Comparison of thermoregulatory responses to heat between Malaysian and Japanese males during leg immersion

The objective of this study was to investigate thermoregulatory responses to heat in tropical (Malaysian) and temperate (Japanese) natives, during 60 min of passive heating. Ten Japanese (mean ages: 20.8 ± 0.9 years) and ten Malaysian males (mean ages: 22.3 ± 1.6 years) with matched morphological characteristics and physical fitness participated in this study. Passive heating was induced through leg immersion in hot water (42°C) for 60 min under conditions of 28°C air temperature and 50% RH. Local sweat rate on the forehead and thigh were significantly lower in Malaysians during leg immersion, but no significant differences in total sweat rate were observed between Malaysians (86.3 ± 11.8 g m⁻² h⁻¹) and Japanese (83.2 ± 6.4  g m⁻² h⁻¹) after leg immersion. In addition, Malaysians displayed a smaller rise in rectal temperature (0.3 ± 0.1°C) than Japanese (0.7 ± 0.1°C) during leg immersion, with a greater increase in hand skin temperature. Skin blood flow was significantly lower on the forehead and forearm in Malaysians during leg immersion. No significant different in mean skin temperature during leg immersion was observed between the two groups. These findings indicated that regional differences in body sweating distribution might exist between Malaysians and Japanese during heat exposure, with more uniform distribution of local sweat rate over the whole body among tropical Malaysians. Altogether, Malaysians appear to display enhanced efficiency of thermal sweating and thermoregulatory responses in dissipating heat loss during heat loading. Thermoregulatory differences between tropical and temperate natives in this study can be interpreted as a result of heat adaptations to physiological function.     

Effects of heat acclimation on sweating during graded exercise until exhaustion

The aim of the present study was to test the hypothesis that the sweating during graded exercise until exhaustion in a temperate environment would be greater after heat acclimation. Six healthy young males performed an exercise–heat stress acclimation protocol during 9 days. Before (PRE) and after (POS) the acclimation protocol they performed a graded exercise until exhaustion and the sweat loss during exercise increased after acclimation (3.94±1.10, PRE, and 4.86±1.70 g m⁻² min⁻¹, POS; p<0.05). The results showed that daily prolonged exposures to exercise-heat stress increased sweating during a graded and short duration exercise in a temperate environment.     

Effects of encouraged water drinking on thermoregulatory responses after 20 days of head-down bed rest in humans

We tested the hypothesis that encouraged water drinking according to urine output for 20 days could ameliorate impaired thermoregulatory function under microgravity conditions. Twelve healthy men, aged 24 ± 1.5 years (mean ± SE), underwent −6° head-down bed rest (HDBR) for 20 days. During bed rest, subjects were encouraged to drink the same amount of water as the 24-h urine output volume of the previous day. A heat exposure test consisting of water immersion up to the knees at 42°C for 45 min after a 10 min rest (baseline) in the sitting position was performed 2 days before the 20-day HDBR (PRE), and 2 days after the 20-day HDBR (POST). Core temperature (tympanic), skin temperature, skin blood flow and sweat rate were recorded continuously. We found that the −6° HDBR did not increase the threshold temperature for onset of sweating under the encouraged water drinking regime. We conclude that encouraged water drinking could prevent impaired thermoregulatory responses after HDBR.     

Heat acclimation does not modify autonomic responses to core cooling and the skin thermal comfort zone

Exercise heat acclimation (HA) is known to magnify the sweating response by virtue of a lower threshold as well as increased gain and maximal capacity of sweating. However, HA has been shown to potentiate the shivering response in a cold-air environment. We investigated whether HA would alter heat loss and heat production responses during water immersion. Twelve healthy male participants underwent a 10-day HA protocol comprising daily 90-min controlled-hyperthermia (target rectal temperature, T_re 38.5 °C) exercise sessions. Preceding and following HA, the participants performed a maximal exercise test in thermoneutral conditions (ambient temperature 23 °C, relative humidity 50%) and were, following exercise, immersed in 28 °C water for 60 min. Thermal comfort zone (TCZ) was also assessed with participants regulating the temperature of a water-perfused suit during heating and cooling. Baseline pre-immersion T_re was similar pre- and post-HA (pre: 38.33 ± 0.33 °C vs post: 38.12 ± 0.36 °C, p = 0.092). The T_re cooling rate was identical pre-to post-HA (−0.03 ± 0.01 °C·min⁻¹, p = 0.31), as was the vasomotor response reflected in the forearm-fingertip temperature difference. Shivering thresholds (p = 0.43) and gains (p = 0.61) were not affected by HA. TCZ was established at similar temperatures, with the magnitude in regulated water temperature being 7.6 (16.3) °C pre-HA and 5.1 (24.7) °C post-HA (p = 0.65). The present findings suggest that heat production and heat loss responses during whole body cooling as well as the skin thermal comfort zone remained unaltered by a controlled-hyperthermia HA protocol.     

