Effects of Atropine and Β-blockade on temperature regulation and performance during prolonged exercise

Summary The effects of intravenous injections of Atropine (1.8 mg) and practolol (15 mg) on the thermoregulatory responses to 1 h of exercise on a motordriven treadmill have been investigated on six healthy subjects.The results show that -blockade had little effect on thermal responses to work except for a small but significant (p<0.05) decrease in mean skin temperature (¯T _sk ) and peripheral tissue heat conductance (K). Metabolic (M) and total heat (H) production, and evaporative sweat loss (E) and rectal temperature (T _re ) were similar to control values. In contrast, atropine, particularly at work loads beyond 60% maximal aerobic power output (VO_{2 max}), raised T _re (p<0.001), ¯T _sk (p<0.001) and reduced E by approximately 50%. At the highest work loads T _re increased as a linear function of time during the latter part of exercise, and at the 60th min was almost independent of relative stress (expressed as % VO_{2 max}) imposed on the subjects. At the lower work loads the majority of subjects reached thermal equilibrium before the end of exercise by maintaining their convective heat transfer from core to periphery by increasing peripheral blood flow (as indicated by K), and raising their heat losses to environment by convection and radiation. The latter pathways for heat dissipation were enhanced by the subjects ability to sustain a ¯T _sk 4 C above control values independently of M. Atropine had no effect on M or H but greatly affected work performance, no subject was able to exercise at loads >70% VO_{2 max} for 1 h. These results demonstrate the ability of the thermoregulatory system to adapt to -adrenergic and to parasympathetic blockade during light exercise, and underline the effects of a reduction in the capacity of the sweating mechanism on physiological performance at higher rates of work.List of Abbreviations used in the Text M Metabolic heat production - H Total heat production - E Evaporative sweat loss - T _re Rectal temperature - ¯T _sk Mean skin temperature - K Peripheral tissue heat conductance - PBF Peripheral blood flow - VO_{2 max} Maximal aerobic power output - f _H Cardiac frequency     

Seasonal changes in circadian rhythms of body temperatures in humans living in a dry tropical climate

Summary Seven volunteers (3 females and 4 males; 3 Caucasians and 4 Africans) participated in two 24 h sessions during the cool dry (CD) and the hot dry (HD) seasons of the sahelian tropical climate. Body temperatures were taken on portable cassette recorders for 24 h. Rectal (T _re) and mean skin (¯T _sk) temperatures decreased in the HD compared to the CD conditions, meeting one of the criteria for adaptation to heat. No ethnic differences in thermal responses were found. Males and females differed in their body temperature rhythms and in their reactions to heat. Body temperatures were higher in females than in males. Males reacted to heat with a decrease in T _re, without change in the T _re-¯T _sk gradient. Females showed a decrease in both T _re and ¯T _sk, more marked for ¯T _sk, with an increase in the T _re-¯T _sk gradient. It was concluded that males showed seasonal acclimatization to heat via a decrease in metabolism confirmed by a decrease in plasma levels of thyroid stimulating hormone (TSH) in the HD condition. Females showed a mixed metabolic and thermolytic type of acclimatization, with an absence of variation in plasma TSH levels. In conclusion, the steady rise in temperature between the CD and HD conditions was sufficient to trigger an acclimatization to heat similar in Caucasian and African subjects, although exposure to the external climate differed widely.     

