Comparison of thermal responses between rest and leg exercise in water

This study examined both the thermal and metabolic responses of individuals in cool (30 degrees C, n = 9) and cold (18 degrees C, n = 7; 20 degrees C, n = 2) water. Male volunteers were immersed up to the neck for 1 h during both seated rest (R) and leg exercise (LE). In 30 degrees C water, metabolic rate (M) remained unchanged over time during both R (115 W, 60 min) and LE (528 W, 60 min). Mean skin temperature (Tsk) declined (P less than 0.05) over 1 h during R, while Tsk was unchanged during LE. Rectal (Tre) and esophageal (Tes) temperatures decreased (P less than 0.05) during R (delta Tre, -0.5 degrees C; delta Tes, -0.3 degrees C) and increased (P less than 0.05) during LE (delta Tre, 0.4 degrees C; Tsk, 0.4 degrees C). M, Tsk, Tre, and Tes were higher (P less than 0.05) during LE compared with R. In cool water, all regional heat flows (leg, chest, and arm) were generally greater (P less than 0.05) during LE than R. In cold water, M increased (P less than 0.05) over 1 h during R but remained unchanged during LE. Tre decreased (P less than 0.05) during R (delta Tre, -0.8 degrees C) but was unchanged during LE. Tes declined (P less than 0.05) during R (delta Tes, -0.4 degrees C) but increased (P less than 0.05) during LE (delta Tes, 0.2 degrees C). M, Tre, and Tes were higher (P less than 0.05), whereas Tsk was not different during LE compared with R at 60 min.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in insulation of body tissue and wet suits during underwater exercise at various atmospheric pressures

To clarify the independent changes of insulations of body tissues (Itissue) and wet suit (Isuit) in the wet-suited subject during underwater exercise, overall heat flow from the skin (Htissue) and wet suit (Hsuit) and esophageal (Tes), skin (Tsk), and wet suit temperatures were measured at 1, 2, and 2.5 atmospheres absolute (ATA) at critical water temperature (Tcw). The average Tcw in nine wet-suited men (23-38 yr) was 22.3 +/- 0.2, 26.3 +/- 0.2, and 28.0 +/- 0.4 degrees C (SE) at 1, 2, and 2.5 ATA, respectively. At Tcw of each pressure male volunteers wearing 5-mm neoprene wet suits completed three 2-h experiments while immersed up to the neck. During one experiment the subjects remained at rest, and in the other two they exercised on an underwater ergometer at two different intensities (2 and 3 met). Tes significantly declined (P less than 0.05) over 2 h from 37.1 to 36.5 degrees C during rest in each pressure. The 2-met exercise prevented Tes from falling in all pressures, and the 3-met exercise elevated Tes by 0.2-0.3 degrees C. There was no exercise-dependent difference in Isuit, but a pressure-dependent difference was remarkable. The Itissue at rest was identical for all pressures; however, it progressively decreased as a function of exercise intensity. It is concluded that overall Itissue is entirely determined by work intensity at Tcw, but not by atmospheric pressure. On the contrary, Isuit at Tcw is solely dependent on the pressure, but not on the work intensity.     

Changes in thermal insulation during underwater exercise in Korean female wet-suit divers

