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.     

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.     

Arterial vs. rectal temperature in ponies: rest, exercise, CO2 inhalation, and thermal stresses

We assessed in ponies the adequacy of using rectal (Tre) rather than arterial temperature (Tar) under conditions common to ventilatory control experiments, i.e., CO2 breathing, thermal stress, and particularly exercise. We were interested in whether, and to what extent, Tar-Tre differences could lead to errors in arterial blood gas corrections. At control environmental temperatures (Ta) of 5 degrees C in the winter and 21 degrees C in the summer, Tar and Tre (37.1 degrees C) did not differ (P greater than 0.05). Elevating winter or summer Ta by 10-18 degrees C for 2-days or lowering summer Ta by 9 degrees C (2-days) did not change Tar or Tre (P greater than 0.05). Furthermore, elevating inspired PCO2 to 42 Torr for 15 min did not alter Tar or Tre from control (P greater than 0.05). During treadmill exercise, at 1.8 mph 5% grade, Tar and Tre did not change significantly (P greater than 0.05) from rest by 11 min of work. At 3 mph 5% grade, Tar increased progressively by 0.3 degrees C (P less than 0.05) while Tre tended to increase 0.1 degree C by 11 min. During moderate exercise at 6 mph 5% grade, Tar increased 0.9 degree C (P less than 0.05) while Tre increased 0.25 degree C (P less than 0.05). Finally, by 6 min of heavy exercise at 8 mph 20% grade, Tar increased 2 degrees C (P less than 0.05) while Tre increased 0.5 degree C (P less than 0.05). The Tar-Tre differences during the latter three work loads were statistically significant (P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)     

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

Thermoregulatory responses were studied in 10 men and 8 women during 36-W exercise for 1 h in air and water at 20, 24, and 28 degrees C. Men were classified as high (27.6%; n = 2), average (16.8%; n = 4), and low (9.2%; n = 4) percent body fat, whereas women were classified as average (25.2%; n = 4) and low (18.5%; n = 4) fat. For both men and women, exercise of about 1.7 l O2 X min-1 was beneficial in either preventing or retarding the fall in rectal temperature (Tre) observed in a previous study for the same subjects at rest. The greatest thermal strain was noted for the leanest subjects. However, in no instance did exercise facilitate a drop in Tre compared with resting conditions. Despite a larger surface area-to-mass ratio (P less than 0.05) and less effective thermoregulation for women at rest compared with men, essentially similar thermoregulatory responses were observed for both sexes during exercise at each water temperature. For both the men and women, the thermoregulatory benefits of exercise were due largely to the added heat production from physical activity. For the female, an additional benefit of exercise may in part be derived from a more favorable distribution of subcutaneous fat over the active musculature.     

Hypohydration and exercise: effects of heat acclimation, gender, and environment

This study examined the effects of heat acclimation and subject gender on treadmill exercise in comfortable (20 degrees C, 40% rh), hot-dry (49 degrees C, 20% rh), and hot-wet (35 degrees C, 79% rh) environments while subjects were hypo- or euhydrated. Six male and six female subjects, matched for maximal aerobic power and percent body fat, completed two exercise tests in each environment both before and after a 10-day heat acclimation program. One exercise test was completed during euhydration and one during hypohydration (-5.0% from baseline body weight). In general, no significant (P greater than 0.05) differences were noted between men and women at the completion of exercise for rectal temperature (Tre), mean skin temperature (Tsk), or heat rate (HR) during any of the experimental conditions. Hypohydration generally increased Tre and HR values and decreased sweat rate values while not altering Tsk values. In the hypohydration experiments, heat acclimation significantly reduced Tre (0.19 degrees C) and HR (13 beats X min-1) values in the comfortable environment, but only HR values were reduced in hot-dry (21 beats X min-1) and hot-wet (21 beats X min-1) environments. The present findings indicated that men and women respond in a physiologically similar manner to hypohydration during exercise. They also indicated that for hypohydrated subjects heat acclimation decreased thermoregulatory and cardiovascular strain in a comfortable environment, but only cardiovascular strain decreased in hot environments.     

