Thermal and metabolic responses to heat exposure in obesity

Summary Thirteen obese and 13 lean women were each exposed to a hot-humid condition (T_db=47 C, T_wb=39 C) for 1 h. Measurements included rates of heart beat, respiration, sweat evaporation, and resting metabolism, and the rectal and mean skin temperatures. Resting metabolic rate was calculated from oxygen consumption and carbon dioxide production. The obese subjects were more tolerant of heat than were lean subjects, as indicated by the smaller thermal and metabolic responses of the obese.     

Effects of brief leg cooling after moderate exercise on cardiorespiratory responses to subsequent exercise in the heat

We investigated the effects of brief leg cooling after moderate exercise on the cardiorespiratory responses to subsequent exercise in the heat. Following 40 min of ergometer cycling [65% peak oxygen uptake (O_2peak)] at 35°C (Ex. 1), seven male subjects [21.9 (1.1) years of age; 170.9 (1.9) cm height; 66.0 (2.0) kg body mass; 46.7 (2.0) ml kg^–1 min^–1 O_2peak] immersed their legs in 35°C (control condition, CONT) or 20°C (cooling condition, COOL) water for 5 min and then repeated the cycling (as before, but for 10 min) (Ex. 2). Just before Ex. 2, esophageal temperature (T _es) was lower in COOL than in CONT [36.9 (0.2) vs 37.5 (0.1)°C] (P<0.01), as also were both mean skin temperature [33.9 (0.2) vs 35.2 (0.2)°C] (P<0.01), and heart rate (HR) [93.2 (6.0) vs 102.7 (4.9) beats min^–1] (P<0.05). During Ex. 2, no differences between CONT and COOL were observed in oxygen uptake, arterial blood pressure, blood lactate concentration, or ratings of perceived exertion; however, T _es, skin temperature, and HR were lower in COOL than in CONT. Further, during the first 5 min of Ex. 2, minute ventilation was significantly lower in COOL than in CONT [50.3 (2.0) vs 53.4 (2.6) l min^–1] (P<0.01). These results suggest that brief leg cooling during the recovery period may be effective at reducing thermal and cardiorespiratory strain during subsequent exercise in the heat.     

Circadian variations in psychophysiological responses to heat exposure and exercise

Summary Ten healthy men were tested at 0600, 1200, 1800 and 2400 hours on different days at rest in a laboratory at room temperature followed by 1 h of heat exposure in a climatic chamber at 42° C, 60% rh (50 min rest and 10 min exercise on a cycle ergometer at 50% ). Heart rate, blood pressure, rectal temperatureT _re, metabolic rate, number connection test, visual and auditory reaction time, flicker test and catecholamine excretion were measured. Heat exposure and exercise caused lower heart rate acceleration at 2400 hours than at 0600 and 1200 hours, the smallest increase ofT _re at 1800 hours, and an increase in metabolic rate greater at 1200 than at 1800 hours. In the afternoon, when, according to the circadian rhythm, the body temperature is highest, the additional heat load produced the smallest physiological effects. Performance efficiency, after heat exposure combined with physical exercise, improved slightly, but diurnal variations did not show significant circadian rhythm. The work was supported from Poland's Central Programme of Basic Research 06-02.III.2.2     

Physiological responses to fire fighting activities

Summary Eight professional fire fighters participated in six fire fighting scenarios at a training facility. Data on heart rate (HR), rectal temperature (T_re), and skin temperatures at the chest and thigh were collected using a portable data acquisition system. Average HR ranged from 122 to 151 beats · min⁻¹ during the six scenarios. Detailed analyses indicated that HR and T_re increases are related to both the physical and environmental stresses of the various activities carried out. The most demanding activity, that of building search and victim rescue, resulted in an average HR of 153 beats · min⁻¹ and T_re rise of 1.3‡ C, while the least demanding activity, that of the crew captain who directs the fire fighting, resulted in an average HR of only 122 beats · min⁻¹ and a T_re rise of only 0.3‡ C. This study shows that fire fighting is strenuous work for those directly entering a building and performing related duties, but that the physical demands of other activities are considerably less. The results further suggest that heat strain injuries in fire fighters could perhaps be reduced by rotating duties frequently with other crew members performing less stressful work.     