The present work was undertaken to examine the effect of wet suits on the pattern of heat exchange during immersion in cold water. Four Korean women divers wearing wet suits were immersed to the neck in water of critical temperature (Tcw) while resting for 3 h or exercising (2-3 met on a bicycle ergometer) for 2 h. During immersion both rectal (Tre) and skin temperatures and O2 consumption (VO2) were measured, from which heat production (M = 4.83 VO2), skin heat loss (Hsk = 0.92 M +/- heat store change based on delta Tre), and thermal insulation were calculated. The average Tcw of the subjects with wet suits was 16.5 +/- 1.2 degrees C (SE), which was 12.3 degrees C lower than that of the same subjects with swim suits (28.8 +/- 0.4 degrees C). During the 3rd h of immersion, Tre and mean skin temperatures (Tsk) averaged 37.3 +/- 0.1 and 28.0 +/- 0.5 degrees C, and skin heat loss per unit surface area 42.3 +/- 2.66 kcal X m-2 X h. The calculated body insulation [Ibody = Tre - Tsk/Hsk] and the total shell insulation [Itotal = (Tre - TW)/Hsk] were 0.23 +/- 0.02 and 0.5 +/- 0.04 degrees C X kcal-1 X m2 X h, respectively. During immersion exercise, both Itotal and Ibody declined exponentially as the exercise intensity increased. Surprisingly, the insulation due to wet suit (Isuit = Itotal - Ibody) also decreased with exercise intensity, from 0.28 degrees C X kcal-1 X m2 X h at rest to 0.12 degrees C X kcal-1 X m2 X h at exercise levels of 2-3 met.(ABSTRACT TRUNCATED AT 250 WORDS)     

Core threshold temperatures for sweating

To detect shifts in the threshold core temperature (Tc) for sweating caused by particular nonthermal stresses, it is necessary to stabilize or standardize all other environmental and physiological variables which cause such shifts. It is, however, difficult to cause progressive changes in Tc without also causing changes in skin temperature (Tsk). This study compares the technique of body warming by immersion in water at 40 degrees C, and subsequent body cooling in water at 28 degrees C, to determine the core threshold for sweating, with one by which Tc was raised by cycling exercise in air at 20 degrees C, and then lowered by immersion in water at 28 degrees C. The first of these procedures involved considerable shifts in Tsk upon immersion in water at 40 degrees C, and again upon transfer to water at 28 degrees C; the second procedure caused only small changes in Tsk. The onset of sweating at a lower esophageal temperature (Tes) during immersion in water at 40 degrees C (36.9 +/- 0.1 degrees C) than during exercise (37.4 +/- 0.3 degree C) is attributed to the high Tsk since Tes was then unchanged. Likewise, the rapid decline in the sweat rate during immersion at 28 degrees C had the same time course to extinction after the pretreatments. This related more to the Tsk, which was common, than to the levels or rates of change of Tes, which both differed between techniques. Tes fell most rapidly, and thus sweating was extinguished at a lower Tes, following 40 degrees C immersion than following exercise.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of body mass and morphology on thermal responses in water

Ten male volunteers were divided into two groups based on body morphology and mass. The large-body mass (LM) group (n = 5) was 16.3 kg heavier and 0.22 cm2 X kg-1 X 10(-2) smaller in surface area-to-mass ratio (AD X wt-1) (P less than 0.05) than the small-body mass (SM) group (n = 5). Both groups were similar in total body fat and skinfold thicknesses (P greater than 0.05). All individuals were immersed for 1 h in stirred water at 26 degrees C during both rest and one intensity of exercise (metabolic rate approximately 550 W). During resting exposures metabolic rate (M) and rectal temperature (Tre) were not different (P greater than 0.05) between the LM and SM groups at min 60. Esophageal temperature (Tes) was higher (P less than 0.05) for the SM group at min 60, although the change in Tes during the 60 min between groups was similar (LM, -0.4 degrees C; SM, -0.2 degrees C). Tissue insulation (I) was lower (P less than 0.05) for SM (0.061 degrees C X m-2 X W-1) compared with the LM group (0.098 degrees C X m-2 X W-1). During exercise M, Tre, Tes, and I were not different (P greater than 0.05) between groups at min 60. These data illustrate that a greater body mass between individuals increases the overall tissue insulation during rest, most likely as a result of a greater volume of muscle tissue to provide insulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of body temperature on the development of fatigue during prolonged exercise in the heat.

