Residual analysis in the determination of factors affecting the estimates of body heat storage in clothed subjects

Body heat storage can be estimated by calorimetry (from heat gains and losses) or by thermometry [from changes (Δ) in mean body temperature (T _b) calculated as a weighted combination of rectal (T _re) and mean skin temperatures (T _sk)]. If an invariant weighting factor ofT _re andT _sk were to be used (for instance, ΔT _b = 0.8 · ΔT _re + 0.2 · ΔT _sk under hot conditions), body heat storage could be over- or underestimated substantially relative to calorimetry, depending on whether the subject was wearing light or protective clothing. This study investigated whether discrepancies between calorimetry and thermometry arise from methodological errors in the calorimetric estimate of heat storage, from inappropriate weightings in the thermometric estimate, or from both. Residuals of calorimetry versus thermometric estimates were plotted against individual variables in the standard heat balance equation, applying various weighting factors toT _re andT _sk. Whether light or protective clothing was worn, the calorimetric approach generally gave appropriate estimates of heat exchange components and thus heat storage. One exception was in estimating latent heat loss from sweat evaporation. If sweat evaporation exceeded 650 g·h⁻¹ when wearing normal clothing, evaporative heat loss was overestimated and thus body heat storage was underestimated. Nevertheless, if data beyond this ceiling were excluded from the analyses, the standard 4:1 weighting matched calorimetric heat storage estimates quite well. When wearing protective clothing, the same 4:1 weighting approximated calorimetric heat storage with errors of less than approximately 10%, but only if environmental conditions allowed a subject to exercise for more than 90 min. The best thermometric estimates of heat storage were provided by using two sets of relative weightings, based upon the individual's metabolic heat production ( in kilojoules per metre squared per hour): {4 − [( )· ] 2}:1 for an initial, thermoneutral environment and {4 + [( ) · ] · 5}: 1 for a final, hot environment; the optimal value of lay between 450 and 500 kJ m⁻² · h⁻¹. We concluded that the accuracy of thermometric estimates of heat storage can be improved by modifying weighting factors ofT _re andT _sk according to the environment, type of clothing, and metabolic rate.     

Determination of body heat storage in clothing: calorimetry versus thermometry

Two methods of estimating body heat storage were compared under differing conditions of clothing, training, and acclimation to heat. Six male subjects underwent 8 weeks of physical training [60–80% of maximal aerobic power ( ) for 30–45 min · day–, 3–4 days · week^–1 at < 25 °C dry bulb (db)] followed by 6 consecutive days of heat acclimation (45–55% for 60 min · day^–1 at 40°C db, 30% relative humidity)]. Nine other male subjects underwent corresponding periods of control observation followed by heat acclimation. Before and after each treatment, subjects walked continuously on a treadmill (1.34 m · s^–1, 2% grade) in a climatic chamber (40°C db, 30% relative humidity) for an average of 118 min (range 92–120 min) when wearing normal light combat clothing and for an average of 50 min (range 32–68 min) when wearing protective clothing resistant to nuclear, biological, and chemical agents. The heat storage was determined calorimetrically (by the balance of heat gains and losses) and thermometrically [by the conventional equations, using one or two set(s) of relative weightings for the rectal temperature (T _re) to mean skin temperature _sk of 4:1 and 4:1, 2:1 and 4:1, or 2:1 and 9:1 in thermoneutral and hot environments, respectively]. _sk was calculated from 12-site measurements, weighted according to the regional distribution of body surface area and the first eigenvectors of principal component analysis. There were only minor differences (< 5%) between the heat storage values calculated by given weighting factors forT _re and _sk, whether the individual coefficients were derived from estimates of regional surface area or principal component methodologies. When wearing normal clothing, no significant differences were found between the two estimates of heat storage (calorimetry vs thermometry with an invariant relative weighting of 4:1) in any experimental condition, with one specific exception: when wearing protective clothing, thermometry underestimated the heat storage by 24–31%. This underestimation was attenuated by using two sets of relative weightings of 2: 1 and 4: 1 or 2: 1 and 9: 1. The results suggest that when subjects wearing protective clothing are transferred from thermoneutral to hot environments, the accuracy of thermometric estimates of heat storage can be improved by using two sets of weighting factors forT _re and _sk     

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.     