Differences between sexes in thermoregulatory responses and exercise time during endurance exercise in a hot environment following pre-cooling with ice slurry ingestion

This study aimed to examine differences between sexes in thermoregulatory responses and exercise time after ice slurry ingestion in a hot environment. Twenty-four healthy adults (male n = 12, body weight (BW) = 65.8 ± 10.3; female n = 12, BW = 58.2 ± 10.0) ingested 7.5 g/kg of either ice slurry at −1 °C (ICE) or control water at 20 °C (CON) before cycling at 55%VO₂ max in a hot environment (controlled at 38 °C, 40% relative humidity). Rectal (Tre) and skin (Tsk) temperature, heart rate, sweat rate, respiratory gases, ratings of thermal sensation (TS), thermal comfort (TC), and rating of perceived exertion (RPE) were measured. Ice slurry did not improve exercise time in both sexes despite Tre was significantly lower in ICE than CON in both sexes. Tre, Tsk, HR, sweat rate and TS did not differ between sexes. TC and RPE in ICE were significantly higher during exercise in males than in females. In conclusion, there were no sex differences in the effects of pre-cooling with ice slurry ingestion; however, pre-cooling with ice slurry may be more effective in mitigating ratings of TC and RPE in females than males.     

Neural control and mechanisms of eccrine sweating during heat stress and exercise.

In humans, evaporative heat loss from eccrine sweat glands is critical for thermoregulation during exercise and/or exposure to hot environmental conditions, particularly when environmental temperature is greater than skin temperature. Since the time of the ancient Greeks, the significance of sweating has been recognized, whereas our understanding of the mechanisms and controllers of sweating has largely developed during the past century. This review initially focuses on the basic mechanisms of eccrine sweat secretion during heat stress and/or exercise along with a review of the primary controllers of thermoregulatory sweating (i.e., internal and skin temperatures). This is followed by a review of key nonthermal factors associated with prolonged heat stress and exercise that have been proposed to modulate the sweating response. Finally, mechanisms pertaining to the effects of heat acclimation and microgravity exposure are presented.     

Hyperhydration: thermoregulatory effects during compensable exercise-heat stress

Latzka, William A., Michael N. Sawka, Scott J. Montain, GaryS. Skrinar, Roger A. Fielding, Ralph P. Matott, and Kent B. Pandolf.Hyperhydration: thermoregulatory effects during compensable exercise-heat stress. J. Appl.Physiol. 83(3): 860-866, 1997.This studyexamined the effects of hyperhydration on thermoregulatory responsesduring compensable exercise-heat stress. The general approach was todetermine whether 1-h preexercise hyperhydration [29.1 ml/kg leanbody mass; with or without glycerol (1.2 g/kg lean body mass)]would improve sweating responses and reduce core temperature duringexercise. During these experiments, the evaporative heat loss required(E_req = 293 W/m²) to maintain steady-statecore temperature was less than the maximal capacity(E_max = 462 W/m²) of the climate forevaporative heat loss(E_req/E_max = 63%). Eight heat-acclimated men completed five trials: euhydration, glycerol hyperhydration, and water hyperhydration both with and withoutrehydration (replace sweat loss during exercise). During exercise inthe heat (35°C, 45% relative humidity), there was no differencebetween hyperhydration methods for increasing total body water (~1.5liters). Compared with euhydration, hyperhydration did not alter coretemperature, skin temperature, whole body sweating rate, local sweatingrate, sweating threshold temperature, sweating sensitivity, or heartrate responses. Similarly, no difference was found between water andglycerol hyperhydration for these physiological responses. These datademonstrate that hyperhydration provides no thermoregulatory advantageover the maintenance of euhydration during compensable exercise-heatstress.

