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.     

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.     

Dynamics of sweating in men and women during passive heating

Summary The dynamics of sweating was investigated at rest in 8 men and 8 women. Electrical skin resistance (ESR), rectal temperature (T_re) and mean skin temperature were measured in subjects exposed to 40‡ C environmental temperature, 30% relative air humidity, and 1 m · s⁻¹ air flow. Sweat rate was computed from continuous measurement of the whole body weight loss. It was found that increases in T_re, and mean body temperature were higher in women than in men by 0.16, 0.38 and 0.21‡ C, but only the difference in δ was significant (p<0.05). The dynamics of sweating in men and women respectively, was as follows: delay (t_d) 7.8 and 18.1 min (p<0.01), time constant (Τ) 7.5 and 8.8 min (N.S.), inertia time (t_i) 15.3 and 26.9 min (p<0.002), and total body weight loss 153 and 111 g · m⁻² · h⁻¹ (p<0.001). Dynamic parameters of ESR did not differ significantly between men and women. Inertia times of ESR and sweat rate correlated in men (r=0.93, p<0.001), and in women (r=0.76, p<0.02). In men, δ T_re correlated with inertia time of sweat rate (r=0.81, p<0.01) as well as with the inertia time of ESR (r=0.83, p<0.001). No relation was found between δ T_re and the dynamics of sweating in women. It is concluded that the dynamics of sweating plays a decisive role in limiting δ T_re in men under dry heat exposure. The later onset of sweating in women does not influence the rectal temperature increase significantly. In women, δ T_re is probably limited by a complex interaction of sweating, skin blood flow increase, and metabolic rate decrease. This work was supported by the Centre National de la Recherche Scientifique and Polish Academy of Siences     

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.     

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

Summary In six male subjects the sweating thresholds, heart rate (f _c, as well as the metabolic responses to exercise of different intensities [40%, 60% and 80% maximal oxygen uptake (VO_2max)], were compared at ambient temperatures (T _a) of 5° C (LT) and 24° 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< 0.05). During 20-min exercise at 40%VO_2max performed in the cold no sweating was recorded, while at higher exercise intensities sweating occurred at similar rectal temperatures (T _re) but at lower mean skin (T _sk) and mean body temperatures (T _b) in LT than MT experiments (P<0.001). The exercise inducedVO₂ increase was greater only at the end of the light (40%VO_2max) exercise in the cold in comparison with MT (P<0.001). Bothf _c and blood lactate concentration [la]_b were lower at the end of LT than MT for moderate (60%VO_2max) and heavy (80%VO_2max) exercises. It was concluded that the sweating threshold during exercise in the cold environment had shifted towards lower (T _b) andT _sk. 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° C since the increases inT _re,f _c and [la]_b were lower at the lowerT _a.     

Heat balance and cumulative heat storage during exercise performed in the heat in physically active younger and middle-aged men

On separate days, eight physically active younger (22 ± 2 years) and eight highly trained middle-aged (45 ± 4 years) men matched for physical fitness and body composition performed 90 min of semi-recumbent cycling at a constant rate of heat production (290 W) followed by 60 min of seated recovery in either a temperate (T, 30°C), warm (W, 35°C) or hot (H, 40°C) ambient condition. Rectal temperature (T _re) was measured continuously, while the rate of whole-body heat loss (H _L), as well as changes in body heat content (∆H _b) was measured simultaneously using direct whole-body and indirect calorimetry. No difference in H _L was observed between age groups for all ambient conditions. Accordingly, the average ∆H _b during the 90-min exercise was similar for the younger (+193 ± 52, 212 ± 82 and +211 ± 44 kJ for T, W and H, respectively) and middle-aged men (+192 ± 119, +225 ± 76 and +217 ± 130 kJ for T, W and H, respectively). This was paralleled by a similar increase in T _re of 0.40 ± 0.20, 0.36 ± 0.14 and 0.34 ± 0.23°C for T, W and H, respectively in the younger men and 0.37 ± 0.23, 0.32 ± 0.19 and 0.28 ± 0.14°C for T, W and H, respectively in the middle-aged men. After 60 min of recovery, ∆H _b was similar for the younger and the middle-aged men, respectively (−45 ± 52 and −38 ± 31 kJ for T; −57 ± 78 and −40 ± 25 kJ for W; and −32 ± 71 and 11 ± 96 kJ for H). End recovery T _re remained elevated to similar levels in both the younger and middle-aged men, respectively, for each of the ambient conditions (0.24 ± 019 and 0.18 ± 0.18°C for T; 0.25 ± 0.11 and 0.24 ± 0.14°C for W and 0.33 ± 0.21 and 0.33 ± 0.13°C for H). We conclude that highly trained middle-aged men demonstrate a similar capacity for heat dissipation when compared with physically active younger men.     

