 30°C and 70% relative humidity after being sprayed with 100 mL of water containing either 0.05 or 0.2% l-menthol, or a control spray. During each trial, participants underwent 15 min of rest, spraying, 15 min of rest and 45 min of exercise at 45% of PO_peak. The following variables were measured: rectal temperature (T _re), sweat rate (SR), skin blood flow (SBF), heart rate (HR), thermal comfort (TC) and sensation (TS) votes, irritation (IRR) and rating of perceived exertion (RPE). Mean skin (MST) and body temperatures ( [`(T)]<sub>\textbody</sub> \bar{T}_{\text{body}} ) were calculated. There was no significant difference in MST, [`(T)]_\textbody \bar{T}_{\text{body}} SR, SBF, HR, TC or RPE between conditions. Spraying with 0.2% menthol significantly (P < 0.05) elevated T _re by 0.2°C compared to the other conditions. Both menthol sprays caused participants to feel significantly cooler than control spraying (P = 0.001), but 0.2% spraying induced significantly cooler sensations (P = 0.01) than 0.05% spraying. Both menthol sprays induced greater irritation (P < 0.001) than control spraying. These findings suggest that 0.05% menthol spraying induced cooler upper body sensations without measurable thermoregulatory impairment. T _re was significantly elevated with 0.2% spraying. Irritation persisted with both menthol sprays while TC remained unchanged, suggesting a causal relationship. The use in sport of a spray similar to those tested here remains equivocal.     

How should body heat storage be determined in humans: by thermometry or calorimetry?

Summary The aim of this study was to determine whether in humans there are differences in the heat storage calculated by partitional calorimetry (S, the balance of heat gains and heat losses) compared to the heat storage obtained by conventional methods (thermometry) via either core temperature or mean body temperatures ( , whereT _c is core temperature and is mean skin temperature) when two different sites are used as an index ofT _c [rectal (T _re) and auditory canal (T _ac) temperatures]. Since women respond to the heat differently than men, both sexes were studied. After a stabilisation period at thermal neutrality, six men and seven women were exposed to a globe temperature of 50°C, relative humidity of 17% and wind speed of 0.8–1.0 m·s^–1 for 90 min semi-nude at rest, whereT _re,T _ac, , metabolic rate, dry (radiant+convective heat exchange) and evaporative heat losses,S, heat storage byT _c ( ) and heat storage by were assessed every minute. In the men,S was equal to 350.8(SEM 49.6) kJ whereas amounted to only 114.6(SEM 16.2) and 196.7(SEM 32.3) kJ forT _re andT _ac, respectively (P<0.05). Final underestimatedS by 49% [177.7(SEM 23.0) kJ;P<0.05] whereas was not significantly different than S [255.7(SEM 37.9) kJ]. In the women,S corresponded to a total of 294.3(SEM 23.2) kJ, a value that was very similar to the 262.6(SEM 31.0) kJ], whereas underpredicted by 35% [190.4(SEM 26.3) kJ;P<0.05]. As in the men,S _{T c} was much lower thanS [116.6(SEM 19.9) and 190.3(SEM 24.2) kJ forT _re andT _ac, respectively;P<0.05]. Using seven other well-known weighting coefficients, could under- and overestimateS by up to 55% and 11%, respectively. In all subjects, a large portion of the variance (68% and 75%) in the difference betweenS and , could be explained primarily by the T _ac. The results demonstrated that although some estimates of thermometric heat storage matched the calorimetricS, other predictions underestimated it by up to 67% during passive heating. It is suggested that these differences can be explained in part by he site chosen to representT _c, the use of eitherT _c or in the heat storage calculation, and the thermoneutral/hot weighting coefficient(s) chosen to determine . Until more representative measurements of body temperatures at different depths (core, shell and intermediate) are possible, the use of and -derived heat storage is difficult to justify.     

Short-term exercise-heat acclimation enhances skin vasodilation but not hyperthermic hyperpnea in humans exercising in a hot environment

