Moderate exercise increases postexercise thresholds for vasoconstriction and shivering

The purpose of this study was to evaluate theeffect of exercise on the subsequent postexercise thresholds forvasoconstriction and shivering. On two separate days, with six subjects(3 women), a whole body water-perfused suit slowly decreased mean skintemperature (~7.0°C/h) until thresholds for vasoconstriction andshivering were clearly established. Subjects were then rewarmed byincreasing water temperature until both esophageal and mean skintemperatures returned to near-baseline values. Subjects eitherperformed 15 min of cycle ergometry (65% maximalO₂ consumption) followed by 30 minof recovery (Exercise) or remained seated with no exercise for 45 min(Control). Subjects were then cooled again. We mathematically compensated for changes in skin temperatures by using the established linear cutaneous contribution of skin to the control ofvasoconstriction and shivering (20%). The calculated core temperaturethreshold (at a designated skin temperature of 30.0°C) forvasoconstriction increased significantly from 36.64 ± 0.20 to 36.89 ± 0.22°C postexercise (P < 0.01). Similarly, the shivering threshold increased from 35.73 ± 0.13 to 36.13 ± 0.12°C postexercise(P < 0.01). In contrast, sequentialmeasurements, without exercise, demonstrate a time-dependent decreasein both the vasoconstriction (0.10°C) and shivering (0.12°C) thresholds. These data indicate that exercise has a prolonged effect byincreasing the postexercise thresholds for both cold thermoregulatoryresponses.

     

Effectiveness of three field treatments for induced mild (33.0 degrees C) hypothermia

Three field applicable treatments for hypothermia were compared. Subjects were cooled in stirred cold water (8.0 degrees C) to a core temperature (Tco) as low as 33 degrees C and rewarmed in a random order by each of three techniques: shivering, external heat, and treadmill exercise. Tco was monitored with an esophageal thermistor probe at the level of the heart. Treatment effectiveness was determined by calculating the amount of Tco afterdrop, length of afterdrop period, rate of Tco increase, and total recovery time. Rate of Tco increase for exercise (4.9 degrees C/h) was significantly higher (P less than 0.05) than shivering (3.5 degrees C/h) but not external heat (3.7 degrees C/h). Exercise afterdrop amount and afterdrop length values (0.95 degrees C and 24 min, respectively) were significantly higher (P less than 0.05) than both shivering (0.33 degrees C, 15 min) and external heat (0.32 degrees C, 14 min). Therefore, although rate of Tco increase during recovery for exercise was faster than for shivering or external heat, as it was preceded by a greater afterdrop length and amount, total recovery time did not differ among the three treatments.     

Effect of water temperature on cooling efficiency during hyperthermia in humans.

We evaluated the cooling rate of hyperthermic subjects, as measured by rectal temperature (T(re)), during immersion in a range of water temperatures. On 4 separate days, seven subjects (4 men, 3 women) exercised at 65% maximal oxygen consumption at an ambient temperature of 39 degrees C until T(re) increased to 40 degrees C (45.4 +/- 4.1 min). After exercise, the subjects were immersed in a circulated water bath controlled at 2, 8, 14, or 20 degrees C until T(re) returned to 37.5 degrees C. No difference in cooling rate was observed between the immersions at 8, 14, and 20 degrees C despite the differences in the skin surface-to-water temperature gradient, possibly because of the presence of shivering at 8 and 14 degrees C. Compared with the other conditions, however, the rate of cooling (0.35 +/- 0.14 degrees C/min) was significantly greater during the 2 degrees C water immersion, in which shivering was seldom observed. This rate was almost twice as much as the other conditions (P < 0.05). Our results suggest that 2 degrees C water is the most effective immersion treatment for exercise-induced hyperthermia.     