Skin AVA and capillary dilatation and constriction induced by local skin heating

In conscious sheep, total femoral blood flow and flow through arteriovenous anastomoses (AVAs) and capillaries (CAP) in skin of the hindleg were measured employing electromagnetic and radioactive microsphere techniques. Core temperature (T _c) was manipulated using intravascular heat exchangers and hindleg skin temperature (T _sk) was manipulated by immersion in temperature controlled water. WithT _c set 1°C above normal, AVA flow was highest at the lowestT _sk tested (34°C); AVAs progressively constricted asT _sk was increased from 34 to 40–41°C, then dilated again asT _sk reached the highest levels tested (42–44°C). Skin CAP flow was not altered byT _sk of 34 to 42°C but was increased at aT _sk of 44°C. Therefore total skin blood flow followed essentially the same pattern as AVA flow; total femoral flow also followed this pattern. WhenT _c was set 0.5°C below normal, AVA flow was low at all levels ofT _sk. It is concluded thatT _c plays a dominant role in control of skin blood flow, however, onceT _c is at a level requiring increased heat loss,T _sk exerts an extremely potent influence on the nature and magnitude of changes in skin blood flow. The pattern of flow changes appears to reflect principally a negative feedback mechanism aimed at maintainingT _sk at approximately 40°C; this may contrast with mechanisms associated with sweating and/or active vasodilation in other species.     

Assessing neonatal heat balance and physiological strain in newborn infants nursed under radiant warmers in intensive care with fentanyl sedation

Purpose

To assess heat balance status of newborn infants nursed under radiant warmers (RWs) during intensive care.

Methods

Heat balance, thermal status and primary indicators of physiological strain were concurrently measured in 14 newborns nursed under RWs for 105 min. Metabolic heat production (M), evaporative heat loss (E), convective (C) and conductive heat flow (K), rectal temperature (T _re) and mean skin temperatures (T _sk) were measured continuously. The rate of radiant heat required for heat balance (R _req) and the rate of radiant heat provided (R _prov) were derived. The rate of body heat storage (S) was calculated using a two-compartment model of ‘core’ (T _re) and ‘shell’ (T _sk) temperatures.

Results

Mean M, E, C and K were 10.5 ± 2.7 W, 5.8 ± 1.1 W, 6.2 ± 0.8 W and 0.1 ± 0.1 W, respectively. Mean R _prov (1.7 ± 2.6 W) and R _req (1.7 ± 2.7 W) were similar (p > 0.05). However, while the resultant mean change in body heat content after 105 min was negligible (–0.1 ± 3.7 kJ), acute time-dependent changes in S were evidenced by a mean positive heat storage component of +6.4 ± 2.6 kJ and a mean negative heat storage component of –6.5 ± 3.7 kJ. Accordingly, large fluctuations in both T _re and T _sk occurred that were actively induced by changes in RW output. Nonetheless, no active physiological responses (heart rate, breathing frequency and mean arterial pressure) to these bouts of heating and cooling were observed.

Conclusions

RWs maintain net heat balance over a prolonged period, but actively induce acute bouts of heat imbalance that cause rapid changes in T _re and T _sk. Transient bouts of heat storage do not exacerbate physiological strain, but could in the longer term.     

Impact of a protective vest and spacer garment on exercise-heat strain

Protective vests worn by global security personnel, and weighted vests worn by athletes, may increase physiological strain due to added load, increased clothing insulation and vapor resistance. The impact of protective vest clothing properties on physiological strain, and the potential of a spacer garment to reduce physiological strain, was examined. Eleven men performed 3 trials of intermittent treadmill walking over 4 h in a hot, dry environment (35°C, 30% rh). Volunteers wore the US Army battledress uniform (trial B), B + protective vest (trial P), and B + P + spacer garment (trial S). Biophysical clothing properties were determined and found similar to many law enforcement, industry, and sports ensembles. Physiological measurements included core (T _c), mean skin (T _sk) and chest (T _chest) temperatures, heart rate (HR), and sweating rate (SR). The independent impact of clothing was determined by equating metabolic rate in all trials. In trial P, HR was +7 b/min higher after 1 h of exercise and +19 b/min by the fourth hour compared to B (P < 0.05). T _c (+0.30°C), T _sk (+1.0°C) and Physiological Strain Index were all higher in P than B (P < 0.05). S did not abate these effects except to reduce T _sk (P > S) via a lower T _chest (−0.40°C) (P < 0.05). SR was higher (P < 0.05) in P and S versus B, but the magnitude of differences was small. A protective vest increases physiological strain independent of added load, while a spacer garment does not alter this outcome.     