Disturbance of thermal homeostasis during post-exercise hyperthermia

The response of core temperature to exercise was investigated during recovery in order to avoid the antagonistic competition between exercise and thermal reflexes for the same effector systems which control skin blood flow. Five healthy, non-training males [mean (SD) age, 23.8 (2.04) years] were habituated to 29° C at relative 50% humidity for more than 2 h and then exercised by treadmill running at about 75% maximum oxygen uptake for 18 min. They then remained at 29° C for up to 65 min of recovery. Oesophageal (T _es), rectal (T _re) and skin temperatures (T _sk) were recorded at 5-s intervals throughout. The abrupt fall of temperature gradient from the forearm to finger was used to identify the T _es for skin vessel dilatation (T _dil) during exercise. Mean (SE) Ts rose from a resting value of 36.67 (0.15)° C to 38.22 (0.24)° C, mean T _re rose from 37.09 (0.25)° C to 38.23 (0.15)° C, and T _dil occurred at 37.39 (0.32)° C. Within 10 min of recovery mean T _es fell to 37.31 (0.24)° C, where it remained a significant 0.64° C above its pre-exercise (PrEx) level (P0.018) but insignificantly different from T _dil for the remaining 55 min of recovery. Meanwhile, T _re fell gradually throughout recovery to 37.64 (0.18)° C. The T _sk at all non-acral sites except the thigh had recovered to PrEx levels by 20–30 min post-exercise (PoEx). The rapid PoEx fall of T _es to the level of T _dil and the subsequent plateau above PrEx values suggests that heat dissipation during recovery was primarily passive once T _es had fallen to T _dil, even though T _es and T _re were significantly elevated. The relationship of these results to the set-point and load error concepts of thermal control is discussed.These data have been presented at the Canadian Physiological Society Winter meeting, January 1993, but have not been previously published     

The effect of postural changes on body temperatures and heat balance

Early studies have demonstrated that rectal temperature (T _re) decreases and mean skin temperature (T _sk) increases in subjects changing their posture from standing to supine, and vice versa. Such changes have important implications insofar as thermal stress experiments are conducted and interpreted. However, the extent of these changes between steady-state conditions is not known. In addition, it is not known whether thermal balance is also affected by postural changes. To examine these questions, 11 healthy males were exposed to a thermoneutral air environment (28.2–28.5°C and 40% relative humidity) in various postures at rest. Body temperatures, heat losses, and metabolic rate were measured. Subjects wore shorts only and began in an upright posture (standing or sitting at an inclination of 7.5°) on a customized tilt-table. They were tilted twice, once into a supine position and then back to the original upright position. Each tilt occurred after steady state was satisfied based on the subject's circadian variation of T _re determined previously in a 4.25 h control supine trial. Times to supine steady state following the first tilt were [mean (SE)] 92.6 (6.4) and 116.6 (5.1) min for the standing and sitting trials, respectively. Times to upright steady state following the second tilt were 107.9 (11.4) and 124.1 (9.0) min. Mean steady-state T _re and T _sk were 36.87 (0.07) and 34.04 (0.14), 37.47 (0.09) and 33.48 (0.14), and 37.26 (0.05) and 33.49 (0.10) °C for supine, standing, and sitting, respectively. Thermal balance was attained in all steady-state conditions, and allowing for a decrease in the weighting factor of T _re for mean body temperature in the upright postures, it also appears that thermal balance was preserved between changes in posture. These results are consistent with no perceived changes by the subjects in their thermal comfort and skin wetness.     

Effects of anesthetic injected into brainstem sites on body temperature and behavioral thermoregulation

The preoptic/anterior hypothalamic region (PO/AH), the posterior (PH) and lateral hypothalamic (LH) regions, the mesencephalic reticular formation (MRF) and the medulla oblongata (MED) have all been implicated in the control of body temperature. The purpose of the present research was to learn whether differential effects upon physiological and behavioral thermoregulation are produced by temporarily depressing the activity of these regions with anesthetic. The rectal temperature (T_re) of rats with central cannulae was recorded after sodium pentobarbital injections while the animals rested in 23°, 10° and 34°C environments. In other experiments the effects of central anesthetic injections on behavioral regulation against heat were measured. Anesthetic injected into the PO/AH region caused changes in T_re and behavior that are consistent with a coordinated rise in the set point of body temperature control. Injections into the MED produced transient and rapid decreases in T_re without affecting behavioral thermoregulation. Bilateral injections into the LH caused hyperthermia in the 10° and 34°C environments, hypothermia in the 23°C environment and had no effect on behavioral temperature regulation. No changes in thermoregulatory responses were observed after PH and MRF injections. These results indicate that there are differences among the 5 brain regions in relative importance to overall temperature control and specific differences in the significance of certain regions to the two forms of temperature control; physiological and behavioral temperature regulation.     