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)     

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)     

Conductive and convective heat flows of exercising humans in cold water

The apparent conductance (Kss, in W.m-2.degrees C-1) of a given region of superficial shell (on the thigh, fat + skin) was determined on four nonsweating and nonshivering subjects, resting and exercising (200 W) in water [water temperature (Tw) 22-23 degrees C] Kss = Hss/(Tsf-Tsk) where Hss is the skin-to-water heat flow directly measured by heat flow transducers and Tsf and Tsk are the temperatures of the subcutaneous fat at a known depth below the skin surface and of the skin surface, respectively. The convective heat flow (qc) through the superficial shell was then estimated as qc = (Tsf - Tsk).(Kss - Kss,min), assuming that at rest Kss was minimal (Kss,min) and resting qc = 0. The duration of immersion was set to allow rectal temperature (Tre) to reach approximately 37 degrees C at the end of rest and approximately 38 degrees C at the end of exercise. Except at the highest Tw used, Kss at the start of exercise was always Kss,min and averaged 51 W.m-2.degrees C-1 (range 33-57 W.m-2.degrees C-1) across subjects, and qc was zero. At the end of exercise at the highest Tw used for each subject, Kss averaged 97 W.m-2.degrees C-1 (range 77-108 W.m-2.degrees C-1) and qc averaged 53% (range 48-61%) of Hss (mean Hss = 233 W.m-2).(ABSTRACT TRUNCATED AT 250 WORDS)     

Influences of age and gender on human thermoregulatory responses to cold exposures

To delineate age- and gender-related differences in physiological responses to cold exposure, men and women between the ages of 20 and 29 yr and 51 and 72 yr, wearing minimal clothing, were exposed at rest for 2 h to 28, 20, 15, and 10 degrees C room temperatures with 40% relative humidity. During the coldest exposure, the rates of increase in metabolic rate (W X m-2 or ml X kg lean body mass-1 X min-1 were similar for all groups. However, older women (n = 7) may have benefited from a larger (P less than 0.05) early metabolic (M) increase (40% within 15 min) than young men (18%) (n = 10), young women (5%) (n = 10), or older men (5%) (n = 10). A similar rapid M response in older women occurred during the 15 degrees C exposure. During all cold exposures, older women maintained constant rectal temperature (Tre) and young women maintained Tre only during the 20 degrees C exposures, whereas Tre of the men declined during all cold exposures (P less than 0.01). Changes in Tre and mean skin temperature (Ts) during cold exposure were largely related to body fat, although age and surface area/mass modified the changes in men. The data suggest that older men are more susceptible to cold ambients than younger people, since they did not prevent a further decline in their initially relatively low Tre. Despite greater insulation from body fat, the older women maintained a constant Tre at greater metabolic cost than men or younger women.     

Tissue temperature profile in the human forearm during thermal stress at thermal stability.

The purpose of the present study was to investigate the effect of a range of water temperatures (Tw from 15 to 36 degrees C) on the tissue temperature profile of the resting human forearm at thermal stability. Tissue temperature (Tti) was continuously monitored by a calibrated multicouple probe during 3 h of immersion of the forearm. The probe was implanted approximately 9 cm distal from the olecranon process along the ulnar ridge. Tti was measured every 5 mm, from the longitudinal axis of the forearm (determined from computed tomography scanning) to the skin surface. Along with Tti, skin temperature (Tsk), rectal temperature (Tre), and blood flow were measured during the immersions. For all temperature conditions, the temperature profile inside the limb was linear as a function of the radial distance from the forearm axis (P less than 0.001). Temperature gradient measured in the forearm ranged from 0.2 +/- 0.1 degrees C C cm (Tw = 36 degrees C) to 2.3 +/- 0.5 degrees C cm (Tw = 15 degrees C). The maximal Tti was measured in all cases at the longitudinal axis of the forearm and was in all experimental conditions lower than Tre. On immersion at Tw less than 36 degrees C, the whole forearm can be considered to be part of the shell of the body. With these experimental data, mathematical equations were developed to predict, with an accuracy of at least 0.6 degrees C, the Tti at any depth inside the forearm at steady state during thermal stress.(ABSTRACT TRUNCATED AT 250 WORDS)     

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)     

The physiological strain index applied to heat-stressed rats.

A physiological strain index (PSI) based on heart rate (HR) and rectal temperature (Tre) was recently suggested to evaluate exercise-heat stress in humans. The purpose of this study was to adjust PSI for rats and to evaluate this index at different levels of heat acclimation and training. The corrections of HR and Tre to modify the index for rats are as follows: PSI = 5 (Tre t - Tre 0). (41.5 - Tre 0)-1 + 5 (HRt - HR0). (550 - HR0)-1, where HRt and Tre t are simultaneous measurements taken at any time during the exposure and HR0 and Tre 0 are the initial measurements. The adjusted PSI was applied to five groups (n = 11-14 per group) of acclimated rats (control and 2, 5, 10, and 30 days) exposed for 70 min to a hot climate [40 degrees C, 20% relative humidity (RH)]. A separate database representing two groups of acclimated or trained rats was also used and involved 20 min of low-intensity exercise (O2 consumption approximately 50 ml. min-1. kg-1) at three different climates: normothermic (24 degrees C, 40% RH), hot-wet (35 degrees C, 70% RH), and hot-dry (40 degrees C, 20% RH). In normothermia, rats also performed moderate exercise (O2 consumption approximately 60 ml. min-1. kg-1). The adjusted PSI differentiated among acclimation levels and significantly discriminated among all exposures during low-intensity exercise (P < 0.05). Furthermore, this index was able to assess the individual roles played by heat acclimation and exercise training.     