Exercise thermoregulation after 6 h of chair rest, 6° head-down bed-rest,and water immersion deconditioning in men

The purpose was to investigate the mechanism for the excessive exercise hyperthermia following deconditioning (reduction of physical fitness). Rectal (T _re) and mean skin ( ) temperatures and thermoregulatory responses were measured in six men [mean (SD) age, 32 (6) years; mass, 78.26 (5.80) kg; surface area, 1.95 (0.11)m²; maximum oxygen uptake ( ), 48 (6) ml·min^–1·kg^–1; whilst supine in air at dry bulb temperature 23.2 (0.6)°C, relative humidity 31.1 (11.1)% and air speed 5.6 (0.1) m·min^–1] during 70 min of leg cycle exercise [51 (4)% ] in ambulatory control (AC), or following 6 h of chair rest (CR), 6° head-down bed rest (BR), and 20° (WI20) and 80° (WI80) foot-down water immersion [water temperature, 35.0 (0.1)°C]. Compared with the AC exercise T _re [mean (SD) 0.77 (0.13)°C], T _re after CR was 0.83 (0.08)°C (NS), after BR 0.92 (0.13)°C (^*P<0.05), after WI80 0.96 (0.13)°C^*, and after WI20 1.03 (0.09)°C^*. All responded similarly to exercise: they decreased (NS) by 0.5–0.7°C in minutes 4–8 and equilibrated at +0.1 to +0.5°C at 60–70. Skin heat conductance was not different among the five conditions (range = 147–159 kJ·m^–2·h^–1·°C^–1). Results from an intercorrelation matrix suggested that total body sweat rate was more closely related toT _re at 70 min (T _re70) than limb sweat rate or blood flow. Only 36% of the variability inT _re70 could be accounted for by total sweating, and less than 10% from total body dehydration. It would appear that multiple factors are involved which may include change in sensitivity of thermo- and osmoreceptors.     

How should body heat storage be determined in humans: by thermometry or calorimetry?

Summary The aim of this study was to determine whether in humans there are differences in the heat storage calculated by partitional calorimetry (S, the balance of heat gains and heat losses) compared to the heat storage obtained by conventional methods (thermometry) via either core temperature or mean body temperatures ( , whereT _c is core temperature and is mean skin temperature) when two different sites are used as an index ofT _c [rectal (T _re) and auditory canal (T _ac) temperatures]. Since women respond to the heat differently than men, both sexes were studied. After a stabilisation period at thermal neutrality, six men and seven women were exposed to a globe temperature of 50°C, relative humidity of 17% and wind speed of 0.8–1.0 m·s^–1 for 90 min semi-nude at rest, whereT _re,T _ac, , metabolic rate, dry (radiant+convective heat exchange) and evaporative heat losses,S, heat storage byT _c ( ) and heat storage by were assessed every minute. In the men,S was equal to 350.8(SEM 49.6) kJ whereas amounted to only 114.6(SEM 16.2) and 196.7(SEM 32.3) kJ forT _re andT _ac, respectively (P<0.05). Final underestimatedS by 49% [177.7(SEM 23.0) kJ;P<0.05] whereas was not significantly different than S [255.7(SEM 37.9) kJ]. In the women,S corresponded to a total of 294.3(SEM 23.2) kJ, a value that was very similar to the 262.6(SEM 31.0) kJ], whereas underpredicted by 35% [190.4(SEM 26.3) kJ;P<0.05]. As in the men,S _{T c} was much lower thanS [116.6(SEM 19.9) and 190.3(SEM 24.2) kJ forT _re andT _ac, respectively;P<0.05]. Using seven other well-known weighting coefficients, could under- and overestimateS by up to 55% and 11%, respectively. In all subjects, a large portion of the variance (68% and 75%) in the difference betweenS and , could be explained primarily by the T _ac. The results demonstrated that although some estimates of thermometric heat storage matched the calorimetricS, other predictions underestimated it by up to 67% during passive heating. It is suggested that these differences can be explained in part by he site chosen to representT _c, the use of eitherT _c or in the heat storage calculation, and the thermoneutral/hot weighting coefficient(s) chosen to determine . Until more representative measurements of body temperatures at different depths (core, shell and intermediate) are possible, the use of and -derived heat storage is difficult to justify.     