We investigated whether fatigue during prolonged exercise in uncompensable hot environments occurred at the same critical level of hyperthermia when the initial value and the rate of increase in body temperature are altered. To examine the effect of initial body temperature [esophageal temperature (Tes) = 35.9 +/- 0.2, 37.4 +/- 0. 1, or 38.2 +/- 0.1 (SE) degrees C induced by 30 min of water immersion], seven cyclists (maximal O2 uptake = 5.1 +/- 0.1 l/min) performed three randomly assigned bouts of cycle ergometer exercise (60% maximal O2 uptake) in the heat (40 degrees C) until volitional exhaustion. To determine the influence of rate of heat storage (0.10 vs. 0.05 degrees C/min induced by a water-perfused jacket), four cyclists performed two additional exercise bouts, starting with Tes of 37.0 degrees C. Despite different initial temperatures, all subjects fatigued at an identical level of hyperthermia (Tes = 40. 1-40.2 degrees C, muscle temperature = 40.7-40.9 degrees C, skin temperature = 37.0-37.2 degrees C) and cardiovascular strain (heart rate = 196-198 beats/min, cardiac output = 19.9-20.8 l/min). Time to exhaustion was inversely related to the initial body temperature: 63 +/- 3, 46 +/- 3, and 28 +/- 2 min with initial Tes of approximately 36, 37, and 38 degrees C, respectively (all P < 0.05). Similarly, with different rates of heat storage, all subjects reached exhaustion at similar Tes and muscle temperature (40.1-40.3 and 40. 7-40.9 degrees C, respectively), but with significantly different skin temperature (38.4 +/- 0.4 vs. 35.6 +/- 0.2 degrees C during high vs. low rate of heat storage, respectively, P < 0.05). Time to exhaustion was significantly shorter at the high than at the lower rate of heat storage (31 +/- 4 vs. 56 +/- 11 min, respectively, P < 0.05). Increases in heart rate and reductions in stroke volume paralleled the rise in core temperature (36-40 degrees C), with skin blood flow plateauing at Tes of approximately 38 degrees C. These results demonstrate that high internal body temperature per se causes fatigue in trained subjects during prolonged exercise in uncompensable hot environments. Furthermore, time to exhaustion in hot environments is inversely related to the initial temperature and directly related to the rate of heat storage.     

Radio frequency (13.56 MHz) energy enhances recovery from mild hypothermia

The rate of warming after hypothermia depends on the method of rewarming. This study compared the effectiveness of radio frequency (RF) energy against hot (41 degrees C) water immersion (HW) and an insulated cocoon (IC) for rewarming hypothermic men. Six men fasted overnight and were rewarmed for 1 h after attaining a 0.5 degree C reduction in rectal temperature (Tre). Tre and esophageal (Tes) temperature were recorded every 5 min with nonmetallic thermal probes. The base-line value for Tre and Tes just before rewarming was subtracted from each 5 min Tre and Tes during rewarming to give delta Tre and delta Tes. The 12 delta Tes values were averaged for each individual and were compared using analysis of variance. The average delta Tes for RF (1.15 +/- 0.22 degrees C/h) was faster (P less than 0.001) than either IC (0.37 +/- 0.16 degrees C/h) or HW (0.18 +/- 0.09 degree C/h). The present study shows the superiority of RF energy for rewarming mildly hypothermic men.     

Critical water temperature during water immersion at various atmospheric pressures