Effects of caffeine ingestion on endurance racing in heat and humidity

A hot and humid environment can be detrimental to race performance. Caffeine, on the other hand, has been shown to be an ergogenic aid for improving endurance performance. To examine the influence of caffeine ingestion on race performance during high heat stress, seven endurance trained competitive road racers aged between 23 and 51 years (five men, two women) performed three maximal effort 21-km road races outdoors in hot and humid conditions. The caffeine dose, randomly assigned in a double-blind fashion, consisted of either 0, 5, or 9 mg · kg⁻¹ body mass. During each run, the subjects were allowed to drink waterad libitum at each 5-km point. Blood samples were obtained immediately before and after each run and analysed for changes in concentrations of Na⁺, K⁺, glucose, lactate, and hematocrit. Pre and postrun data were also collected for body mass and tympanic membrane temperature. Race times were not significantly different among the races or caffeine doses, with the average times within 1.1% of each other. In addition, none of the other variables measured varied significantly among the races or caffeine doses. In summary, caffeine intake did not affect race performance. Therefore it was concluded from our study that caffeine is not of ergogenic benefit in endurance races during high heat stress.     

Physiological adaptations to thermal stress in tropical asians

Summary Young sedentary adult males of Malay, Indian, and Chinese origin who had established continuous residence in tropical Malaysia and presumed to be naturally acclimatized to heat, were studied to evaluate their physiological responses to a standard heat stress test. The Malay and Indian races have evolved in hot and humid geographical zones, whereas the Chinese originated from a temperate area. Subjects exercised at 50% alternating 18 minutes walking and 2 min rest during a 2-h exposure to an ambient of 34.9°C dry bulb and 32.1°C wet bulb. Heart rates, core and skin temperatures, sweat rates, and oxygen uptakes were measured during the heat exposure. The subjects of Malay origin exhibited the least circulatory stress of the three ethnic groups. The data obtained on these long-term residents of a hot-wet climate and who were considered acclimatized to this environment were compared to experimental data obtained by other investigators and other ethnic groups.     

The effect of hypohydration severity on the physiological,psychological and renal hormonal responses to hypoxic exercise

Evidence of the effect of dehydration on physiological responses to hypoxia is limited. The purpose of this study was to determine the effect of hypohydration severity on physiological, renal hormonal and psychological responses to acute hypoxia. Eight males completed intermittent walking tests under normobaric hypoxic conditions (FI O₂ = 0.13) after completing four separate hypohydration protocols, causing change in body mass of approximately 0% (EU), −1% (H1), −2% (H2) and −3% (H3). Physiological and psychological markers were monitored throughout the 125 min test. Fluid controlling hormones were measured pre and post exposure. Heart rate, core temperature, peripheral arterial oxygen saturation (SpO₂), minute ventilation and urine osmolality were found to be significantly different between hydration conditions and correlated with Lake Louise Questionnaire score (LLQ) (P < 0.05). LLQ score increased with hypohydration severity above H2 (EU 1.3 ± 1; H1 1.2 ± 1; H2 2.7 ± 2; H3 3.9 ± 2) (P < 0.001). Antidiuretic hormone and aldosterone increased over the test, but were not different between hydration conditions (P < 0.05). Atrial natriuretic peptide showed no change over time, or with conditions. Therefore, renal hormones are not influenced by hypohydration severity during moderate intensity hypoxic exercise. Hypohydration less than −2% induces greater physiological strain during hypoxic exercise and may cause rise in symptoms such as, fatigue, headache, nausea and lightheadedness.     

Solar heat load: heat balance during exercise in clothed subjects

Summary Six subjects exercised for 60 min on a cycle ergometer. Their backs were exposed to an artificial sun with a spectral distribution similar to sunlight and an intensity of 724 W m^–2. Each subject took part in four experiments in random order: wearing suits of polyester (insulation value = 0.5 clo), white (WP) or black (BP), or cotton (0.6 clo), white (WC) or black (BC). Measured by partitional calorimetry, the calculated heat losses and gains for the four conditions balanced within less than 10%. The differences between the short-wave radiation gains of subjects in white or black garments were small. This is due to the transparency of the white materials, which allows a larger percentage of the radiation to penetrate the clothing. The surface temperatures of the sun-exposed areas were very high, especially in the black suits. This promotes dry heat loss. Therefore the sweat loss in the black suits and the differences between the black and white clothes became relatively small. The physiological strain in steady-state exercise, as expressed by average heart rates, was 142 (WP), 154 (BP), 151 (WC), and 160 (BC) beats min^–1; the sweat losses were 649 (WP), 666 (BP), 704 (WC), and 808 (BC) g. For both of these measures values for white polyester were significantly less than those for black cotton.     