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.     

Muscle glycogen availability and temperature regulation in humans

The effects of intramuscular glycogen availability on human temperature regulation were studied in eight seminude subjects immersed in 18 degrees C water for 90 min or until rectal temperature (Tre) decreased to 35.5 degrees C. Each subject was immersed three times over a 3-wk period. Each immersion followed 2.5 days of a specific dietary and/or exercise regimen designed to elicit low (L), normal (N), or high (H) glycogen levels in large skeletal muscle groups. Muscle glycogen concentration was determined in biopsies taken from the vastus lateralis muscle before and after each immersion. Intramuscular glycogen concentration before the immersion was significantly different among the L, N, and H trials (P less than 0.01), averaging 247 +/- 15, 406 +/- 23, and 548 +/- 42 (SE) mmol glucose units.kg dry muscle-1, respectively. The calculated metabolic heat production during the first 30 min of immersion was significantly lower during L compared with N or H (P less than 0.05). The rate at which Tre decreased was more rapid during the L immersion than either N or H (P less than 0.05), and the time during the immersion at which Tre first began to decrease also appeared sooner during L than N or H. The results suggest that low skeletal muscle glycogen levels are associated with more rapid body cooling during water immersion in humans. Higher than normal muscle glycogen levels, however, do not increase cold tolerance.     

Rectal and rectal vs. esophageal temperatures in paraplegic men during prolonged exercise

This study investigated the rectal (Tre), esophageal (Tes), and skin (Tsk) temperature changes in a group of trained traumatic paraplegic men pushing their own wheelchairs on a motor-driven treadmill for a prolonged period in a neutral environment. There were two experiments. The first experiment (Tre and Tsk) involved a homogeneous group (T10-T12/L3) of highly trained paraplegic men [maximum O2 uptake (VO2max) 47.5 +/- 1.8 ml.kg-1.min-1] exercising for 80 min at 60-65% VO2max.Tre and Tsk (head, arm, thigh, and calf) and heart rate (HR) were recorded throughout. O2 uptake (VO2), minute ventilation (VE), CO2 production (VCO2), and heart rate (HR) were recorded at four intervals. During experiment 1 significant changes in HR and insignificant changes in VCO2, VE, and VO2 occurred throughout prolonged exercise. Tre increased significantly from 37.1 +/- 0.1 degrees C (rest) to 37.8 +/- 0.1 degrees C after 80 min of exercise. There were only significant changes in arm Tsk. Experiment 2 involved a nonhomogeneous group (T5-T10/T11) of active paraplegics (VO2max 39.9 +/- 4.3 ml.kg-1.min-1) exercising at 60-65% VO2max for up to 45 min on the treadmill while Tre and Tes were simultaneously recorded. Tes rose significantly faster than Tre during exercise (dT/dt 20 min: Tes 0.050 +/- 0.003 degrees C/min and Tre 0.019 +/- 0.005 degrees C/min), and Tes declined significantly faster than Tre at the end of exercise. Tes was significantly higher than Tre at the end of exercise. Our results suggest that during wheelchair propulsion by paraplegics, Tes may be a better estimate of core temperature than Tre.     

In vivo thermal conductivity of the human forearm tissues

The effective thermal conductivities of the skin + subcutaneous (keff skin + fat) and muscle (keff muscle) tissues of the human forearm at thermal steady state during immersion in water at temperatures (Tw) ranging from 15 to 36 degrees C were determined. Tissue temperature (Tt) was continuously monitored by a calibrated multicouple probe during a 3-h immersion of the resting forearm. Tt was measured every 5 mm from the longitudinal axis of the forearm (determined from computed-tomography scanning) to the skin surface. Skin temperature (Tsk), heat loss (Hsk), and blood flow (Q) of the forearm, as well as rectal temperature (Tre) and arterial blood temperature at the brachial artery (Tbla), were measured during the experiments. When the keff values were calculated from the finite-element (FE) solution of the bioheat equation, keff skin + fat ranged from 0.28 +/- 0.03 to 0.73 +/- 0.14 W.degrees C-1.m-1 and keff muscle varied between 0.56 +/- 0.05 and 1.91 +/- 0.19 W.degrees C-1.m-1 from 15 to 36 degrees C. The values of keff skin + fat and keff muscle, calculated from the FE solution for Tw less than or equal to 30 degrees C, were not different from the average in vitro values obtained from the literature. The keff values of the forearm tissues were linearly related (r = 0.80, P less than 0.001) to Q for Tw greater than or equal to 30 degrees C. It was found that the muscle tissue could account for 92 +/- 1% of the total forearm insulation during immersion in water between 15 and 36 degrees C.     