The present work was undertaken to determine the effect of atmospheric pressure [ranging from a high altitude of 4,300 m above sea level or 0.6 atmospheres absolute (ATA) to depths of 10 m deep or 2 ATA] on the critical water temperature (Tcw), defined as the lowest water temperature a subject can tolerate at rest for 2 h without shivering, of the unprotected subject during water immersion. Nine healthy males wearing only shorts were subjected to immersion to the neck in water at 0.6, 1, and 2 ATA while resting for 2 h. Continuous measurements included esophageal (Tes) and skin (Tsk) temperatures, direct heat loss from the skin (Htissue), and insulation of the tissue (Itissue). The Tcw was significantly higher at 0.6 ATA than 1 and 2 ATA: however, Tcw at 1 ATA was identical to that at 2 ATA. The metabolic heat production remained unchanged among the pressures. During the 2-h immersion in Tcw, Tes was identical among all atmospheric pressures: however, Tsk was significantly higher (P less than 0.05) at 0.6 ATA and was identical between 1 and 2 ATA. The overall mean Itissue was near maximal during immersion in Tcw in each pressure, and no difference was detected among the pressures. However, Itissue at the acral extremities (arm, hand, and foot) decreased significantly at 0.6 ATA, and subsequently heat loss from these parts was increased, which elevated an extremity-to-trunk heat loss ratio to 1.4 at 0.6 ATA from 1.1 at 1 and 2 ATA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of occluded venous return on core temperature during cold water immersion

Recent studies using inanimate and animal models suggest that the afterdrop observed upon rewarming from hypothermia is based entirely on physical laws of heat flow without involvement of the returning cooled blood from the limbs. During the investigation of thermoregulatory responses to cold water immersion (15 degrees C), blood flow to the limbs (minimized by the effects of hydrostatic pressure and vasoconstriction) was occluded in 17 male subjects (age, 29.0 +/- 3.3 yr). Comparisons of rectal (Tre) and esophageal temperature (Tes) responses were made during the 5 min before occlusion, during the 10-min occlusion period, and for 5 min immediately after the release of the cuffs (postocclusion). In the preocclusion phase, Tre and Tes showed similar cooling rates. The occlusion of blood flow to the extremities significantly arrested the cooling of Tes (P less than 0.05) with little effect on Tre. Upon release of the pressure cuffs, the returning extremity blood flow resulted in an increased rate of cooling, that was three times greater at the esophageal site (-0:149 +/- 0.052 vs. -0.050 +/- 0.026 degrees C.min-1). These results suggest that the cooled peripheral circulation, minimized during cold water immersion, may dramatically affect esophageal temperature and the complete neglect of the circulatory component to the afterdrop phenomenon is not warranted.     

Effect of saline infusion during exercise on thermal and circulatory regulations

To quantify the effect of an acute increase in plasma volume (PV) on forearm blood flow (FBF), heart rate (HR), and esophageal temperature (Tes) during exercise, we studied six male volunteers who exercised on a cycle ergometer at 60% of maximal aerobic power for 50 min in a warm [(W), 30 degrees C, less than 30% relative humidity (rh)] or cool environment [(C), 22 degrees C, less than 30% rh] with isotonic saline infusion [Inf(+)] or without infusion [Inf(-)]. The infusion was performed at a constant rate of 0.29 ml.kg body wt-1.min-1 for 20-50 min of exercise to mimic fluid intake during exercise. PV decreased by approximately 5 ml/kg body wt within the first 10 min of exercise in all protocols. Therefore, PV in Inf(-) was maintained at the same reduced level by 50 min of exercise in both ambient temperatures, whereas PV in Inf(+) increased toward the preexercise level and recovered approximately 4.5 ml/kg body wt by 50 min in both temperatures. The restoration of PV during exercise suppressed the HR increase by 6 beats/min at 50 min of exercise in W; however, infusion had no effect on HR in C. In W, FBF in Inf(+) continued to increase linearly as Tes rose to 38.1 degrees C by the end of exercise, whereas FBF in Inf(-) plateaued when Tes reached approximately 37.7 degrees C. The infusion in C had only a minor effect on FBF.(ABSTRACT TRUNCATED AT 250 WORDS)     

Relationship between ventilatory response and body temperature during prolonged submaximal exercise.