Control of heat-induced cutaneous vasodilatation in relation to age

Summary Well matched unacclimatised older (age 55–68, 4 women, 2 men) and younger (age 19–30, 4 women, 2 men) subjects performed 75 min cycle exercise (40% ) in a hot environment (37°C, 60% rh). Rectal temperature (T _re), mean skin temperature (¯T _sk), arm blood flow (ABF, strain gauge plethysmography), and cardiac output (Q, CO₂ rebreathing) were measured to examine age-related differences in heat-induced vasodilatation.T _re and¯T _sk rose to the same extent in each group during the exposure. There was no significant intergroup difference in sweat rate (older: 332±43 ml · m^–2 · h^–1, younger: 435±49 ml · m^–2 · h^–1; mean±SEM). However, the older subjects responded to exercise in the heat with a lower ABF response which could be attributed to a lower for the same exercise intensity. The slope of the ABF-T _re relationship was attenuated in the older subjects (9.3±1.3 vs 17.9±3.3 ml · 100 ml^–1 · min^–1 · °C^–1,p <0.05), but theT _re threshold for vasodilatation was about 37.0°C for both groups. These results suggest an altered control of skin vasodilatation during exercise in the heat in older individuals. This attenuated ABF response appears to be unrelated to , and may reflect an age-related change in thermoregulatory cardiovascular function.     

Cold thermoregulatory changes induced by sleep deprivation in men

The aim of this study was to evaluate the thermoregulatory changes induced by 27-h of sleep deprivation (SD) in men at rest both in a comfortable ambient temperature and in cold air. A group of 12 male subjects were placed in a comfortable ambient temperature (dry bulb temperature,T _db = 25° C, relative humidity, rh = 40%–50% , clothing insulation = 1 clo) for 1 h and then they were submitted to a standard cold air test in a climatic chamber for 2h (T _db=1° C, rh = 40%–50%, wind speed = 0.8 m·s^–1, nude), before and after 27 h of sleep deprivation. Thermoregulatory changes (rectal temperature,T _re; mean skin temperature, _sk; metabolic heat production ) were monitored continuously. At comfortable ambient temperature, no significant change was observed after SD forT _re, _sk and . During the cold test,T _re did not change but _sk and were higher after SD (P<0.05). Increased (+ 6%,P < 0.05) was related to earlier and higher shivering, with a possible increase in the sensitivity of the thermoregulatory system as shown by the shorter time to onset of continous shivering (d): 8.66 (SEM 1.33) min versus 28.20 (SEM 1.33) min (P < 0.001) and by a higher _sk observed at d: 27.60 (SEM 1.40)° C versus 21.40 (SEM 0.60)° C (P < 0.001). These results were associated with higher cold sensations and shivering following SD. They also suggested that SD modified thermoregulatory responses at a central level especially in a cold environment.     

Serotonin release in the preoptic area and anterior hypothalamus is not involved in thermoregulation during low-intensity exercise in a warm environment