The effects of pre-warming on the metabolic and thermoregulatory responses to prolonged submaximal exercise in moderate ambient temperatures

To determine the effects of pre-warming on the human metabolic and thermoregulatory responses to prolonged steady-rate exercise in moderate ambient temperatures and relative humidities [means (SD) 21.7 (2.1)° C and 36.7 (5.4)%, respectively], six healthy men each ran at a steady-rate (70% maximal oxygen uptake) on a treadmill until exhausted after being actively pre-warmed (AH), passively pre-warmed (PH), and rested (Cont). Exercise time to exhaustion was significantly reduced following both AH and PH compared to Cont [AH 47.8 (14.0) min, PH 39.6 (16.0) min, Cont 62.0 (8.8) min; P<0.05]. During exercise there were no significant differences in oxygen uptake, total sweat loss, mean skin temperature (T_sk) and the thermal gradient (T _re–T_sk, where T _re is rectal temperature) following the three conditions. Serum prolactin, plasma catecholamine and plasma free fatty acid concentrations were also similar between all three trials. In contrast, T _re, mean body temperature, heart rate and ratings of perceived exertion were significantly greater during the initial 25 min of exercise following both AH and PH, compared with Cont (P<0.05). At exhaustion, there were no significant differences in the metabolic and thermoregulatory responses to exercise between the trials. The current findings demonstrate that AH and PH promote a reduction in prolonged submaximal endurance performance under moderate environmental temperatures compared with pre-exercise rest. Such observations appear likely to have been mediated through mechanisms associated with the earlier development of high internal body temperature which resulted in changes in the capacity for heat storage. Electronic Publication     

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.     

Regulation of local sweating in sleep-deprived exercising humans

Summary Thermoregulatory sweating [total body (m _sw,b), chest (m _sw,c) and thigh (m _sw,t) sweating], body temperatures [oesophageal (T _oes) and mean skin temperature (T _sk)] and heart rate were investigated in five sleep-deprived subjects (kept awake for 27 h) while exercising on a cycle (45 min at approximately 50% maximal oxygen consumption) in moderate heat (T _air andT _wall at 35° C. Them _sw,c andm _sw,t were measured under local thermal clamp (T _sk,1), set at 35.5° C. After sleep deprivation, neither the levels of body temperatures (T _oes,T _sk) nor the levels ofm _{sw, b},m _{sw, c} orm _{sw, t} differed from control at rest or during exercise steady state. During the transient phase of exercise (whenT _sk andT _sk,1 were unvarying), them _{sw, c} andm _{sw, t} changes were positively correlated with those ofT _oes. The slopes of them _{sw, c} versusT _oes, orm _{sw, t} versusT _oes relationships remained unchanged between control and sleep-loss experiments. Thus the slopes of the local sweating versusT _oes, relationships (m _{sw, c} andm _{sw, t} sweating data pooled which reached 1.05 (SEM 0.14) mg·cm^–2·min^–1°C^–1 and 1.14 (SEM 0.18) mg·cm^–2·min^–1·°C^–1 before and after sleep deprivation) respectively did not differ. However, in our experiment, sleep deprivation significantly increased theT _oes threshold for the onset of bothm _{sw, c} andm _{sw, t} (+0.3° C,P<0.001). From our investigations it would seem that the delayed core temperature for sweating onset in sleep-deprived humans, while exercising moderately in the heat, is likely to have been due to alterations occurring at the central level.     

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.     