     

Hydration effects on physiological strain of horses during exercise-heat stress

This study examined the effects ofhyperhydration, exercise-induced dehydration, and oral fluidreplacement on physiological strain of horses during exercise-heatstress. On three occasions, six horses completed a 90-min exerciseprotocol (50% maximal O₂ uptake,34.5°C, 48% relative humidity) divided into two 45-min periods(exercise I andexercise II) with a 15-min recoverybetween exercise bouts. In random order, horses receivedno fluid (NF), 10 liters of water (W), or a carbohydrate-electrolytesolution (CE) 2 h before exercise and between exercise bouts. Compared with NF, preexercise hyperhydration (W and CE) did not alter heart rate, cardiac output (), stroke volume (SV), corebody temperature, sweating rate (SR), or sweating sensitivity duringexercise I. In contrast, afterexercise II, exercise-induceddehydration in NF (decrease in body mass: NF, 5.6 ± 0.8%; W, 1.1 ± 0.4%; CE, 1.0 ± 0.2%) resulted in greater heat storage,with core body temperature ~1.0°C higher compared with W and CE.In exercise II, the greater thermalstrain in NF was associated with significant(P < 0.05) decreases in (10 ± 2%), SV (9 ± 3%), SR, and sweatingsensitivity. We concluded that 1)preexercise hyperhydration provided no thermoregulatory advantage;2) maintenance of euhydration byoral fluid replacement (~85% of sweat fluid loss) during exercise inthe heat was reflected in higher , SV, and SR withdecreased heat storage; and 3) W oran isotonic CE solution was equally effective in reducing physiological strain associated with exercise-induced dehydration and heat stress.

     

Transition duration of ingested deuterium oxide to eccrine sweat during exercise in the heat

The time necessary for the initial appearance of ingested water as sweat during exercise in the heat remains unknown. Based on the current literature, we estimated fluid transition through the body, from ingestion to appearance as sweat, to have a minimum time duration of approximately three minutes. The purpose of this study was to test this prediction and identify the time necessary for the initial enrichment of deuterium oxide (D₂O) in sweat following ingestion during exercise in the heat. Eight participants performed moderate intensity (40% of maximal oxygen uptake) treadmill exercise in an environmental chamber (40 °C, 40% rH) to induce active sweating. After fifteen minutes, while continuing to walk, participants consumed D₂O (0.15 ml kg⁻¹) in a final volume of 50 ml water. Scapular sweat samples were collected one minute prior to and ten minutes post-ingestion. Samples were analyzed for sweat D₂O concentration using isotope ratio mass spectrometry and compared to baseline. Mean±SD ∆ sweat D₂O concentration at minutes one and two post-ingestion were not significantly higher than baseline (0 min). Minutes three (9±3 ppm) through ten (23±11 ppm) post-ingestion had ∆ sweat D₂O concentrations significantly (P<0.05) higher than baseline. Such results suggest that ingested water rapidly transports across the mucosal membrane of the alimentary canal into the vasculature space, enters the extravascular fluid, and is actively secreted by the eccrine sweat glands onto the surface of the skin for potential evaporation in as little as three minutes during exercise in the heat.     

Influence of season on plasma antidiuretic hormone, angiotensin II, aldosterone and plasma renin activity in young volunteers

We investigated seasonal changes in hormonal and thermoregulatory responses. Eight volunteers were subjected to the experiment at four times of the year: around the vernal and autumnal equinoxes, and at the summer and winter solstices at latitude 35° N. Plasma antidiuretic hormone (ADH), angiotensin II (ANG II), aldosterone (ALD) and plasma renin activity (PRA) were analyzed before and after water immersion. Seasonal changes in thermoregulatory responses were assessed by measuring core temperature and sweat rate during immersion of the leg in hot water (at 42°C) for 30 min in a room maintained at 26°C. The concentration of plasma ADH and ALD before water immersion was significantly higher in summer than in other seasons. The concentrations of ANG II and PRA did not show seasonal variations. Changes in tympanic temperature during water immersion showed significant differences between seasons, and were higher in winter than in other seasons. The sweat rate was significantly higher in summer than in other seasons. In summary, ADH and ALD concentrations displayed a seasonal rhythm with marked elevation in summer; this may be a compensative mechanism to prevent dehydration from increased sweat loss during summer due to heat acclimatization.     