We tested the hypothesis that short-term exercise-heat acclimation (EHA) attenuates hyperthermia-induced hyperventilation in humans exercising in a hot environment. Twenty-one male subjects were divided into the two groups: control (C, n = 11) and EHA (n = 10). Subjects in C performed exercise-heat tests [cycle exercise for ~75 min at 58% [(V)\dot]_{\textO 2 \textpeak} \dot{V}_{{{\text{O}}_{{ 2 {\text{peak}}}} }} (37°C, 50% relative humidity)] before and after a 6-day interval with no training, while subjects in EHA performed the tests before and after exercise training in a hot environment (37°C). The training entailed four 20-min bouts of exercise at 50% [(V)\dot]_{\textO 2 \textpeak} \dot{V}_{{{\text{O}}_{{ 2 {\text{peak}}}} }} separated by 10 min of rest daily for 6 days. In C, comparison of the variables recorded before and after the no-training period revealed no changes. In EHA, the training increased resting plasma volume, while it reduced esophageal temperature (T _es), heart rate at rest and during exercise, and arterial blood pressure and oxygen uptake ( [(V)\dot]_\textO2 \dot{V}_{{{\text{O}}_{2} }} ) during exercise. The training lowered the T _es threshold for increasing forearm vascular conductance (FVC), while it increased the slope relating FVC to T _es and the peak FVC during exercise. It also lowered minute ventilation ( [(V)\dot]_\textE \dot{V}_{\text{E}} ) during exercise, but this effect disappeared after removing the influence of [(V)\dot]_\textO2 \dot{V}_{{{\text{O}}_{2} }} on [(V)\dot]_\textE \dot{V}_{\text{E}} . The training did not change the slope relating ventilatory variables to T _es. We conclude that short-term EHA lowers ventilation largely by reducing metabolism, but it does not affect the sensitivity of hyperthermia-induced hyperventilation during submaximal, moderate-intensity exercise in humans.     

Circadian rhythms in mitotic activity and 3H-thymidine uptake in the duodenum: Effect of isoproterenol on the mitotic rhythm

Mitotic activity in the duodenum of the rat and mouse exhibits a circadian periodicity with a peak in the rat between 1200 and 1500 hours and a sustained trough between 1800 and 0600. Scintillation counts revealed a similar rhythm in the total uptake of ³H-thymidine by the rat duodenum with a sustained but fluctuating crest occurring between 0800 and 1800 and a trough between 1900 and 0100. In the mouse the peak mitotic activity occurred at 0900 and the trough at 1700. Isoproterenol completely abolishes the rhythm in mitosis in mouse duodenum, when injected exactly 28 hours previous to sacrifice. The results are discussed in relation to reports that deny a circadian rhythm in mitotic activity in the duodenum.     

Reproducibility of relationships between human ventilation, its components and oesophageal temperature during incremental exercise

For human exercise at intensities greater than ~70 to 85% of maximal levels of exertion, ventilation (V _E) increases proportionately to core temperature (T _C) following distinct T _C thresholds. This suggested T _C in humans could be a modulator of exercise-induced ventilation. This study tested the reproducibility of relationships between oesophageal temperature (T _oes), ventilation and its components during incremental exercise. On two nonconsecutive days, at an ambient temperature of 22.1±0.3°C and RH of 45±5%, seven untrained adult males of normal physique pedaled on a seated cycle ergometer in an incremental exercise protocol from rest to the point of exhaustion. In each exercise session, ventilatory equivalents for oxygen consumption and carbon dioxide production plus the components of V _E, tidal volume (V _T) and frequency of respiration (ƒ), were expressed as a function of T _oes. Results indicated the reproducibility criteria of Bland and Altman were met for the relationships between T _oes and both and as well as for relationships between T _oes and each of V _T and f. Intraclass correlation coefficients (R) for between-trial T _oes thresholds for (R=0.91, P<0.05) and (R=0.88, P<0.05) were also high and significant. In both trials, after T _oes increased by ~0.3°C, V _T demonstrated a distinct plateau point at a reproducible T _oes (R=0.93, P<0.05) and ƒ demonstrated a distinct and reproducible T _oes threshold (R=0.84, P<0.05). In conclusion, the results illustrate that for humans, ventilation has a significant and reproducible relationship with core temperature during incremental exercise.     

Thermal responses of men and women during cold-water immersion: influence of exercise intensity

Summary The influence of exercise intensity on thermoregulation was studied in 8 men and 8 women volunteers during three levels of arm-leg exercise (level I: 700 ml oxygen (O₂) · min^–1; level II: 1250 ml O₂ · min^–1; level III: 1700 ml O₂ · min^–1 for 1 h in water at 20 and 28°C (T _w). For the men inT _w 28°C the rectal temperature (T _re) fell 0.79°C (P<0.05) during immersion in both rest and level-I exercise. With level-II exercise a drop inT _re of 0.54° C (P < 0.05) was noted, while at level-III exerciseT _re did not change from the pre-immersion value. AtT _w of 20°C,T _re fell throughout immersion with no significant difference in finalT _re observed between rest and any exercise level. For the women at rest atT _w 28°C,T _re fell 0.80°C (P<0.05) below the pre-immersion value. With the two more intense levels of exercise,T _re did not decrease during immersion. InT _w 20°C, the women maintained higherT _re (P<0.05) during level-II and level-III exercise compared to rest and exercise at level I. The_{T re} responses were related to changes in tissue insulation (I _t) between rest and exercise with the largest reductions inI _t noted between rest and level-I exercise acrossT _w and gender. For men and women of similar percentage body fat, decreases inT _re were greater for the women at rest and level-I exercise inT _w 20°C (P< 0.05). With more intense exercise, the women maintained a higherT _re than the men, especially in the colder water. These findings indicate that exercise is not always effective in offsetting the decrease inI _t and facilitated heat loss in cool or cold water compared to rest. The factors of exercise intensity,T _W, body fat, and gender influence the thermoregulatory responses.     