The reproducibility of closed-pouch sweat collection and thermoregulatory responses to exercise–heat stress

Seven active male subjects cycled for 60 min at 29.5 (0.8)% peak work rate on three separate occasions in a hot environmental condition [36.0 (0.1)°C, 60 (1)% relative humidity] in order to determine the reproducibility of a closed-pouch sweat collection technique for sweat composition at the scapula, forearm and thigh. To confirm that sweat composition was not influenced by between-trial variations in sudomotor drive, local sweat rate, whole-body sweat rate, heart rate (HR), rectal temperature (T_re) and mean skin temperature (T_sk) responses were also measured, consequently reproducibility was also established for these variables. Sweat composition did not differ among trials, with the mean coefficients of variation (CVs) for sweat [Na⁺], [K⁺] and pH being 10.4 (7.4)%, 8.1 (6.5)% and 1.3 (1.1)%, respectively. Local sweat rates did not differ among the three trials (P>0.05) although whole-body sweat rate was reduced in the third trial (P<0.05). The mean CVs were 11.0 (7.8)% and 4.7 (1.6)% for local and whole-body sweat rates, respectively. Between-trial differences were not evident for T_re, T_sk or HR with mean CVs of 0.3 (0.2)%, 0.7 (0.6)% and 3.9 (1.7)%, respectively, although HR tended to be greater in the first trial (P=0.08). It is proposed that moderate variations in sweat composition were influenced by variations in the local sweat rate, which were induced by application of the pouch.     

Thermal face protection delays finger cooling and improves thermal comfort during cold air exposure

When people dress for cold weather, the face often remains exposed. Facial cooling can decrease finger blood flow, reducing finger temperature (T _f). This study examined whether thermal face protection limits finger cooling and thereby improves thermal comfort and manual dexterity during prolonged cold exposure. T _f was measured in ten volunteers dressed in cold-weather clothing as they stood for 60 min facing the wind (−15°C, 3 m s⁻¹), once while wearing a balaclava and goggles (BAL), and once with the balaclava pulled down and without goggles (CON). Subjects removed mitts, wearing only thin gloves to perform Purdue Pegboard (PP) tests at 15 and 50 min, and Minnesota Rate of Manipulation (MRM) tests at 30 and 55 min. Subjects rated their thermal sensation and comfort just before the dexterity tests. T _f decreased (p < 0.05 for time × trial interaction) by 15 min of cold exposure during CON (33.6 ± 1.4–28.7 ± 2.0°C), but not during BAL (33.2 ± 1.4–30.6 ± 3.2°C); and after 30 min T _f remained warmer during BAL (23.3 ± 5.9°C) than CON (19.2 ± 3.5); however, by 50 min, T _f was no different between trials (14.1 ± 2.7°C). Performance on PP fell (p < 0.05) by 25% after 50 min in both trials; MRM performance was not altered by cold on either trial. Subjects felt colder (p < 0.05) and more uncomfortable (p < 0.05) during CON, compared to BAL. Thermal face protection was effective for maintaining warmer T _f and thermal comfort during cold exposure; however, local cooling of the hands during manual dexterity tests reduced this physiological advantage, and performance was not improved.     

Time-of-day effect on nonthermal control of sweating response to maintained static exercise in humans

To investigate the influence of nonthermal factors in the time-of-day effect on the sweating response to maintained static exercise, eight healthy male subjects performed handgrip exercise at 20%, 35% and 50% maximal voluntary contraction (MVC) for 60 s at 0600 hours (morning) and at 1800 hours (evening). Oesophageal temperature (T _oes) before the experiment showed a diurnal rhythm [mean (SEM)] [36.3 (0.1) (morning) compared to 36.8 (0.1) °C (evening), P<0.01]. Experiments were conducted with subjects in a state of mild hyperthermia during which the mean skin temperature (T _sk) was kept constant at 35.5–36.5 °C using a water-perfused suit to activate sudomotor responses. The T _oes and mean T _sk remained stable during the pre-exercise, handgrip exercise and recovery periods. The response in sweating rate (ΔSR) on the chest and forearm to handgrip exercise increased significantly with increasing exercise intensity in both the morning and evening tests (P<0.05). The ΔSR on the palm did not change significantly with increasing exercise intensity in the morning test (P>0.1). During handgrip exercise at 50% MVC only, ΔSR on the chest, forearm and palm in the evening was significantly higher than in the morning (P<0.05). On the other hand, mean arterial blood pressure and the rating of perceived exertion during 50% MVC handgrip exercise were not significantly different between the morning and evening (P>0.1). These results indicate the presence of a time-of-day effect on nonthermal control of the sweating response to isometric handgrip exercise, and that this effect is dependent on exercise intensity. Electronic Publication     