Relationship of osmotic inhibition in thermoregulatory responses and sweat sodium concentration in humans

Heat acclimatization improves thermoregulatory responses to heat stress and decreases sweat sodium concentration ([Na(+)](sweat)). The reduced [Na(+)](sweat) results in a larger increase in plasma osmolality (P(osmol)) at a given amount of sweat output. The increase in P(osmol) inhibits thermoregulatory responses to increased body core temperature. Therefore, we hypothesized that the inhibitory effect of plasma hyperosmolality on the thermoregulatory responses to heat stress should be attenuated with the reduction of [Na(+)](sweat) due to heat acclimatization. Eleven subjects (9 male and 2 female) were passively heated by immersing their lower legs into water at 42 degrees C (room temperature 28 degrees C and relative humidity 30%) for 50 min following isotonic or hypertonic saline infusion. We determined the increase in the esophageal temperature (T(es)) required to elicit sweating and cutaneous vasodilation (CVD) (DeltaT(es) thresholds for sweating and CVD, respectively) in each condition and calculated the elevation of the T(es) thresholds per unit increase in P(osmol) as the osmotic inhibition of sweating and CVD. The osmotic shift in the DeltaT(es) thresholds for both sweating and CVD correlated linearly with [Na(+)](sweat) (r = 0.858 and r = 0.628, respectively). Thus subjects with a lower [Na(+)](sweat) showed a smaller osmotic elevation of the DeltaT(es) thresholds for sweating and CVD. These results suggest the possibility that heat acclimatization attenuates osmotic inhibition of thermoregulatory responses as well as reducing [Na(+)](sweat).     

Tropical Malaysians and temperate Koreans exhibit significant differences in sweating sensitivity in response to iontophoretically administered acetylcholine

Natives of the tropics are able to tolerate high ambient temperatures. This results from their long-term residence in hot and often humid tropical climates. This study was designed to compare the peripheral mechanisms of thermal sweating in tropical natives with that of their temperate counterparts. Fifty-five healthy male subjects including 20 native Koreans who live in the temperate Korean climate (Temperate-N) and 35 native tropical Malaysian men that have lived all of their lives in Malaysia (Tropical-N) were enrolled in this study after providing written informed consent to participate. Quantitative sudomotor axon reflex testing after iontophoresis (2 mA for 5 min) with 10% acetylcholine (ACh) was used to determine directly activated (DIR) and axon reflex-mediated (AXR) sweating during ACh iontophoresis. The sweat rate, activated sweat gland density, sweat gland output per single gland activated, and oral and skin temperature changes were measured. The sweat onset time of AXR (nicotinic-receptor-mediated) was 56 s shorter in the Temperate-N than in the Tropical-N subjects (P < 0.0001). The nicotinic-receptor-mediated sweating activity AXR (1), and the muscarinic-receptor-mediated sweating activity DIR, in terms of sweat volume, were 103% and 59% higher in the Temperate-N compared to the Tropical-N subjects (P < 0.0001). The Temperate-N group also had a 17.8% (P < 0.0001) higher active sweat gland density, 35.4% higher sweat output per gland, 0.24°C higher resting oral temperature, and 0.62°C higher resting forearm skin temperature compared to the Tropical-N subjects (P < 0.01). ACh iontophoresis did not influence oral temperature, but increased skin temperature near where the ACh was administered, in both groups. These results suggest that suppressed thermal sweating in the Tropical-N subjects was, at least in part, due to suppressed sweat gland sensitivity to ACh through both recruitment of active sweat glands and the sweat gland output per each gland. This physiological trait guarantees a more economical use of body fluids, thus ensuring more efficient protection against heat stress.     