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)     

Thermal and metabolic responses to cold by men and by eumenorrheic and amenorrheic women

Previous work has suggested that men (M) are more sensitive to cold stress than women. There have also been observations that suggest that amenorrheic women (AW) are less thermally responsive than eumenorrheic women (EW). We investigated the hypothesis that M, EW, and AW would have different responses to cold stress. The subjects (6/group) were tested four times: twice at rest for 60 min (5 and 22 degrees C) and twice in a progressive exercise test (5 and 22 degrees C). At rest at 22 degrees C AW had a lower O2 uptake (VO2) than M and lower rectal (Tre) and finger temperatures than EW. At rest at 5 degrees C both AW and EW had lower skin temperature (Tsk) than M, but there were no group differences in peripheral Tsk sites. M increased VO2 after 10 min and EW after 20 min of cold stress; however, AW did not increase metabolism until 60 min. In the two exercise tests Tre increased in proportion to relative work load; in the 5 degrees C test there was little evidence that exercise increased Tsk sites above rest levels. Few of the metabolic or thermal differences could be accounted for by body fatness, body surface area (BSA), or BSA/kg. The data support the hypothesis that M, EW, and AW have different responses to cold stress.     

Forearm temperature profile during the transient phase of thermal stress.

The transient temperature response of the resting human forearm immersed in water at temperatures (Tw) ranging from 15 to 36 degrees C was investigated. Tissue temperature (Tt) was continuously monitored by a calibrated multicouple probe during the 3-h immersions. Tt was measured every 5 mm, from the longitudinal axis of the forearm to the skin surface. Skin temperature, rectal temperature, and blood flow (Q) were also measured during the immersions. The maximum rate of change of the forearm mean tissue temperature (Tt, max) occurred during the first 5 min of the immersion. Tt, max was linearly dependent on Tw (P less than 0.001), with mean values (SEM) ranging from -0.8 (0.1) degrees C.min-1 at 15 degrees C to 0.2 (0.1) degrees C.min-1 at 36 degrees C. The maximum rate of change of compartment mean temperature was dependent (P less than 0.001) on the radial distance from the longitudinal axis of the forearm. The half-time for thermal steady state of the forearm mean tissue temperature was linearly dependent on Tw between 30 and 36 degrees C (P less than 0.01), with mean values (SEM) ranging from 15.6 (0.6) min at 30 degrees C to 9.7 (1.2) min at 36 degrees C and not different between 15 and 30 degrees C, averaging 16.2 (0.6) min. There was a significant linear relationship between the half-time for thermal steady-state of the compartment mean temperature and the radial distance from the longitudinal axis of the forearm for each value of Tw tested (P less than 0.001). The data of the present study suggest that the forearm Q is an important determinant of the transient thermal response of the forearm tissue during thermal stress.     

Muscle glycogen utilization during shivering thermogenesis in humans

The purpose of the present study was to clarify the importance of skeletal muscle glycogen as a fuel for shivering thermogenesis in humans during cold-water immersion. Fourteen seminude subjects were immersed to the shoulders in 18 degrees C water for 90 min or until rectal temperature (Tre) decreased to 35.5 degrees C. Biopsies from the vastus lateralis muscle and venous blood samples were obtained before and immediately after the immersion. Metabolic rate increased during the immersion to 3.5 +/- 0.3 (SE) times resting values, whereas Tre decreased by 0.9 degrees C to approximately 35.8 degrees C at the end of the immersion. Intramuscular glycogen concentration in the vastus lateralis decreased from 410 +/- 15 to 332 +/- 18 mmol glucose/kg dry muscle, with each subject showing a decrease (P less than 0.001). Plasma volume decreased (P less than 0.001) markedly during the immersion (-24 +/- 1%). After correcting for this decrease, blood lactate and plasma glycerol levels increased by 60 (P less than 0.05) and 38% (P less than 0.01), respectively, whereas plasma glucose levels were reduced by 20% after the immersion (P less than 0.001). The mean expiratory exchange ratio showed a biphasic pattern, increasing initially during the first 30 min of the immersion from 0.80 +/- 0.06 to 0.85 +/- 0.05 (P less than 0.01) and decreasing thereafter toward basal values. The results demonstrate clearly that intramuscular glycogen reserves are used as a metabolic substrate to fuel intensive thermogenic shivering activity of human skeletal muscle.