Thermoregulatory responses of young and older men to cold exposure

Summary Nine young (20–25 years) and ten older (60–71 years) men, matched for body fatness and surface area :mass ratio, underwent cold tests in summer and winter. The cold tests consisted of a 60-min exposure, wearing only swimming trunks, to an air temperature of 17°C (both seasons) and 12°C (winter only). Rectal (T _re) and mean skin ( _sk) temperatures, metabolic heat production (M), systolic (BP_S) and diastolic (BP_d) blood pressures and heart rate (f _c) were measured. During the equilibrium period (28°C air temperature) there were no age-related differences inT _re, _sk, BP_S, BP_d, orf _c regardless of season, although M of the older men was significantly lower (P<0.003). The decrease inT _re and _sk (due to the marked decrease in six of the older men) and the increase in BPS and BPd were significantly greater (P<0.004) for the older men during all the cold exposures. The rate of increase inM was significantly greater (P<0.01) for the older group when exposed to 12°C in winter and 17°C in summer (due to the marked increase in four of the older men). This trend was not apparent during the 17°C exposure in winter. There was no age-related difference in f_c during the exposures. Significant decreases inT _re and _sk and increases inM, BP_S and BP_d during the 12°C exposure were observed for the older group (P< 0.003) compared to their responses during the 17°C exposure in winter. In contrast,T _re,M, BP_S in the young group were not affected as much by the colder environment. It was concluded that older men have more variable responses and some appear more or less responsive to mild and moderate cold air than young men.     

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     

Human sudomotor responses to heating and cooling upper-body skin surfaces: cutaneous thermal sensitivity

The influence of local skin temperature (T_skl) on the control of local and whole-body sweating was evaluated in eight healthy males. A water-perfusion garment (37 °C) and a climatic chamber (36.45 ± 0.78 °C; [±SD]; relative humidity 60.3 ± 1.6%) were used to raise and clamp skin and core temperatures. Warm and cool stimuli were applied to four upper-body skin regions (face, arm, forearm, hand) using perfusion patches (249.0 ± 0.2 cm²). Heating elevated, while cooling suppressed sweat rate (m˙_sw) locally, and at other skin surfaces. However, the tendency for T_skl manipulations to induce localized sweat responses was no more powerful than it was at stimulating sweating in non-treated regions (P > 0.05). Accordingly, neither thermal stimulus produced significantly greater local sudomotor influences than were elicited contralaterally (P > 0.05). No statistical support was found for the notion of inter-regional differences in upper-body cutaneous thermal sensitivity for sudomotor control, and, regardless of the stimulation site, whole-body sudomotor responses to localized thermal treatments were equivalent (P > 0.05).     

Thermoregulatory responses to intermittent exercise are influenced by knit structure of underwear

Summary The purpose of this study was to evaluate the role of knit structure in underwear on thermoregulatory responses. Underwear manufactured from 100% polypropylene fibres in five different knit structures (1-by-1 rib, fleece, fishnet, interlock, double-layer rib) was evaluated. All five underwear prototypes were tested as part of a prototype clothing system. Measured on a thermal manikin these clothing systems had total thermal resistances of 0.243, 0.268, 0.256, 0.248 and 0.250 m² · K · W^–1, respectively (including a value for the thermal resistance of the ambient environment of 0.104 m² · K · W^–1). Human testing was done on eight male subjects and took place at ambient temperature (T _a)=5°C, dew point temperature (T _dp)=–3.5° C and air velocity (V _a)=0.32 m · s^–1. The test comprised a repeated bout of 40-min cycle exercise (315 W · m^–2; 52%, SD 4.9% maximal oxygen uptake) followed by 20 min of rest (62 W · m^–2). The oxygen uptake, heart rate, oesophageal temperature, skin temperature,T _a,T _dp at the skin and in the ambient air, onset of sweating, evaporation rate, non-evaporated sweat accumulated in the clothing and total evaporative loss of mass were measured. Skin wettedness was calculated. The differences in knit structure of the underwear in the clothing systems resulted in significant differences in mean skin temperature, local and average skin wettedness, non-evaporated and evaporated sweat during the course of the intermittent exercise test. No differences were observed over this period in the core temperature measurements.The views, opinions and/or findings in this report are those of the authors and should not be construed as an official Department of the Army position, policy, or decision, unless so designated by other official documentation