Effects of modafinil on heat thermoregulatory responses in humans at rest

The effects of modafinil on heat thermoregulatory responses were studied in 10 male subjects submitted to a sweating test after taking 200 mg of modafinil or placebo. Sweating tests were performed in a hot climatic chamber (45 degrees C, relative humidity <15%, wind speed = 0.8 m x s(-1), duration 1.5 h). Body temperatures (rectal (Tre) and 10 skin temperatures (Tsk)), sweat rate, and metabolic heat production (M) were studied as well as heart rate (HR). Results showed that modafinil induced at the end of the sweating test higher body temperatures increases (0.50 +/- 0.04 versus 0.24 +/- 0.05 degrees C (P < 0.01) for deltaTre and 3.64 +/- 0.16 versus 3.32 +/- 0.16 degrees C (P < 0.05) for deltaTsk (mean skin temperature)) and a decrease in sweating rate throughout the heat exposure (P < 0.05) without change in M, leading to a higher body heat storage (P < 0.05). AHR was also increased, especially at the end of the sweating test (17.95 +/- 1.49 versus 12.52 +/- 1.24 beats/min (P < 0.01)). In conclusion, modafinil induced a slight hyperthermic effect during passive dry heat exposure related to a lower sweat rate, probably by its action on the central nervous system, and this could impair heat tolerance.     

Effects of isotonic and isometric exercises with mist sauna bathing on cardiovascular,thermoregulatory, and metabolic functions

To clarify the effects of isometric and isotonic exercise during mist sauna bathing on the cardiovascular function, thermoregulatory function, and metabolism, six healthy young men (22?±?1 years old, height 173?±?4 cm, weight 65.0?±?5.0 kg) were exposed to a mist sauna for 10 min at a temperature of 40 °C, and relative humidity of 100 % while performing or not performing ～30 W of isometric or isotonic exercise. The effect of the exercise was assessed by measuring tympanic temperature, heart rate, systolic and diastolic blood pressure, chest sweat rate, chest skin blood flow, and plasma catecholamine and cortisol, glucose, lactate, and free fatty acid levels. Repeated measures ANOVA showed no significant differences in blood pressure, skin blood flow, sweat rate, and total amount of sweating. Tympanic temperature increased more during isotonic exercise, and heart rate increase was more marked during isotonic exercise. The changes in lactate indicated that fatigue was not very great during isometric exercise. The glucose level indicated greater energy expenditure during isometric exercise. The free fatty acid and catecholamine levels indicated that isometric exercise did not result in very great energy expenditure and stress, respectively. The results for isotonic exercise of a decrease in lactate level and an increase in plasma free fatty acid level indicated that fatigue and energy expenditure were rather large while the perceived stress was comparatively low. We concluded that isotonic exercise may be a more desirable form of exercise during mist sauna bathing given the changes in glucose and free fatty acid levels.     

Female anthropometric variability and their effects on predicted thermoregulatory responses to work in the heat

The use of thermoregulatory models for assessing physiological responses of workers in thermally stressful situations has been increasing because of the risks and costs related to human studies. In a previous study (Yokota et al. Eur J Appl Physiol 104:297–302, 2008), the effects of anthropometric variability on predicted physiological responses to heat stress in U.S. Army male soldiers were evaluated. Five somatotypes were identified in U.S. Army male multivariate anthropometric distribution. The simulated heat responses, using a thermoregulatory model, were different between somatotypes. The present study further extends this line of research to female soldiers. Anthropometric somatotypes were identified using multivariate analysis [height, weight, percent body fat (%BF)] and the predicted physiological responses to simulated exercise and heat stress using a thermoregulatory model were evaluated. The simulated conditions included walking at ~3 mph (4.8 km/h) for 300 min and wearing battle dress uniform and body armor in a 30°C, 25% relative humidity (RH) environment without solar radiation. Five major somatotypes (tall-fat, tall-lean, average, short-lean, and short-fat), identified through multivariate analysis of anthropometric distributions, showed different tolerance levels to simulated heat stress: lean women were predicted to maintain their core temperatures (T_c) lower than short-fat or tall-fat women. The measured T_c of female subjects obtained from two heat studies (data1: 30°C, 32% RH, protective garments, ~225 w·m⁻² walk for 90 min; data2: 32°C, 75% RH, hot weather battle dress uniform, ~378 ± 32 w·m⁻² for 30 min walk/30 min rest cycles for 120 min) were utilized for validation. Validation results agreed with the findings in this study: fat subjects tended to have higher core temperatures than medium individuals (data2) and lean subjects maintained lower core temperatures than medium subjects (data1).     