We examined whether an increase in skin temperature or the rate of increase in core body temperature influences the relationship between minute ventilation (Ve) and core temperature during prolonged exercise in the heat. Thirteen subjects exercised for 60 min on a cycle ergometer at 50% of peak oxygen uptake while wearing a suit perfused with water at 10 degrees C (T10), 35 degrees C (T35), or 45 degrees C (T45). During the exercise, esophageal temperature (Tes), skin temperature, heart rate (HR), Ve, tidal volume, respiratory frequency (f), respiratory gases, blood pressure (BP), and blood lactate were all measured. We found that oxygen uptake, carbon dioxide output, BP, and blood lactate did not differ among the sessions. Tes, HR, Ve, and f remained nearly constant from minute 10 onward in the T10 session, but all of these parameters progressively increased in the T35 and T45 sessions, and significantly higher levels were seen in the T45 than the T35 session. For all but two subjects in the T35 and T45 sessions, plotting Ve as a function of Tes revealed no threshold for hyperventilation; instead, increases in Ve were linearly related to Tes, and there were no significant differences in the slopes or intercepts between the T35 and T45 sessions. Thus, during prolonged submaximal exercise in the heat, Ve increases with core temperature, and the influences of skin temperature and the rate of increase in Tes on the relationship between Ve and Tes are apparently small.     

Thermal responses during arm and leg and combined arm-leg exercise in water

Thermal and metabolic responses were examined during exposures in stirred water at approximately 20, 26, and 33 degrees C while subjects were performing 45 min of either arm (A), leg (L), or combined arm-leg (AL) exercise. Eight males immersed to the neck completed a low exercise intensity for A exercise and both a low and high exercise intensity for L and AL exercise. During low-intensity exercise, final metabolic rate (M) for A, L, and AL exercise was not different (P greater than 0.05) between exercise type for each water temperature (Tw). In contrast final rectal temperatures (Tre) for A and AL exercise were significantly lower than L values for each Tw during low-intensity exercise. These findings were supported by both mean weighted skin temperature (Tsk) and mean weighted heat flow (Hc) values, which were greater during A than L for each Tw. During high-intensity exercise, final Tre values were lower (P less than 0.05) during AL compared with L exercise across all Tw. Final Tsk and Hc values were not different between each type of exercise, although M was significantly lower during L exercise in 20 degrees C water. These data suggest a greater conductive and convective heat loss during exercise utilizing the arms when compared with leg-only exercise.     

Influence of beta-adrenergic blockade on the control of sweating in humans

To evaluate the role of beta-adrenergic receptors in the control of human sweating, we studied six subjects during 40 min of cycle-ergometer exercise (60% maximal O2 consumption) at 22 degrees C 2 h after oral administration of placebo or nonselective beta-blockade (BB, 80 mg propranolol). Internal temperature (esophageal temperature, Tes), mean skin temperature (Tsk), local chest temperature (Tch), and local chest sweat rate (msw) were continuously recorded. The control of sweating was best described by the slope of the linear relationship between msw and Tes and the threshold Tes for the onset of sweating. The slope of the msw-Tes relationship decreased 27% (P less than 0.01), from 1.80 to 1.30 mg X cm-2 X min-1 X degree C-1 during BB. The Tes threshold for sweating (36.8 degrees C) was not altered as the result of BB. These data suggest that BB modified the control of sweating via some peripheral interaction. Since Tsk was significantly (P less than 0.05) reduced during BB exercise, from a control value of 32.8 to 32.2 degrees C, we evaluated the influence of the reduction in local skin temperature (Tsk) in the altered control of sweating. Reductions in Tch accounted for only 45% of the decrease in the slope of the msw-Tes relationship during BB. Since evaporative heat loss requirement during exercise with BB, as estimated from the energy balance equation, was also reduced 18%, compared with control exercise, we concluded that during BB the reduction in sweating at any Tes is the consequence of both a decrease in local Tsk and a direct effect on sweat gland.     