Individual variability in the peripheral and core interthreshold zones

The purpose of the study was to investigate the degree of subject variability in the peripheral and core temperature thresholds of the onset of shivering and sweating. Nine healthy young male subjects participated in three trials. In the first two trials, wearing only shorts, they were exposed to air temperatures of 5 degrees C and 40 degrees C until the onset of shivering and sweating, respectively. In the second experiment, subjects wore a water perfused suit that was perfused with 25 degrees C water at a rate of 600 cc/min. They exercised on an ergometer at 50% of their maximum work rate for 10-15 min. At the onset of sweating, the exercise was terminated, and they remained seated until the onset of shivering, as reflected in oxygen uptake. In the first two trials, rectal temperature (Tre) was stable, despite displacements in skin temperature (Tsk), whereas in the third trial, Tsk (measured at four sites) was almost constant (30-32 degrees C), and the thermoregulatory responses were initiated due to changes in Tre alone. The results of the first two trials established the peripheral interthreshold zone, whereas the results of the third trial established the core interthreshold zone. The results demonstrated individual variability in the peripheral and core interthreshold zones, a proportional correlation between both zones (r=0.87), and a relatively higher contribution of adiposity in both zones as compared with those of other non-thermal factors such as height, weight, body surface area, surface area-to mass ratio, and the maximum work load.     

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)     

Significance of skin temperature changes in surface electromyography.

Differing results have been reported concerning the direction and quantity of the electromyogram (EMG) amplitude response to changes in tissue temperature. The EMG signals from the soleus muscle of six healthy human subjects were therefore recorded during dynamic exercise (concentric contractions) at ambient temperatures of 30 degrees C and 14 degrees C. The mean skin temperature (Tsk) above the muscle investigated was 32.9 degrees C and 21.7 degrees C, respectively. The core temperature, estimated by rectal temperature, was unchanged. The cooling of the superficial tissues caused approximately a doubling of the EMG amplitude. For the probability level 0.9 in the amplitude probability distribution function, the average signal level increased from 73 microV to 135 microV (P = 0.02). The average mean power frequency of the EMG signal was reduced from 142 Hz to 83 Hz (P = 0.004). The amplitude increase was not due to shivering but other possible explanations are presented. As the changes in Tsk investigated were within the range which may occur normally during the working hours, it was concluded that Tsk should be carefully controlled in vocational EMG studies.     

A second postcooling afterdrop: more evidence for a convective mechanism.

An attempt was made to demonstrate the importance of increased perfusion of cold tissue in core temperature afterdrop. Five male subjects were cooled twice in water (8 degrees C) for 53-80 min. They were then rewarmed by one of two methods (shivering thermogenesis or treadmill exercise) for another 40-65 min, after which they entered a warm bath (40 degrees C). Esophageal temperature (Tes) as well as thigh and calf muscle temperatures at three depths (1.5, 3.0, and 4.5 cm) were measured. Cold water immersion was terminated at Tes varying between 33.0 and 34.5 degrees C. For each subject this temperature was similar in both trials. The initial core temperature afterdrop was 58% greater during exercise (mean +/- SE, 0.65 +/- 0.10 degrees C) than shivering (0.41 +/- 0.06 degrees C) (P < 0.005). Within the first 5 min after subjects entered the warm bath the initial rate of rewarming (previously established during shivering or exercise, approximately 0.07 degrees C/min) decreased. The attenuation was 0.088 +/- 0.03 degrees C/min (P < 0.025) after shivering and 0.062 +/- 0.022 degrees C/min (P < 0.025) after exercise. In 4 of 10 trials (2 after shivering and 2 after exercise) a second afterdrop occurred during this period. We suggest that increased perfusion of cold tissue is one probable mechanism responsible for attenuation or reversal of the initial rewarming rate. These results have important implications for treatment of hypothermia victims, even when treatment commences long after removal from cold water.     

Menstrual cycle phase affects temperature regulation during endurance exercise.

We investigated whether menstrual cycle phase would affect temperature regulation during an endurance exercise bout performed at room temperature (Ta) of 22 degrees C and 60% relative humidity. Nine eumenorrheic women [age 27.2 +/- 3.7 yr, peak O2 uptake (VO2) 2.52 +/- 0.35 l/min] performed 60 min of cycle exercise at 65% of peak VO2. Subjects were tested in both midfollicular (F) and midluteal (L) phases, although one woman did not show a rise in serum progesterone (P4) that is typically evident 1 wk after ovulation. VO2, rectal (Tre) and skin (Tsk) temperatures, heart rates (HR), and ratings of perceived exertion (RPE) were measured throughout exercise. Sweat loss (SL) was estimated from pre- and postexercise body weight differences. VO2, SL, and Tsk were not affected by menstrual cycle phase. Preexercise Tre was 0.3 degrees C higher during L than during F conditions, and this difference increased to 0.6 degrees C by the end of exercise (P less than 0.01). Compared with F, HRs during L were approximately 10 beats/min greater (P less than 0.001) at all times, whereas RPE responses were significantly greater (P less than 0.01) by 50 min of cycling. No differences in any measured values were found in the subject whose P4 was low in both test conditions. Results indicate that thermoregulation (specifically, regulation of Tre), as well as cardiovascular strain and perception of exercise, was adversely affected during the L phase.     