Effect of age on heat-activated sweat gland density and flow during exercise in dry heat

Physiological responses of eight postmenopausal older women (age 52-62 yr) and eight younger women (age 20-30 yr) were compared during moderate intensity exercise in a hot dry environment (48 degrees C dry bulb, 25 degrees C wet bulb). The age groups were matched on the basis of maximal O2 consumption (VO2max), body surface area, and body fatness. After heat acclimation the women walked at 40% VO2max for up to 2 h in the hot dry environment while heart rate (HR), rectal temperature (Tre), mean skin temperature (Tsk), whole-body sweating rate (Msw), and local sweating rates (msw; forearm, chest, and scapula) were measured. Additionally, the density of heat-activated sweat glands (HASG) was determined and average sweat gland flow (SGF) was calculated for the scapular area. Although no differences between age groups were found in HR response (when analyzed as percent of maximal HR) or Tsk, the older women had a significantly higher Tre throughout the heat-exercise session. The greater heat storage of the older women may be explained by their significantly lower Msw and msw. There were no differences between the younger and older women in the density of HASG after 30 min; therefore, the lower msw reflects a diminished output per HASG rather than a decrease in the number of sweat glands recruited. The diminished thermoregulatory ability of the older women, unrelated to differences in VO2max, appears to reflect either 1) a diminished response of the sweat glands to central and/or peripheral stimuli, or 2) an age-related structural alteration in the eccrine glands or surrounding skin cells.     

Repeatability of a running heat tolerance test

At present there is no standardised heat tolerance test (HTT) procedure adopting a running mode of exercise. Current HTTs may misdiagnose a runner's susceptibility to a hyperthermic state due to differences in exercise intensity. The current study aimed to establish the repeatability of a practical running test to evaluate individual's ability to tolerate exercise heat stress. Sixteen (8M, 8F) participants performed the running HTT (RHTT) (30 min, 9 km h⁻¹, 2% elevation) on two separate occasions in a hot environment (40 °C and 40% relative humidity). There were no differences in peak rectal temperature (RHTT1: 38.82±0.47 °C, RHTT2: 38.86±0.49 °C, Intra-class correlation coefficient (ICC)=0.93, typical error of measure (TEM)=0.13 °C), peak skin temperature (RHTT1: 38.12±0.45, RHTT2: 38.11±0.45 °C, ICC=0.79, TEM=0.30 °C), peak heart rate (RHTT1: 182±15 beats min⁻¹, RHTT2: 183±15 beats min⁻¹, ICC=0.99, TEM=2 beats min⁻¹), nor sweat rate (1721±675 g h⁻¹, 1716±745 g h⁻¹, ICC=0.95, TEM=162 g h⁻¹) between RHTT1 and RHTT2 (p>0.05). Results demonstrate good agreement, strong correlations and small differences between repeated trials, and the TEM values suggest low within-participant variability. The RHTT was effective in differentiating between individuals physiological responses; supporting a heat tolerance continuum. The findings suggest the RHTT is a repeatable measure of physiological strain in the heat and may be used to assess the effectiveness of acute and chronic heat alleviating procedures.     

Supplemental intermittent-day heat training and the lactate threshold

Heat acclimation over consecutive days has been shown to improve aerobic-based performance. Recently, it has been suggested that heat training can improve performance in a temperate environment. However, due to the multifactorial training demands of athletes, consecutive-day heat training may not be suitable. The current study aimed to investigate the effect of brief (8×30 min) intermittent (every 3–4 days) supplemental heat training on the second lactate threshold point (LT₂) in temperate and hot conditions. 21 participants undertook eight intermittent-day mixed-intensity treadmill exercise training sessions in hot (30 °C; 50% relative humidity [RH]) or temperate (18 °C; 30% RH) conditions. A pre- and post-incremental exercise test occurred in temperate (18 °C; 30% RH) and hot conditions (30 °C; 50% RH) to determine the change in LT₂. The heat training protocol did not improve LT₂ in temperate (Effect Size [ES]±90 confidence interval=0.10±0.16) or hot (ES=0.26±0.26) conditions. The primary finding was that although the intervention group had a change greater than the SWC, no statistically significant improvements were observed following an intermittent eight day supplemental heat training protocol comparable to a control group training only in temperate conditions. This is likely due to the brief length of each heat training session and/or the long duration between each heat exposure.     