Time course of deacclimatization to cold water immersion in Korean women divers

Seasonal basal metabolic rates (BMR), critical water temperature (Tcw), maximal body insulations (Imax), and finger blood flow during hand immersion in 6 degrees C water (Q finger) were measured periodically during the course of a 3-yr longitudinal study (1980-1982) of modern Korean diving women (ama), who have been wearing wet suits since 1977 to avoid cold stress during work. Methods and protocols were identical to previous studies of cotton-suited ama from 1961-1974. The BMR of modern ama did not undergo seasonal fluctuation (1980-1981) and was within the DuBois standard and comparable to nondivers year around Tcw of ama was still reduced by 2-3 degrees C in 1980 but increased progressively to equal that of nondivers in 1982, when compared at comparable subcutaneous fat thickness (SFT). Since modern ama and nondivers have 2.4 times thicker SFT (i.e., 4-13 mm) than in 1962 the absolute Tcw is significantly reduced. Q finger of ama was also significantly lower than controls in 1980 but in 1981-1982 was identical to controls. Imax of modern ama was identical to controls of comparable SFT in 1980-1982. The time course of cold deacclimatization thus was BMR, 3 yr; Imax, 3 yr; Q finger, 4 yr; and Tcw, 5 yr. This longitudinal study provides further evidence that acclimatization to cold did at one time exist in these diving women.     

Effects of human menstrual cycle on thermoregulatory vasodilation during exercise

To investigate the effects of the menstrual cycle and of exercise intensity on the relationship between finger blood flow (FBF) and esophageal temperature (Tes), we studied four women, aged 20-32 years. Subjects exercised at 40% and 70% VO2max in the semi-supine posture at an ambient temperature of 20 degrees C. Resting Tes was higher during the luteal phase than the follicular phase (P less than 0.01). There were no significant differences between the two phases in FBF, oxygen consumption, carbon dioxide production, heart rate or minute ventilation at rest and during exercise, respectively. Each regression line of the FBF-Tes relationship consists of two distinct segments of FBF change to Tes (slope 1 and 2). FBF increased at a threshold Tes for vasodilation ([Tes 0]) and the rate of FBF rise became greater at ([Tes 0]) and the rate of FBF rise became greater at another Tes above this threshold ([Tes 0']). For both levels of exercise, [Tes 0] and [Tes 0'] were shifted upward during the luteal phase, but the slopes of the FBF-Tes relationship were almost the same in the two phases of the menstrual cycle. Increasing exercise intensity induced a significant decrease in slope 1 of the FBF-Tes relationship during the follicular (P less than 0.01) and the luteal phases (P less than 0.02), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Exercise thermoregulation after prolonged wakefulness

The effect of 33 h of wakefulness on the control of forearm cutaneous blood flow and forearm sweating during exercise was studied in three men and three women. Subjects exercised for 30 min at 60% peak O2 consumption while seated behind a cycle ergometer (Ta = 35 degrees C, Pw = 1.0 kPa). We measured esophageal temperature (Tes), mean skin temperature, and arm sweating continuously and forearm blood flow (FBF) as an index of skin blood flow, twice each minute by venous occlusion plethysmography. During steady-state exercise, Tes was unchanged by sleep loss. The sensitivity of FBF to Tes was depressed an average of 30% (P less than 0.05) after 33 h of wakefulness with a slight decrease (-0.15 degrees C, P less than 0.05) in the core temperature threshold for vasodilatory onset. Sleep loss did not alter the Tes at which the onset of sweating occurred; however, sensitivity of arm sweating to Tes tended to be lower but was not significant. Arm skin temperature was not different between control and sleep loss experiments. Reflex cutaneous vasodilation during exercise appeared to be reduced by both central and local factors after 33 h of wakefulness.     