Combined effect of heat stress, dehydration and exercise on neuromuscular function in humans

This study examined the combined effect of exercise induced hyperthermia and dehydration on neuromuscular function in human subjects. Six trained male runners ran for 40 min on a treadmill at 65% of their maximal aerobic velocity while wearing a tracksuit covered with an impermeable jacket and pants to impair the evaporation of sweat. These stressful experimental running conditions led the runners to a physiological status close to exhaustion. On average, the 40 min run ended at a heart rate of 196 (SD 8) beats · min⁻¹, a tympanic temperature of 40 (SD 0.3) °C and with a loss of body mass of 2 (SD 0.5)%. Pre- and post-running strength tests included measurements of maximal knee extension and flexion torques in both isometric and isokinetic (at 60 and 240° · s⁻¹) conditions. A 20 s endurance test at 240° · s⁻¹ was also performed. Surface electromyographic (EMG) activity was recorded from six knee extensor and flexor muscles during the entire protocol. The treadmill run led to clear decrements in maximal extension torque and EMG activity both in isometric and at the slowest isokinetic velocity (60° · s⁻¹). However, no differences in these parameters were observed at 240° · s⁻¹. Furthermore, the EMG patterns of the major knee extensor and flexor muscles remained remarkably stable during the treadmill run. These results demonstrate that the exercise-induced hyperthermia and dehydration in the present experiments had only minor effects on the neuromuscular performance. However, it is also suggested that high internal body temperature per se could limit the production of high force levels. Accepted: 26 September 2000     

The influence of heat stress on plasma volume and intravascular proteins in sedentary females

Summary 23 untrained female subjects underwent a standardised thermal dehydration. Plasma volume (PV), hematocrit (HCT), total intravascular protein concentration (TPC), albumin concentration (AC), total globulin concentration (TGC) were determined before, immediately, 90 min and 180 min after the heat stress. The intravascular protein masses (IPM) were calculated from PV and protein concentration. In comparison to men the loss of body weight and PV was smaller. Consequently the TPC does not increase to the same amount in men. A tendency for a diminution of IPM could be observed but this shift was not significant, compared to males. According to these findings the colloid osmotic capacity remained on a relatively high level. Opposite to men the IPM of globulins increased in females after thermal dehydration.This study was supported by the Bundesinstitut für Sportwissenschaft and the Deutsche Forschungsgemeinschaft     

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.     

Hormonal and metabolic responses to slow movement resistance exercise with different durations of concentric and eccentric actions

We examined hormonal responses to slow movement exercise involving concentric (CON) and eccentric (ECC) actions. Nine men performed knee extension exercises: (1) low-intensity exercise with slow CON contractions (5-1; 5 s for CON and 1 s for ECC); (2) low-intensity exercise with slow ECC contractions (1-5; 1 s for CON and 5 s for ECC); (3) low-intensity exercise with slow CON and ECC contractions (3-3; 3 s for each contraction); and (4) high-intensity exercise at normal velocity (1-1; 1 s for each contraction). Lactate concentration was significantly higher after the 5-1 than after the 1-5 (P < 0.05). Slow movement exercises significantly raised the concentrations of plasma epinephrine, serum growth hormone, and free testosterone (P < 0.05). Serum growth hormone concentration increased to a greater extent after the three slow movement trials compared with the normal movement trial (1-1). However, serum cortisol concentration was significantly higher after the 5-1 than after the 1-5 and 1-1 (P < 0.05). Average  throughout the exercise session (divided by the time to complete exercise session) was significantly higher in the 1-1 (P < 0.05), with no significant difference among the slow movement trials. In conclusion, low-intensity exercises with slow movement acutely increased anabolic hormone concentrations regardless of the time to complete CON and ECC actions. In contrast, low-intensity exercise with slower ECC action stimulated smaller changes in lactate and cortisol compared with low-intensity exercise with slower CON action.     

Thermoregulatory responses of firemen to exercise in the heat

Summary Twelve volunteer (VF) and 12 professional firemen (PF) wearing only brief trunks exercised on an electrically-braked cycle ergometer at three-five exercise intensities. After 45 min of exercise at 75 W, the exercise intensity was elevated in steps of 25 W every 15 min until the subject was exhausted. Air temperature was regulated to equal skin temperature (36°–38°C) and relative humidity was regulated at 52%. The two groups of firemen were comparable in terms of body mass, age and maximum oxygen consumption. Their oxygen consumption, rectal and skin temperatures, sweating and heart rate were measured during the tests. Blood lactate concentration was measured before, during and after the test. The physiological strain was higher in VF as indicated by higher heat storage, heart rate, skin and rectal temperatures. Sweat rate tended to be lower in VF than PF. The results indicated a better adaptation of the professional compared to the volunteer firemen to work in the heat, although the degree of heat acclimatization was considered to be equally minimal in both groups.     