The relative influence of body characteristics on humid heat stress response

The present study was designed to determine the relative importance of individual characteristics such as maximal oxygen uptake ( O_2max), adiposity, DuBois body surface area (A _D), surface to mass ratio (A _D: mass) and body mass, for the individual's reaction to humid heat stress. For this purpose 27 subjects (19 men, 8 women), with heterogeneous characteristics ( O_2max 1.86–5.28 1 · min^–1; fat% 8.0%–31.9%; mass 49.8–102.1 kg; A _D 1.52–2.33 m²) first rested (30 min) and then exercised (60 W for 1 h) on a cycle ergometer in a warm humid climate (35°C, 80% relative humidity). Their physiological responses at the end of exercise were analysed to assess their relationship with individual characteristics using a stepwise multiple regression technique. Dependent variables (with ranges) included final values of rectal temperature (T _re 37.5–39.0°C), mean skin temperature (T _sk 35.7–37.5°C), body heat storage (S 3.2–8.1 J · g^–1), heart rate (HR 100–172 beat · min^–1), sweat loss (397–1403g), mean arterial blood pressure (BP_a, 68–96 mmHg), forearm blood flow (FBF, 10.1–33.9 ml · 100ml^–1 · min^–1) and forearm vascular conductance (FVC = FBF/BP_a, 0.11–0.49 ml · 100 ml^–1 · min^–1 · mmHg^–1). The T _re, T _sk and S were (34%–65%) determined in the: main by ( O_2max), or by exercise intensity expressed as a percent age of O_2max (% O_2max). For T _re, A _D: mass ratio also contributed to the variance explained, with about half the effect of ( O_2max), For T _sk, fat% contributed to the variance explained with about two-third the effect of O_2max. Total body sweat loss was highly dependent (50%) on body size (A _D or mass) with regular activity level having a quarter of the effect of body size on sweat loss. The HR, similar to T _re, was determined by O_2max (48%–51%), with less than half the effect of A _D or A _D :mass (20%). Other circulatory parameters (FBF, BP_a, FVC) showed little relationship with individual characteristics ( < 36% of variance explained). In general, the higher the ( O_2max), and/or the bigger the subject, the lower the heat strain observed. The widely accepted concept, that body core temperature is determined by exercise intensity expressed as % O_2max and sweat loss by absolute heat load, was only partially supported by the results. For both variables, other individual characteristics were also shown to contribute.     

Effects of estrus cycle on thermoregulatory responses during exercise in rats

Summary In female rats, rectal temperature (T _re), tail vasomotor response, oxygen uptake , and carbon dioxide production were measured in proestrus and estrus stages during treadmill running at two different speeds at an ambient temperature (T _a) of 24° C. Experiments were performed at 2.00–6.00 a.m., when the difference inT _re was greatest between the two stages;T _re at rest in the estrus stage was 0.54° C higher than in the proestrus stage. In a mild warm environment, thresholdT _re for a rise in tail skin temperature (T _tail) was also higher in the estrus stage than in the proestrus stage. In contrast, no difference was seen in the thresholdT _re and steady stateT _re at the end of exercise between proestrus and estrus stages. These values were higher at the higher work intensity. was also similar between the two stages, except in the second 5 min after the beginning of exercise, when was greater andT _re rose more steeply in the proestrus stage. These data indicate that deep body temperature during exercise is regulated at a certain level depending on the work intensity and is not influenced by the estrus cycle.This study was supported in part by a Grant-in Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan (Grant No. 62480114)     

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.     

Thermoregulatory responses of prepubertal boys and young men in changing temperature linearly from 28 to 15°C

To examine thermoregulatory responses of prepubertal children to cold stress, 11 boys (aged 8 years) and 11 young men (aged 19–23 years), wearing only trunks, participated in this study. They sat in air at 28°C for 30 min (equilibrium period) and then in conditions where air temperature (T _a) was decreased linearly from 28 to 15°C (at a constant rate of 0.22°C · min^–1) for 60 min, at a fixed relative humidity of 65%. In the equilibrium period there was no significant difference between the groups for rectal temperature [T _re, mean 37.30 (SEM 0.10) and mean 37.43 (SEM 0.14)°C in the boys and the men, respectively] or for the respective skin temperatures (except for the forehead), but metabolic heat production ( ) was significantly greater for the boys [mean 57.1 (SEM 1.2) and mean 52.0 (SEM 0.9)W. m^–2,P <0.005]. With decliningT _a, the skin temperatures decreased in both groups (P <0.001), but the decrease was significantly greater for the boys (P < 0.05), especially on the limbs as represented by the thigh and forearm. No significant correlations were observed between the limb skin temperatures compared to surface area-to-mass ratio or limb skinfold thicknesses in either group. The rate of increase in asT _a decreased was significantly lower for the boys (P < 0.01) largely because of a higher before the cold exposure. Thus, the mean during the cold exposure did not differ between the groups [mean 63.6 (SEM 1.1) and mean 61.6 (SEM 1.1) W · m^–2 in boys and men, respectively]. When theT _a was lowered,T _re in the boys started falling (P < 0.001), whereas theT _re in the young men did not change for 60 min. TheT _re during the 60-min exposure was significantly lower (P < 0.001) for the boys [mean 37.01 (SEM 0.13) and mean 37.48 (SEM 0.18)°C at the end of the exposure]. It was concluded that whenT _a was lowered, the prepubertal boys appeared to vasoconstrict more in their limbs and to be somewhat more hypothermic, compared to the young men.     