Muscle blood flow is not reduced in humans during moderate exercise and heat stress

The effect of heat stress on circulation in an exercising leg was determined using one-legged knee extension and two-legged bicycle exercise, both seated and upright. Subjects exercised for three successive 25-min periods wearing a water-perfused suit: control [CT, mean skin temperature (Tsk) = 35 degrees C], hot (H, Tsk = 38 degrees C), and cold (C, Tsk = 31 degrees C). During the heating period, esophageal temperature increased to a maximum of 37.91, 39.35, and 39.05 degrees C in the three types of exercise, respectively. There were no significant changes in pulmonary O2 uptake (VO2) throughout the entire exercise period with either one or two legs. Leg blood flow (LBF), measured in the femoral vein of one leg by thermodilution, remained unchanged between CT, H, and C periods. Venous plasma lactate concentration gradually declined over time, and no trend for an increased lactate release during the heating period was found. Similarly, femoral arteriovenous O2 difference and leg VO2 remained unchanged between the three exercise periods. Although cardiac output (acetylene rebreathing) was not significantly higher during H, there was a tendency for an increase of 1 and 2 l/min in one- and two-legged exercise, respectively, which could account for part of the increase in total skin blood flow during heating (gauged by changes in forearm blood flow). Because LBF was not reduced during exercise and heat stress in these experiments, the additional increase in skin blood flow must have been met by redistribution of blood away from vascular beds other than active skeletal muscle.     

Effects of metabolic rate on thermal responses at different air velocities in -10 degrees C

The effects of exercise intensity on thermoregulatory responses in cold (-10 degrees C) in a 0.2 (still air, NoWi), 1.0 (Wi1), and 5.0 (Wi5) m x s(-1) wind were studied. Eight young and healthy men, preconditioned in thermoneutral (+20 degrees C) environment for 60 min, walked for 60 min on the treadmill at 2.8 km/h with different combinations of wind and exercise intensity. Exercise level was adjusted by changing the inclination of the treadmill between 0 degrees (lower exercise intensity, metabolic rate 124 W x m(-2), LE) and 6 degrees (higher exercise intensity, metabolic rate 195 W x m(-2), HE). Due to exercise increased heat production and circulatory adjustments, the rectal temperature (T(re)), mean skin temperature (Tsk) and mean body temperature (Tb) were significantly higher at the end of HE in comparison to LE in NoWi and Wi1, and T(re) and Tb also in Wi5. Tsk and Tb were significantly decreased by 5.0 m x s(-1) wind in comparison to NoWi and Wi1. The higher exercise intensity was intense enough to diminish peripheral vasoconstriction and consequently the finger skin temperature was significantly higher at the end of HE in comparison to LE in NoWi and Wi1. Mean heat flux from the skin was unaffected by the exercise intensity. At LE oxygen consumption (VO2) was significantly higher in Wi5 than NoWi and Wi1. Heart rate was unaffected by the wind speed. The results suggest that, with studied exercise intensities, produced without changes in walking speed, the metabolic rate is not so important that it should be taken into consideration in the calculation of wind chill index.     

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.     

Blood-brain barrier integrity may be threatened by exercise in a warm environment

Seven active men were recruited to examine changes in the serum concentration of S100beta, a proposed peripheral marker of blood-brain barrier permeability, following prolonged exercise in temperate (T) and warm (W) conditions. Subjects were seated immersed to the neck in water at 35.0 (0.1) degrees C (T) or 39.0 (0.1) degrees C (W) for 30 min. Subjects then entered a room maintained at either 18.3 (1.8) degrees C (T) or 35.0 (0.3) degrees C (W) and completed 60 min of cycle exercise at 60% peak oxygen uptake. Serum S100beta concentration was elevated after exercise in the W trial (+0.12 (0.10) microg/l; P = 0.02) but not after the T trial (P = 0.238). Water immersion and exercise elevated core temperature by 2.1 (0.5) degrees C to 39.5 (0.3) degrees C at the end of exercise in the W trial compared with a 0.9 (0.2) degrees C increase during the T trial (P < 0.001). Weighted mean skin temperature was higher throughout the W trial compared with the T trial (P < 0.001). Heart rate (P < 0.001) and blood glucose (P < 0.001) and lactate (P < 0.001) concentrations were elevated to a greater extent during exercise in the W trial than in the T trial. Ratings of perceived exertion (P < 0.001) and thermal comfort (P < 0.001) were markedly higher throughout the W trial than in the T trial. The results of this study demonstrate that serum S100beta was elevated after water immersion and prolonged exercise in a warm environment, suggesting that blood-brain barrier permeability may be altered.     