Effects of physical training on heat loss responses of young women to passive heating in relation to menstrual cycle

To examine the effects of physical training on cutaneous vasodilation and sweating responses of young women in the follicular and luteal phase, 11 physically trained (T group) and 13 untrained (U group) women were passively heated by lower-leg immersion into hot water of 42°C (ambient temperature of 30°C and 45%RH) for 60 min in their mid-follicular and mid-luteal phases of the menstrual cycle. Female hormones increased significantly from the mid-follicular to the mid-luteal phase in T and U groups, but the degree of increase was significantly lower in T group. Mean body temperature thresholds for cutaneous vasodilation and sweating responses were significantly lower in T group than in U group, in both the menstrual phases, and the differences between the groups were greatest during the mid-luteal phase. The slope of the relationship between frequency of sweat expulsion (F_sw) and and between local sweating rate and F_sw was significantly greater in T group, although the slope of the relationship between cutaneous blood flow and did not differ between the groups, regardless of body site or menstrual phase. These results suggest that regular physical activity enhanced sweating and cutaneous vasodilation in young women. The enhancement of sweating was due to both central and peripheral mechanisms, and the enhancement of cutaneous vasodilation was possibly due to a central mechanism. Enhancement of heat loss responses via central mechanisms was greater during the mid-luteal phase than in the mid-follicular phase because the elevation of female reproductive hormone levels during the mid-luteal phase was relatively low in T group.     

Comparative physiological responses of normotensive and essentially hypertensive men to exercise in the heat

Summary Six essentially hypertensive men (average resting arterial pressure of 150/97 mm Hg) and eight normotensive controls (average resting arterial pressure of 115/73 mm Hg) were tested during 1 h of dynamic leg exercise in a warm environment. The groups were well matched for age, max, body surface area, weight, and body fat. Environmental conditions were 38 C dry-bulb, 28 C wet-bulb; exercise intensity was approximately 40% max (85–90 W). There were no significant intergroup differences in core or mean skin temperatures, calculated heat exchange variables, heart, or sweat rates. Blood pressure differences between the groups were maintained (P<0.01). The hypertensive group responded with a significantly lower stroke index (P<0.01) and cardiac index (P<0.01), and a decreased slope of the rise in forearm blood flow (P<0.01) due to an higher vascular resistance (P<0.01). The combined heat load (M + R + C) presented was not sufficient to override the hypertensives' higher cutaneous vasoconstrictor tone. However, on a practical basis, the hypertensives were able to tolerate exercise in the heat as well as their normotensive counterparts.     

Assessment of male anthropometric trends and the effects on simulated heat stress responses

Assessing temporal changes in anthropometrics and body composition of US Army soldiers is important because these changes may affect fitness, performance, and safety. This study investigated differences in body dimensions (height, weight, percent body fat (%BF)) of US Army male soldiers by comparing 2004 and 1988 databases. Anthropometric somatotypes were identified and physiological responses of the different somatotypes to simulated heat stress (35 degrees C/50%rh, approximately 550 W work rate, carrying 12 kg load including battle dress uniform and body armor, rest for 30 min and walk for 70 min) using a thermal regulatory model were evaluated. A significant increase in body weight (2.4 kg) was observed between the 2004 and 1988 data (P < 0.05, after Bonferroni correction). However, changes in height and circumference measurements for %BF were insignificant, with the magnitude of the changes not exceeding inter-observer errors. Multivariate analyses demonstrated that anthropometric distributions did not differ between the two databases and identified five primary somatotypes: "tall-fat", "tall-lean", "average", "short-lean", and "short-fat." Within each database, anthropometric values differed among the somatotypes. However, simulated physiological responses to heat stress in each somatotype were similar in the 2004 and 1988 populations. In conclusion, an increase in body weight was the primary change observed in this sample of US Army male soldiers. Temporal changes in somatotypes of soldiers over a 16-year period had minimal impact on simulated physiological response to heat stress using a thermal regulatory model.     