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     

Influence of the menstrual cycle and oral contraceptives on thermoregulatory responses to exercise in young women

Summary Thermoregulatory responses to exercise in relation to the phase of the menstrual cycle were studied in ten women taking oral contraceptives (P) and in ten women not taking oral contraceptives (NP). Each subject was tested for maximal aerobic capacity ( ) and for 50% exercise in the follicular (F) and luteal (L) phases of the menstrual cycle. Since the oral contraceptives would have prevented ovulation a quasi-follicular phase (q-F) and a quasi-luteal phase (q-L) of the menstrual cycle were assumed for P subjects. Exercise was performed on a cycle ergometer at an ambient temperature of 24° C and relative air humidity of 50%. Rectal (T _re), mean skin ( ), mean body ( ) temperatures and heart rate (f _c) were measured. Sweat rate was estimated by the continuous measurement of relative humidity of air in a ventilated capsule placed on the chest, converted to absolute pressure (PH₂O_chest). Gain for sweating was calculated as a ratio of increase inPH₂O_chest to the appropriate increase inT _re for the whole period of sweating (G) and for unsteady-state (G_u) separately. The did not differ either between the groups of subjects or between the phases of the menstrual cycle. In P, rectal temperature threshold for sweating (T _{re, td}) was 37.85° C in q-L and 37.60° C in q-F (P < 0.01) and corresponded to a significant difference fromT _re at rest. TheT _re, andf _c increased similarly during exercise in q-F and q-L. No menstrual phase-related differences were observed either in the dynamics of sweating or in G. In NP,T _{re, td} was shorter in L than in F (37.70 vs 37.47° C,P<0.02) with a significantly greater value fromT _re at rest. The dynamics and G for sweating were also greater in L than in F. The G_u was 36.8 versus 16.6 kPa · ° C^–1 (P<0.01) while G was 6.4 versus 3.8 kPa · ° C^–1 (P<0.05), respectively. TheT _re, andf _c increased significantly more in phase F than in phase L. It was concluded that in these women performing moderate exercise, there was a greater temperature threshold and larger gains for sweating in phase L than in phase F. Intake of oral contraceptives reduced the differences in the gains for sweating making the thermoregulatory responses to exercise more uniform.     

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.     

Seasonal variation in sweating responses of older and younger men

Eight older (60–65 years) and six younger (20–25 years) men were exposed to a standard heat stress for 60 min in summer, autumn, winter, and spring. The test consisted of placing the lower legs and feet in a 42°C water bath while sitting in constant environmental conditions (30°C and 45% relative humidity). The increase of rectal temperature (T _re) was significantly greater (P < 0.05) in autumn, winter, and spring than in summer for the older group, but significantly greater only in winter than in summer for the younger group (P < 0.05). The T _re was greater for the older group in all seasons, but of significance only in autumn and spring (P < 0.01). There were no significant season-related differences for metabolic heat production (m) and mean skin temperature ( _sk) during the heat test in the respective groups, although the m and _sk were lower for the older group in all seasons (P < 0.01). In the older group total body sweating rate (m_sw) divided by T _re (total m_sw/T _re) decreased from summer to winter (P < 0.02) and did not differ between winter and spring, whereas total m_sw/T _re in the younger group increased in spring after decreasing from autumn to winter (P < 0.03). The variations of the value, local sweating rate on the back and thigh divided by T _re (back m_sw/T _re and thigh m_sw/T _re), were similar to those of the total m_sw/T _re in each group, except for back m_sw/T _re in the younger group, which did not increase from winter to spring. The total m_sw/T _re, back m_sw/T _re and thigh m_sw/T _re were significantly less for the older group in summer, autumn and spring (P < 0.05). The range of seasonal variations was significantly less for the older group (P < 0.001). The results indicated that, compared with younger men in older men, the enhancement of sweating function toward summer occurred later and its reduction toward winter occurred earlier despite a smaller range of seasonal variation and that older men had a somewhat lesser capability to maintainT _re when challenged by heat stress in all seasons.     