The aim of the present study was to assess the response of serotonin (5-hydroxytryptamine; 5-HT) in the preoptic area and anterior hypothalamus (PO/AH) to thermoregulation during exercise in a warm environment. In addition, it was investigated whether a rapid increase in extracellular 5-HT levels in the PO/AH modifies the thermoregulatory response under the same exercise conditions. Rats were made to run for 120 min at 10 m min⁻¹ on a treadmill at the ambient temperature of 30 °C. Body core temperature (T_core) was monitored using a biotelemetry system, and tail skin temperature (T_tail) was simultaneously measured as an index of heat loss response. Microdialysis in combination with HPLC was used to measure concentrations of monoamines in the PO/AH. Both T_core and T_tail increased during the first 20 min of exercise and remained stable until the end of the exercise period. Low-intensity exercise did not induce any changes in 5-HT release in the PO/AH, although the levels of norepinephrine and dopamine were increased. Moreover, increased extracellular 5-HT by local perfusion of 1 μM citalopram (selective 5-HT reuptake inhibitor; SSRI) in the PO/AH had no effect on the thermoregulatory response during acute low-intensity exercise in a warm environment. These results suggest that enhanced release of only 5-HT in the PO/AH may not intervene thermoregulation during exercise in a warm environment.     

Varied and repeated atropine dosages and exercise-heat stress

Summary Comparisons of physiological responses to 0, 0.5, 1, and 2 mg atropine (IM) were made in seven males ( ± SD: age, 24±3 years; ht, 174±12 cm; wt, 76±3 kg) while they exercised (~ 390 W) in a hot-dry (40 C, 20% rh) environment. Responses to 4 mg, as well as repeatability of responses to 2 mg, were studied in two and six of these subjects, respectively. On 8 test days an intramuscular injection of atropine or saline control was administered 20 min before subjects walked on a treadmill for two 50-min bouts. Heart rate (HR) during exercise did not change in the control trial but by min 50 increased during all atropine trials (P<0.01). Rectal temperature T_re) increased (P<0.01) in all trials by min 50 and continued increasing (P<0.01) in the 2-mg trial during the second exercise bout. For the two subjects tested with all dosages (0.5–4 mg atropine), the change in HR and T_re between the atropine and control trials at 50 min of exercise was regressed against the various atropine dosages. The relationship (r=0.92) for HR was curvilinear while the relationship (r=0.99) for T_re was linear. Mean weighted skin temperature ( _sk) was relatively constant during exercise and was warmer (P<0.05) with increasing atropine dosage. In a repeat 2 mg trial, HR was 6 bt·min^–1 lower (P<0.05) on the second exposure but T_re was the same (P>0.05) on both days. For subjects walking in the heat, three new observations were: 1) 0.5 mg of atropine resulted in increased HR and _sk compared to control values; 2) HR was elevated but the magnitude of change decreased with increasing dosage, while the elevation in T_re was consistent with increasing dosage; and 3) rectal temperatures (in trials with and without atropine) were unaffected by previous days of atropine administration.     

Metabolic and temperature responses to physical exercise in thyroidectomized dogs

Summary The purpose of the present work was to further elucidate the role of thyroid hormones in the control of body temperature and metabolism during physical exercise. Changes in rectal temperature (T _re), some parameters of exercise-metabolism and in the plasma noradrenaline (NA) levels were examined in eight dogs performing submaximal treadmill exercise to exhaustion before and after thyroidectomy (THY). The metabolic responses to adrenaline (A) infusion were also compared in intact and THY dogs. During the exercise performed by THY dogs T _re increases were markedly attenuated, plasma FFA level increases were reduced and the pattern of plasma NA changes was modified in comparison with control runs. The reduced exercise-induced FFA mobilization in THY dogs might be attributed to a lower activation of the adrenergic system in the later stage of exercise and to the weaker lipolytic action of catecholamines. The attenuated T _re increases during exercise performed by THY dogs and the exercise-hyperthermia described previously in dogs treated with thyroid hormones suggest that an optimum level of thyroid hormones is necessary to induce typical changes in body temperature during physical exercise.     