Metabolic and endocrine responses to graded exercise under acute hypoxia

Summary Eight male subjects (24±1 years old) performed graded ergocycle exercises in normoxic (N) and acute hypoxic (H) conditions (14.5% O₂). decreased from 55.5±1.3 to 45.8±1.4 ml · kg⁻¹ · min⁻¹ in H condition. Plasma glucose and free fatty acid concentrations remained unchanged throughout exercise in both conditions. Increase in blood lactate concentration was associated with relative workload in both conditions. At lactate concentrations were similar in the two conditions, plasma insulin, glucagon, and LH concentrations did not significantly change in either. Plasma δ4-androstenedione and testosterone increased in a similar manner in both conditions. Finally plasma norepinephrine concentration reached at was significantly lower in hypoxia. These results suggest that acute moderate hypoxia does not affect metabolic and hormonal responses to short exercise performed at similar relative workloads, i.e. when the reduction of due to hypoxia is taken into consideration. The lower catecholamine response to maximal exercise under acute hypoxia might suggest that the sympathetic response could be related to relative as well as absolute workloads. Supported by grants from FCAC and FRSQ, Québec and NSERC, Canada     

Rehydration with drinks differing in sodium concentration and recovery from moderate exercise-induced hypohydration in man

To investigate how differing moderate sodium chloride concentrations affect rehydration after exercise and subsequent exercise capacity, eight males lost 1.98 +/- 0.1% body mass exercising in the heat, then consumed one of four drinks in a volume equivalent to 150% of mass loss. Drinks were identical except for sodium chloride content (1 +/- 1, 31 +/- 1, 40 +/- 1, 50 +/- 1 mmol/l). After 4 h recovery subjects cycled for 5 min at 70% VO(2peak) then at 95% VO(2peak) until volitional fatigue. Urine output was inversely related to sodium intake: more was produced with the 1 than the 40 and 50 mmol/l drinks (P < 0.01). Time to exhaustion in the exercise capacity test was not different between treatments (P = 0.883). The addition of 40 or 50 mmol/l of sodium chloride to a rehydration beverage reduced subsequent urine output, thereby providing more effective rehydration than a sodium-free drink. This did not, however, result in improved performance 4 h after the end of the rehydration period.     

Pinealectomy in ground squirrels: effect on behavioral and physiological responses to heat stress

Richardson's ground squirrels, 1 year after pinealectomy, showed altered behavioral and physiological responses to heat stress when compared to an intact group. Pinealectomized squirrels bar-pressed more often for a cool temperature reward in a hot environment. When deprived of behavioral control of the hot environment, pinealectomized squirrels increased their oxygen consumption, had a higher body temperature, and displayed signs of greater thermal stress, including death, as compared to intact animals. When the intact group was pinealectomized and the experiments were repeated, the difference in behavioral and physiological responses to heat stress were not as great as with the 1 year pinealectomized group. A speculative explanation of the pineal gland's influence on central and peripheral control of evaporative water loss and peripheral blood circulation are offered. The pineal gland may exert a subtle influence on heat transfer mechanisms and adaptations to thermal stress.     

Physiological effects of dehydration and rehydration with water and acidic or neutral carbohydrate electrolyte solutions

Summary Five healthy young men exercised on an ergocycle for six 25-min periods separated by 5-min rest intervals in a warm dry environment (36°C). After 1 h of exercise without fluid intake, the subjects continued to be dehydrated or were rehydrated either with water (W) or with isosmotic electrolyte carbohydrate solutions, either acidic (AISO) or close to neutrality (NISO). The average amount of the fluid ingested progressively every 10 min (120 ml) at 20°C was calculated so as to compensate for 80% of the whole body water loss due to exercise in the heat. Dehydration associated with hyperosmotic hypovolaemia elicited large increases in heart rate (HR), and in rectal temperature (T _re), while no decrease was found in either whole body or local sweat rates. Rehydration with water significantly reduced the observed disturbances, except for plasma osmolality and Na⁺ concentration which were significantly lower than normal. With both AISO and NISO there was no plasma volume reduction and osmolality increase. Although a plasma volume expansion was induced by NISO ingestion, the cardiac cost was not improved, as reflected by the absence of a decrease in HR. With NISO, sweating was not enhanced andT _re tended to remain higher. It is concluded that efficient rehydration requires the avoidance of plasma volume expansion at the expense of interstitial and intracellular rehydration. During rehydration by oral ingestion of fluid, the pH of the drink may be an important factor; its effect remains unclear, however.