Age predicts cardiovascular,but not thermoregulatory,responses to humid heat stress

Cross-section comparisons of the effect of age on physiological responses to heat stress have yielded conflicting results, in part because of the inability to separate chronological age from factors which change in concert with the biological aging process. The present study was designed to examine the relative influence of age on cardiovascular and thermoregulatory responses to low intensity cycle exercise (60 W for 1 h) in a warm humid environment (35°C, 80% relative humidity). Specifically, the relative importance of age compared to other individual characteristics [maximal oxygen uptake ( _max), physical activity level, anthropometry, and adiposity] was determined by multiple regression analysis in a heterogeneous sample of 56 subjects in which age (20–73 years) and _max (1.864–44 l · min^–1) were not interrelated. Dependent variables (with ranges) included final values of thermoregulatory responses [rectal temperature (T _re, 37.8–39.2°C), calculated heat storage (S, 3.4–8.1 J · g^–1), sweat loss (238–847 g · m^–2)] and cardiovascular responses [heart rate (HR, 94–176 beats min^–1), forearm blood flow (FBF, 5.3–31.3 ml · 100 ml^–1 · min^–1), mean arterial blood pressure (MAP, 68–122 mmHg), and forearm vascular conductance (FVC = FBF · MAP^–1, 0.06–0.44 ml · 100 ml^–1 · min^–1 · mmHg^–1). Age had no significant influence onT _re,S, or sweat loss, all of which were closely related to _max. On the other hand, HR, MAP, FBF, and FVC were related to both age and _max. Anthropometric variables and adiposity had secondary, but statistically significant, effects on MAP, FBF, FVC, and sweat loss. With respect to exercise in a warm humid environment, it was concluded that the effect of age on body temperature and sweating was negligible compared to effects related to _max, but that chronological age had an independent effect on cardiovascular effector responses.     

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.     

Does feeding regime affect physiologic and thermal responses during exposure to 8, 20, and 27° C?

Summary When the loss of body heat is accelerated by exposure to low environmental temperatures, additional substrates must be oxidized to provide energy to sustain temperature homeostasis. Therefore, the present investigation examined the relation between feeding regime [pre-experimental carbohydrate feeding (FED) vs a fast (FAST)], during 120 min of exposure to 8, 20, and 27° C in well-nourished men. The following were examined: tissue insulation (I; °C · m² · W^–1), rectal temperature (T _re; °C), and oxygen consumption ( O₂; ml · kg^–1 · min^–1). O₂, T _re, and I revealed no significant differences between treatments (FED vs FAST) at any temperature. At 27° C, I was less (P < 0.05) than at 20 and 8° C, and decreased (P < 0.05) as exposure time increased. At 8° C, O₂was higher (P < 0.5) than at 20 or 27°C, and O₂increased as time increased (P < 0.05). T _re decreased (P < 0.05) as time increased for all conditions. Respiratory exchange ratio (R) differed (P < 0.05) between treatments (FED vs FAST), temperature (8 vs 20° C), and across time. Values for R suggests that carbohydrate accounted for 56% and 33% of caloric utilization during the FED vs FAST conditions, respectively. At 8 vs 20° C, R represented 54% vs 30% of cabohydrate utilization. Across time, R demonstrated that in both conditions (FED vs FAST) there was a decreased reliance on carbohydrate utilization for energy provision. From these data it appears that while substrate utilization differed between dietary treatment and across time this did not differentially affect O₂or T _re during protracted exposure to 8, 20, and 27° C. The higher R in the 8° C condition for both dietary treatments demonstrates that carbohydrate utilization is increased in shivering cold-exposed humans. However, the reduction in R across time suggests that fat oxidation is also involved in metabolic heat production and core temperature maintenance during shivering in the cold.     