Predicting military working dog core temperature during exertional heat strain: Validation of a Canine Thermal Model

Military working dogs (MWDs) operate under a wide range of conditions, including hot environments. Predicting how long a MWD can safely work without overheating is important for both health and performance. A Canine Thermal Model (CTM) was developed to predict core temperature (Tc) of MWDs. The CTM calculates heat storage from the balance of heat production from metabolism and heat exchange with the environment. Inputs to the CTM are: meteorological conditions (ambient temperature, relative humidity, solar radiation and wind speed), physical characteristics of the dog (mass, length), and metabolic activity (MET level, estimated from accelerometer data). The CTM was validated against Tc measured in 23 MWDs during training sessions (11.6 ± 5.0 min (mean ± standard deviation), range 4–26 min) in October (24 °C, 52% RH), March (14 °C, 74% RH), or August (28 °C, 64% RH), and 24 kennel MWDs during a standard exercise walk (11.4 ± 3.3 min, range 5.6–18 min) in July (26 °C, 77% RH). The CTM was considered acceptable if predicted Tc was within ±0.5 °C of measured Tc at the end of exercise. Compared to Tc at the end of training sessions (39.8 ± 0.6 °C, range 38.4–41.1 °C) and exercise walks (40.0 ± 0.7 °C, range 38.9–41.4 °C), the CTM-predicted Tc was within ±0.5 °C for 71 of 84 cases (85%) and 19 of 24 cases (79%), respectively. The mean difference between CTM-predicted and measured final Tc during training was -0.04 ± 0.43 °C, with 80 of 84 cases (95%) within the range of ±2 SD (Bland Altman comparison). During exercise walks the mean difference was -0.15 °C ± 0.57, with 23 of 24 cases (96%) within ±2 SD. These results support the use of the CTM to predict Tc of MWDs for the types of physical activities described above.     

Exercise- and methylcholine-induced sweating responses in older and younger men: effect of heat acclimation and aerobic fitness

 The purpose of this investigation was to examine the effects of aging and aerobic fitness on exercise- and methylcholine-induced sweating responses during heat acclimation. Five younger [Y group – age: 23±1 (SEM) years; maximal oxygen consumption (V.O_2max): 47±3 ml·kg^–1·min^–1], four highly fit older (HO group – 63±3 years; 48±4 ml·kg^–1·min^–1) and five normally fit older men (NO group – 67±3 years; 30±1 ml·kg^–1·min^–1) who were matched for height, body mass and percentage fat, were heat acclimated by daily cycle exercise (≈35% V.O_2max for 90 min) in a hot (43°C, 30% RH) environment for 8 days. The heat acclimation regimen increased performance time, lowered final rectal temperature (T _re) and percentage maximal heart rate (%HR_max), improved thermal comfort and decreased sweat sodium concentration similarly in all groups. Although total body sweating rates (M._sw) during acclimation were significantly greater in the Y and HO groups than in the NO group (P<0.01) (because of the lower absolute workload in the NO group), the M._sw did not change in all groups with the acclimation sessions. Neither were local sweating rates (m. _sw) on chest, back, forearm and thigh changed in all groups by the acclimation. The HO group presented greater forearm m. _sw (30–90 min) values and the Y group had greater back and thigh m. _sw (early in exercise) values, compared to the other groups (P<0.001). In a methylcholine injection test on days immediately before and after the acclimation, the order of sweat output per gland (SGO) on chest, back and thigh was Y>HO>NO, and on the forearm Y=HO>NO. No group differences were observed for activated sweat gland density at any site. The SGO at the respective sites increased in the post-acclimation test regardless of group (P<0.01), but on the thigh the magnitude of the increase was lower in the NO (P<0.02) and HO (P=0.07) groups than in the Y group. These findings suggest that heat tolerance and the improvement with acclimation are little impaired not only in highly fit older but also normally fit older men, when the subjects exercised at the same relative exercise intensity. Furthermore, the changes induced by acclimation appear associated with an age-related decrease in V.O_2max. However methylcholine-activated SGO and the magnitude of improvement of SGO with acclimation are related not only to V.O_2max but also to aging, suggesting that sensitivity to cholinergic stimulation decreases with aging. Received: 8 May 1998/Accepted: 5 October 1998