One night of sleep deprivation decreases treadmill endurance performance

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

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

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

Efficacy of body ventilation system for reducing strain in warm and hot climates

This study determined whether a torso-vest forced ambient air body ventilation system (BVS) reduced physiological strain during exercise-heat stress. Seven heat-acclimated volunteers attempted nine, 2-h treadmill walks at 200 W m⁻² in three environments, −40°C, 20% rh (HD), 35°C, 75% rh (HW), and 30°C, 50% rh, (WW) wearing the Army Combat Uniform, interceptor body armor (IBA) and Kevlar helmet. Three trials in each environment were BVS turned on (BVS_On), BVS turned off (BVS_Off), and no BVS (IBA). In HD, BVS_On significantly lowered core temperature (T _re), heart rate (HR), mean skin temperature (T _sk), mean torso skin temperature (T _torso), thermal sensation (TS), heat storage (S), and physiological strain index (PSI), versus BVS_Off and IBA (P < 0.05). For HW (n = 6), analyses were possible only through 60 min. Exercise tolerance time (min) during HW was significantly longer for BVS_On (116 ± 10 min) versus BVS_Off (95 ± 22 min) and IBA (96 ± 18 min) (P < 0.05). During HW, BVS_On lowered HR at 60 min versus IBA, T _sk from 30 to 60 min versus BVS_Off and IBA, and PSI from 45 to 60 min versus BVS_Off and at 60 min versus IBA (P < 0.05). BVS_On changes in T _re and HR were lower in HD and HW. During WW, BVS_On significantly lowered HR, T _sk, and T _torso versus BVS_Off and IBA (P < 0.05) during late exercise. Sweating rates were significantly lower for BVS_On versus BVS_Off and IBA in both HD and WW (P < 0.05), but not HW. These results indicate that BVS_On reduces physiological strain in all three environments by a similar amount; however, in hot-dry conditions the BVS_Off increases physiological strain.     

Responses of young and older men during prolonged exercise in dry and humid heat

Summary Eight young, sedentary men (aged 34 years, SD 3) and six older moderately active, unacclimated men (aged 57 years, SD 2) walked on a treadmill at 30% of their maximum oxygen consumption up to 3.5 h in a thermoneutral [dry bulb temperature (T _db) 21°C, relative humidity (r.h.) 43%)], a warm humid (T _db 30°C, r.h. 80%) and a hot dry (T _db 40°C, r.h. 20%) environment while wearing ordinary working clothes (0.7 c/o). Their oxgen consumption, heart rate (f _c), rectal (T _re) and mean skin temperature (T_sk), sweat rate (SR), and evaporative rate (ER) were measured during the tests. The ratings of thermal sensation (TS) and perceived exertion (RPE) were assessed using standard scales. In the heat stress tests, the number of experiments discontinued did not significantly differ between the two groups. The mean levels and end-exercise values of T _re, T_sk, f _c, TS and RPE were not significantly different between the young and older subjects in any of the environments. In the warm humid environment, however, the T _re and RPE of the older subjects increased continuously (P<0.05) during the test compared to the young subjects. No significant difference between the groups was observed in SR or in ER. In the hot dry environment, however, the ER of older men increased more slowly compared to the young men. In spite of some time-related differences observed in T _re, RPE, and ER, the older subjects did not exhibit higher f _c during exercise in the heat, they were not more hyperthermic and their performance times were similar to the young subjects. Therefore, it was concluded that older calendar age is not necessarily associated with a reduced ability to exercise in a hot environment and other factors, such as physical activity habits and aerobic capacity, may be equally important in determining heat tolerance in the elderly.     