Mechanisms of potentiation in sweating induced by long-term physical training

To evaluate the mechanism of potentiation of sweating after long-term physical training, we compared sweating function in trained and untrained subjects using the frequency of sweat expulsion (f _sw) as an indicator of central sudomotor activity. Nine trained male subjects (trained group) and eight untrained male subjects (untrained group) performed 30-min cycle exercise at 35% maximal oxygen uptake at 25°C ambient temperature and 35% relative humidity. Oesophageal temperature (T _oes), mean body temperature _b, chest sweating rate ( _sw,chest), forearm sweating rate ( _forearm), andf _sw were measured. The slopes of the _sw,chest versus body temperature (T _oes and _b) and versusf _sw relationships in the trained group were significantly greater than those in the untrained group (both,P < 0.05), while there was no difference between the groups in the slopes of the _sw,chest versus body temperature or versusf _sw relationships. Neither the body temperature threshold for initiation of chest or forearm sweating nor the slope of thef _sw- _b relationship differed between groups. We concluded that, during light exercise at moderate ambient temperature, the _sw,chest in the subjects who had undergone long-term physical training was greater than that in the untrained subjects while the _sw,forearm was not changed. The greater _sw,chest in the trained subjects was concluded to be due to an increase of sensitivity of peripheral mechanisms.     

Heat storage and body temperature during cooling and rewarming

Summary During calorimetric experiments with forced cooling and rewarming, changes in rectal temperature (T _re) and mean skin temperature ( _sk) allowed calculations of Burton's (1935) weighting coefficient a, which relates body temperature change to change in mean body temperature ( _b). Calculating _b from change in body heat content (H _b), which was determined from direct and indirect calorimetry, included individualized values for body specific heat based on body fat content. In five different cooling procedures there were two with cooling by exposure to cold water and three with cooling in a tubing suit; two of the procedures included mild exercise. The H _b ranged from –335 to –1600 kJ; rewarming restored body heat content. The mean (SEM) value of a in 119 determinations was 0.75 (0.01). This small variability in the coefficient probably came from the large values of H _b and from the use of maximal changes in _sk andT _re, including afterdrop. Change inT _re by itself correlated with _b, but with much variability. In forced body cooling and rewarming, 0.75(T _re) + 0.25 ( _sk) gives an accurate estimate of _b, hence change in body heat storage.     

Forearm temperature profile during the transient phase of thermal stress

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

The relationship between test protocol and the development of exercise-induced hypoxemia (EIH) in highly trained athletes

Healthy male endurance-trained cyclists [n = 11, age = 27.3 (3.9) years; mass = 73.0 (9.3) kg; height = 180.5 (6.9) cm; maximal oxygen consumption ( = 71.1 (5.8) ml · kg^–1 · min^–1, mean ± (SD)] were recruited to assess the relationship between test protocol and the development of desaturation of arterial hemoglobin with oxygen, during incremental exercise tests to maximal aerobic capacity . All subjects demonstrated resting pulmonary function within normal limits [forced vital capacity (FVC) = 6.0 (0.9); forced expiratory volume (FEV_1.0) = 4.9 (0.6); FEV_1.0/FVC = 0.8 (0.1)] and completed three ramped tests (Mijnhardt KEM-3 electronically braked cycle ergometer) beginning at 0 W with increments of either 20, 30 or 40 W · min^–1. All periods of testing were separated by a minimum of 72 h. , peak minute ventilation (Medical Graphics, CPX-D), peak heart rate (ƒ_cpeak)), peak power output , and minimum percentage arterial oxyhemoglobin saturation (%S _aO_2min) (Omeda Biox 3740 pulse oximeter) were determined. There were no significant differences (p > 0.05) in [191.5 (26.2), 196.0 (24.4), 194.3 (23.9) 1 · min^–1] ƒ_cpeak [191.4 (7.0), 190.3 (5.5), 187.8 (5.9) beats · min^–1], [5.0 (0.5), 5.1 (0.4), 5.1 (0.5) 1 · min^–1] or %S _aO_2min [89.5 (1.5), 89.6 (1.3), 90.0 (2.3)] between protocols. The 20-W protocol [417 (27) W] demonstrated significantly lower (P < 0.05) than the 30-W [434 (36) W] and 40-W [453 (38) W] protocols, indicating that peripheral fatigue may play an important factor in response to these tests. The results of this study demonstrate that arterial desaturation occurs as a result of intense exercise in highly trained athletes independent of the rate of attainment of .