Effect of exercise hemoconcentration and hyperosmolality on exercise responses

We investigated the effects of a decrease in plasma volume (PV) and an increase in plasma osmolality during exercise on circulatory and thermoregulatory responses. Six subjects cycled at approximately 65% of their maximum O2 uptake in a warm environment (30 degrees C, 40% relative humidity). After 30 min of control (C) exercise (no infusion), PV decreased 13.0%, or 419 +/- 106 (SD) ml, heart rate (HR) increased to 167 +/- 3 beats/min, and esophageal temperature (Tes) rose to 38.19 +/- 0.09 degrees C (SE). During infusion studies (INF), infusates were started after 10 min of exercise. The infusates contained 5% albumin suspended in 0.45, 0.9, or 3.0% saline. The volume of each infusate was adjusted so that during the last 10 min of exercise PV was maintained at the preexercise level and osmolality was allowed to differ. HR was significantly lower (10-16 beats/min) during INF than during C. Tes was reduced significantly during INF, with trends for increased skin blood flow and decreased sweating rates. No significant differences in HR, Tes, or sweating rate occurred between the three infusion conditions. We conclude that the decrease in PV, which normally accompanies moderate cycle exercise, compromises circulatory and thermal regulations. Increases in osmolality appear to have small if any effects during such short-term exercise.     

Thermal adjustment to cold-water exposure in resting men and women

Thermoregulatory responses were studied in 10 men and 8 women at rest in air and during 1-h immersion in water at 20, 24, and 28 degrees C. For men of high body fat (27.6%), rectal temperature (Tre) and oxygen consumption (VO2) were maintained at air values at all water temperatures (Tw). For men of average (16.8%) and low (9.2%) fat the change in Tre (delta Tre) was inversely related to body fat at all Tw with VO2 increasing to 1.07 l X min-1 for a -1.6 degrees C delta Tre for lean men. For women of average (25.2%) and low (18.5%) fat Tre decreased steadily during immersion at all Tw. The greatest changes occurred at 20 degrees C with little differences in delta Tre and VO2 noted between these groups of women. In comparison with males of similar percent fat, Tre dropped to a greater extent (P less than 0.05) in females at 20 and 24 degrees C. Stated somewhat differently, lean women with twice the percentage of fat have similar delta Tre as lean men at all Tw. For delta Tre greater than -1.0 degree C men showed significantly greater (P less than 0.05) thermogenesis compared with women. The differences in thermoregulation between men and women during cold stress at rest may be due partly to the sensitivity of the thermogenic response as well as the significant differences in lean body weight and surface area-to-mass ratio between the sexes.     

Finger vasodilation correlates better with tympanic than esophageal temperature

The relationship of finger blood flow (FBF) measured by venous occlusion plethysmography to tympanic temperature (Tty) was compared with that of FBF to esophageal temperature (Tes) during exercise at 50% VO2max for 40 min at an ambient temperature of 25 degrees C. The relationship of FBF to Tes showed an inflexion as Tes increased during exercise. The slope of the regression line showing the relationship between FBF and Tes was initially moderate, and then suddenly became steeper at the inflexion point. The relationship of FBF to Tty, however, was linear, without an inflexion. The results suggest that finger vasodilation during moderate exercise correlates better with tympanic than esophageal temperature.     

Control of sweating during the human menstrual cycle

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 (Tes), mean skin temperature (Tsk), oxygen uptake and forearm sweating (ms) were made during all experiments. Protocol I involved both passive heat exposure (3 h) and cycle exercise at approximately 80% VO2 peak during which the environmental chamber was controlled at Ta = 50.0 degrees C, rh = 14% (Pw = 1.7 kPa). In protocol II subjects were tested during thirty-five minutes of exercise at approximately 85% VO2 peak at Ta = 35 degrees C and rh = 25% (Pw = 1.4 kPa). The normal L increase in resting Tes (approximately 0.3 degrees C) occurred in all seven subjects. Tsk was higher during L than F in all experiments conducted at 50 degrees C. During exercise and passive heat exposure, the Tes threshold for sweating was higher in L, with no change in the thermosensitivity (slope) of ms to Tes between menstrual cycle phases. This rightward or upward shift in Tes threshold for initiation of sweating averaged 0.5 degrees 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.