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

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

The effect of ambient temperature on gross-efficiency in cycling

Time-trial performance deteriorates in the heat. This might potentially be the result of a temperature-induced decrease in gross-efficiency (GE). The effect of high ambient temperature on GE during cycling will be studied, with the intent of determining if a heat-induced change in GE could account for the performance decrements in time trial exercise found in literature. Ten well-trained male cyclists performed 20-min cycle ergometer exercise at 60% (power output at which VO_2max was attained) in a thermo-neutral climate (N) of 15.6 ± 0.3°C, 20.0 ± 10.3% RH and a hot climate (H) of 35.5 ± 0.5°C, 15.5 ± 3.2% RH. GE was calculated based on VO₂ and RER. Skin temperature (T _sk), rectal temperature (T _re) and muscle temperature (T _m) (only in H) were measured. GE was 0.9% lower in H compared to N (19.6 ± 1.1% vs. 20.5 ± 1.4%) (P < 0.05). T _sk (33.4 ± 0.6°C vs. 27.7 ± 0.7°C) and T _re (37.4 ± 0.6°C vs. 37.0 ± 0.6°C) were significantly higher in H. T _m was 38.7 ± 1.1°C in H. GE was lower in heat. T _m was not high enough to make mitochondrial leakage a likely explanation for the observed reduced GE. Neither was the increased T _re. Increased skin blood flow might have had a stealing effect on muscular blood flow, and thus impacted GE. Cycling model simulations showed, that the decrease in GE could account for half of the performance decrement. GE decreased in heat to a degree that could explain at least part of the well-established performance decrements in the heat.     

Thermoregulation during exercise in relation to sex and age

Summary The thermoregulatory responses to 1 h exercise of 14 male (age range 18–65 year) and 7 female (age range 18–46 year) athletes and 4 (3 and 1 ) non-athletic subjects have been investigated in a moderate environment (T _db=21 C, T _wb=15 C and rh<50%) and analysed in relation to age, sex, and maximum aerobic power output (VO₂ max).The maximal sweat loss (M _{sw max}) under the given conditions was closely related (r=+0.90) to VO_{2 max} and for a given relative work load (%VO_{2 max}), rectal (T _re) and mean skin (¯T _sk) temperatures was the same in all subjects.Sweat loss (004d _sw) was linearly related to total heat production (H) and to peripheral tissue heat conductance (K) and if expressed in relative terms (%M _{sw max}) was linearly related to T _re. For a given T _re relative sweat rate was identical in the groups studied. From these results it would seem that during exercise T _re rises to meet the requirements of heat dissipation by establishing a thermal gradient from core to skin and stimulating sweating in proportion to maximal capacity of the system. Thus provided the thermal responses to work were standardised using the appropriate physiological variables, there was no evidence to be found for differences in thermoregulatory function which could be ascribed to sex or age.     

Influence of hydration status and fluid replacement on heat tolerance while wearing NBC protective clothing

The purpose of the present study was to investigate the influence of hypohydration and fluid replacement on tolerance to an uncompensable heat stress. Eight healthy young males completed a matrix of six trials in an environmental chamber, set at 40°C and 30% relative humidity, while wearing nuclear, biological, and chemical protective clothing. Subjects performed either light (3.5 km · h⁻¹, 0% grade, no wind) or heavy (4.8 km · h⁻¹, 4% grade, no wind) treadmill exercise combined with three hydration states [euhydration with fluid replacement (EU/F), euhydration without fluid replacement (EU/NF), and hypohydration with fluid replacement (H/F)]. Hypohydration of 2.2% body mass was achieved by exercise and fluid restriction on the day preceding the trials. No differences in the endpoint mean skin temperature (Tˉ_sk), sweat rate, or rectal temperature (T _re) were observed among the hydration conditions for either work rate. During light exercise, the change in T _re (ΔT _re) was significantly higher with H/F than EU/F after 40 min, and heart rate was greater after 25 min. The heart rate was greater during EU/NF than during EU/F after 60 min. Tolerance times were significantly greater for EU/F than for either EU/NF or H/F. With heavy exercise, no differences in ΔT _re were observed across hydration conditions. Compared to EU/F, heart rates were higher after 10 and 30 min for H/F and EU/NF, respectively. Tolerance times were significantly less during H/F than with either of the EU conditions. Stroke volume was significantly decreased in H/F trials compared to EU/F trials for both light and heavy work rates, but no differences in cardiac output were observed. It was concluded that even minor levels of hypohydration significantly impaired exercise tolerance in a severely uncompensable heat stress environment at both light and heavy exercise intensities. Accepted: 17 June 1997     