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

In six male subjects the sweating thresholds, heart rate (fc), as well as the metabolic responses to exercise of different intensities [40%, 60% and 80% maximal oxygen uptake (VO2max)], were compared at ambient temperatures (Ta) of 5 degrees C (LT) and 24 degrees 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 less than 0.05). During 20-min exercise at 40% VO2max performed in the cold no sweating was recorded, while at higher exercise intensities sweating occurred at similar rectal temperatures (Tre) but at lower mean skin (Tsk) and mean body temperatures (Tb) in LT than MT experiments (P less than 0.001). The exercise induced VO2 increase was greater only at the end of the light (40% VO2max) exercise in the cold in comparison with MT (P less than 0.001). Both fc and blood lactate concentration [1a]b were lower at the end of LT than MT for moderate (60% VO2max) and heavy (80% VO2max) exercises. It was concluded that the sweating threshold during exercise in the cold environment had shifted towards lower Tb and Tsk. 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 degrees C since the increases in Tre, fc and [1a]b were lower at the lower Ta.     

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)     

Core temperature "null zone".

An experimental protocol was designed to investigate whether human core temperature is regulated at a "set point" or whether there is a neutral zone between the core thresholds for shivering thermogenesis and sweating. Nine male subjects exercised on an underwater cycle ergometer at a work rate equivalent to 50% of their maximum work rate. Throughout an initial 2-min rest period, the 20-min exercise protocol, and the 100-min recovery period, subjects remained immersed to the chin in water maintained at 28 degrees C. On completion of the exercise, the rate of forehead sweating (Esw) decayed from a mean peak value of 7.7 +/- 4.2 (SD) to 0.6 +/- 0.3 g.m-2.min-1, which corresponds to the rate of passive transpiration, at core temperatures of 37.42 +/- 0.29 and 37.39 +/- 0.48 degrees C, as measured in the esophagus (Tes) and rectum (Tre), respectively. Oxygen uptake (VO2) decreased rapidly from an exercising level of 2.11 +/- 0.25 to 0.46 +/- 0.09 l/min within 4 min of the recovery period. Thereafter, VO2 remained stable for approximately 20 min, eventually increased with progressive cooling of the core region, and was elevated above the median resting values determined between 15 and 20 min at Tes = 36.84 +/- 0.38 degrees C and Tre = 36.80 +/- 0.39 degrees C. These results indicate that the core temperatures at which sweating ceases and shivering commences are significantly different (P less than 0.001) regardless of whether core temperature is measured within the esophagus or rectum.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanism of vasoconstriction in the rat's tail when warmed locally.

The vascular response of the tail to local warming was investigated in urethan-anesthetized rats whose colonic temperature was maintained at 39.5 degrees C with an intravenous thermode at an ambient temperature of 23 degrees C. The tail, covered with thin latex tubing, was immersed in temperature-controlled water initially kept at 35 degrees C. The tail was warmed by raising the water bath temperature from 35 to 44 degrees C at a constant rate. Tail blood flow (BF), mean arterial blood pressure (BP), and tail skin temperature (Tsk) were measured before and during the local warming. Tail vascular conductance (VC) was computed as 100 x tail BF/BP. When Tsk exceeded 37 degrees C, tail BF and VC significantly decreased from the levels at Tsk of 35 degrees C, and significant reductions in tail BF and VC occurred until Tsk reached 42 degrees C. Surgical deafferentation of the tail, chemical sympathectomy with 6-hydroxydopamine (100 mg/kg), and alpha-blockade with phentolamine (7 or 40.1-45.5 mg/kg) or phenoxybenzamine (5 mg/kg) failed to stop the decrease in tail BF and VC during the local warming. These results suggest that a reflex via the central nervous system and the alpha-adrenergic sympathetic nervous system is not indispensable for heat-induced vasoconstriction (HIVC). It is therefore assumed that, at least in the rat's tail, HIVC predominantly originates from a local vascular response to high temperature.     