Differences in rectal temperatures measured at depths of 4–19 cm from the anal sphincter during exercise and rest

The purpose of the present study was to examine the discrepancies in rectal temperature (T _re) at various depths. Nineteen young males performed two bouts of bicycle exercise and recovery. T _re was simultaneously measured at depth of 4, 6, 8, 10, 13, 16, and 19 cm, alongside the measurement of skin temperatures. We found small but statistically significant differences by depth in the absolute T _re, the magnitude of rise in T _re and the lag of response in T _re. During the stabilization stage before exercise, T _re at 4 cm-depth was 0.5°C lower than T _re at 16 cm-depth (p < 0.05). As the depth measured in the rectum was shallower, the rise in T _re during exercise was greater. However the rise in T _re at 10, 13, 16 and 19 cm showed no systemic difference. Among seven depths, T _re at 16 cm-depth had the most stable feature with the longest latent period (3.1 ± 1.3 min) and the smallest rise (0.8 ± 0.3°C), while T _re at 4 cm-depth was the most responsive to the change of exercise and rest with the shortest latent period (1.0 ± 0.6 min) and the greatest rise (1.2 ± 0.5°C). The differences observed in the depths from 4 to 19 cm were offset by exercise to some extent. In summary, T _re appeared in different manners according to the seven depths during the repetition of exercise and rest, but T _re deeper than 10 cm-depth seemed to have no systematic differences.     

Dynamics of sweating in men and women during passive heating

Summary The dynamics of sweating was investigated at rest in 8 men and 8 women. Electrical skin resistance (ESR), rectal temperature (T_re) and mean skin temperature were measured in subjects exposed to 40‡ C environmental temperature, 30% relative air humidity, and 1 m · s⁻¹ air flow. Sweat rate was computed from continuous measurement of the whole body weight loss. It was found that increases in T_re, and mean body temperature were higher in women than in men by 0.16, 0.38 and 0.21‡ C, but only the difference in δ was significant (p<0.05). The dynamics of sweating in men and women respectively, was as follows: delay (t_d) 7.8 and 18.1 min (p<0.01), time constant (Τ) 7.5 and 8.8 min (N.S.), inertia time (t_i) 15.3 and 26.9 min (p<0.002), and total body weight loss 153 and 111 g · m⁻² · h⁻¹ (p<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<0.001), and in women (r=0.76, p<0.02). In men, δ T_re correlated with inertia time of sweat rate (r=0.81, p<0.01) as well as with the inertia time of ESR (r=0.83, p<0.001). No relation was found between δ T_re and the dynamics of sweating in women. It is concluded that the dynamics of sweating plays a decisive role in limiting δ T_re in men under dry heat exposure. The later onset of sweating in women does not influence the rectal temperature increase significantly. In women, δ T_re is probably limited by a complex interaction of sweating, skin blood flow increase, and metabolic rate decrease. This work was supported by the Centre National de la Recherche Scientifique and Polish Academy of Siences     

The effect of increased body temperature due to exercise on the heart rate and on the maximal aerobic power

The effect of an increased body temperature (T _r) elicited by prolonged heavy exercise at normal ambient temperature in absence of any heat stress, on the maximal aerobic power ( \(\dot V_{O_2 \max } \) ) and on heart rate (HR) has been studied. The prolonged exercise consisted in running for 1 hr on a motor driven treadmill, this leading to an average increase of T _r of 1.2° C. Oxygen consumption ( \(\dot V_{O_2 } \) ), ventilation (V _I ), HR and T _r were measured at rest and every 10 min during the prolonged exercise. Before and after this exercise indirect measurement of \(\dot V_{O_2 \max } \) were made. After the exercise, HR in submaximal exercise was increased, the increase being less pronounced the heavier the exercise. The HR increment was 17.5 beats/min per 1° C rise in T _r in the exercise involving an oxygen comsumption of 22 ml/kg·min and it dropped to 7.5 b/min · °C when the O₂ consumption increased to 32.4 ml/kg · · min. \(\dot V_{O_2 \max } \) as calculated indirectly from HR values in submaximal exercise remained essentially the same before and after the treadmill run.