Effects of training and acclimation on heat tolerance in exercising men wearing protective clothing

This study examined the effectiveness of endurance training and heat acclimation in reducing the physiological strain imposed by exercising in the heat while wearing protective clothing. Seven young men underwent 8 weeks of physical training [60–80% maximal aerobic power (VO_2max) for 30–45 min · day^–1, 3–4 days · week^–1 at < 25° C] followed by 6 days of heat acclimation (45–55% VO_2max for 60 min · day^–1 at 40° C, 30% relative humidity). Nine other young men underwent corresponding periods of control observation and heat acclimation. Before and after each treatment, subjects completed a treadmill walk (4.8 km · h^–1, 2% grade) in a climatic chamber (40° C, 30% relative humidity), wearing in turn normal combat clothing or clothing protecting against nuclear, biological, and chemical (NBC) agents. Criteria for halting this test were: (1) a rectal temperature (T _re) of 39.3° C; (2) a heart rate (f _c) 95% of the subject's observed maximum, maintained for 3 min; (3) unwillingness of the subject to continue; (4) the elapse of 120 min. The training regimen increased mean VO_2max by 16% and mean plasma volume by 8%. When tested in normal combat clothing, the rates of increase in T _re and f _c were slower after training. However, when wearing NBC protective clothing, the only significant change induced by training was a higher mean skin temperature (T _sk) in the early part of the test. Heat acclimation increased the mean plasma volume of untrained subjects by 8%, but their VO_2max remained unchanged. When tested in normal combat clothing, acclimation decreased their mean values of T _re, T _sk, f _c, and metabolic rate. When wearing NBC protective clothing, the only significant decrease after acclimation was in overall T _re. In trained subjects, heat acclimation induced no further improvement in any physiological variable when wearing normal combat clothing, but reduced overall T _re and T _sk when wearing NBC protective clothing. Training- or acclimation-induced increases of sweat secretion (an average increment of 0.14–0.23 kg · h^–1) were not accompanied by any statistically significant increase in sweat evaporation when wearing NBC protective clothing. Moreover, tolerance times were unchanged in either normal combat (116–120 min) or NBC protective clothing (47–52 min). We conclude that neither endurance training nor heat acclimation do much to improve exercise tolerance when wearing NBC protective clothing in hot environments, because any added sweat secretion decreases blood volume and increases discomfort without augmenting body cooling.     

Thermoregulation during heat exposure of young children compared to their mothers

The study was conducted to investigate the thermoregulation of young children compared to that of adults. A group of 19 children (ages 9 months-4.5 years), with only 3 children aged 3 years or above, and 16 adults first rested in a thermoneutral room (air temperature 25°C relative humidity 50%, air velocity 0.2 m·s^–1). They were then exposed to a hot room (air temperature 35°C, relative humidity 70%, air velocity 0.3 m·s^–) next door for 30 min, and then returned to the thermoneutral room where they stayed for a further 30 min. The rectal temperature (T _re), skin temperatures (T _sk) at seven sites, heart rate (HR), total sweat rate ( ), local sweat rate ( ) and the Na⁺ concentration of the sweat were measured. There was no significant difference inT _re between the children and their mothers in the rest phase. However, theT _re of the children increased as soon as they entered the hot room and was significantly higher than during the control period, and than that of the mothers during heat exposure. MeanT _sk, forehead, abdomen and instepT _sk were significantly higher in the children during both the thermoneutral and heat exposure. The was significantly higher and Na⁺ concentrations in the sweat on the back and upperarm were significantly lower for the children during the heat exposure. They had a greater body surface area-to-mass ratio than the mothers by 64%, which indicated that they had advantages for thermal regulation. However, the sweating andT _sk responses of the children were not enough to prevent a rise in body temperature. These results would suggest that the young children had the disadvantage of heating up easily due to their smaller body sizes and there may be maturation-related differences in thermoregulation during the heat exposure between young children and mothers.