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

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

Thermoregulatory responses to cold water at different times of day.

This study examined how time of day affects thermoregulation during cold-water immersion (CWI). It was hypothesized that the shivering and vasoconstrictor responses to CWI would differ at 0700 vs. 1500 because of lower initial core temperatures (T(core)) at 0700. Nine men were immersed (20 degrees C, 2 h) at 0700 and 1500 on 2 days. No differences (P > 0.05) between times were observed for metabolic heat production (M, 150 W. m(-2)), heat flow (250 W. m(-2)), mean skin temperature (T(sk), 21 degrees C), and the mean body temperature-change in M (DeltaM) relationship. Rectal temperature (T(re)) was higher (P < 0.05) before (Delta = 0.4 degrees C) and throughout CWI during 1500. The change in T(re) was greater (P < 0. 05) at 1500 (-1.4 degrees C) vs. 0700 (-1.2 degrees C), likely because of the higher T(re)-T(sk) gradient (0.3 degrees C) at 1500. These data indicate that shivering and vasoconstriction are not affected by time of day. These observations raise the possibility that CWI may increase the risk of hypothermia in the early morning because of a lower initial T(core).     

Thermal responses of unclothed men exposed to both cold temperatures and high altitudes.

Six resting men were exposed to three temperatures (15.5, 21, 26.5 degrees C) for 120 min at three altitudes (sea level, 2,500 m, 5,000 m). A 60-min sea-level control at the scheduled temperature preceded the nine altitude episodes. Comparison of the base-line results at any one temperature showed no differences between rectal temperatures (Tre) or mean weighted skin temperatures (Tsk). After 120 min, Tre and Tsk not only depended on ambient temperature but also altitude. The initial rate of fall in Tre increased with altitude and equilibrium occurred earlier. At 15.5 degrees C, Tre was 0.3 degrees C lower at 5,000 m and 0.2 degrees C lower at 2,500 m than at sea level. Tsk was almost 2 degrees C higher at 15.5 degrees C at 5,000 m and 1 degrees C higher at 2,500 m than at sea level. Similar, smaller differences were observed at 21 degrees C. Mean weighted body temperature showed no change with altitude, but, since the gradient between core and shell was reduced, a shift of blood toward the periphery is implied.     

Heat debt as an index for cold adaptation in men

Several types of cold adaptation in men have been described in the literature (metabolic, insulative, hypothermic). The aim of this study is to show that the decrease of heat debt can be considered as a new index for cold adaptation. Ten male subjects were acclimated by water immersions (temperature 10-15 degrees C, 4 immersions/wk over 2 mo). Thermoregulatory responses before and after acclimation were tested by a standard cold test in a climatic chamber for 2 h at rest [dry bulb temperature (Tdb): 10 degrees C; relative humidity (rh): 25%]. After adaptation, four thermoregulatory modifications were observed: an increase in the delay for the onset of shivering (32.7 +/- 7.99 instead of 14.1 +/- 5.25 min); a decrease of body temperature levels for the onset of shivering [rectal temperature (Tre): 37.06 +/- 0.08 instead of 37.31 +/- 0.06 degrees C; mean skin temperature (Tsk): 24.83 +/- 0.56 instead of 26.86 +/- 0.46 degrees C; mean body temperature (Tb): 33.03 +/- 0.20 instead of 34.16 +/- 0.37 degrees C); a lower level of body temperatures in thermoneutrality (Tre = 37.16 +/- 0.08 instead of 37.39 +/- 0.06 degrees C; Tsk = 31.29 +/- 0.21 instead of 32.01 +/- 0.22 degrees C; Tb = 35.92 +/- 0.08 instead of 36.22 +/- 0.05 degrees C); a decrease of heat debt calculated from the difference between heat gains and heat losses (5.66 +/- 0.08 instead of 8.33 +/- 0.38 kJ/kg). The different types of cold adaptation observed are related to the physical characteristics of the subjects (percent body fat content) and the level of physical fitness (VO2max).(ABSTRACT TRUNCATED AT 250 WORDS)