Reduced sweating threshold during exercise-induced hyperthermia

Two methods have been proposed previously for evaluating thermoregulatory responses to isolated core temperature perturbations. One involves clamping skin temperature at 28°C by water immersion and elevating core temperature by exercise. In the other, core temperature is reduced by central venous infusion of cold fluid while skin temperature is kept constant near 36°C. The sweating-to-shivering temperature range determined using the first protocol is 0.6±0.2°C, but is 1.4±0.6°C using the second. The authors tested the hypothesis that the disparity results from an exercise-induced reduction in the sweating threshold. The sweating threshold was determined three times each in six men, with skin temperature kept constant (36.5–37.0°C) throughout. The first threshold (control) was in response to gradual core hyperthermia that resulted from a skin temperature sufficiently high to prevent dissipation of metabolic heat. The second threshold (exercise) was in response to exercise-induced hyperthermia, and followed a period of core cooling ( 1°C) induced by central venous infusion of cold lactated Ringer's solution. The purpose of the central venous cooling was to reduce core temperature to a level well below the sweating threshold. The third threshold (recovery) again resulted from a skin temperature sufficiently high to prevent dissipation of metabolic heat. The control sweating threshold was 0.7±0.2°C greater than during exercise (P<0.01), but virtually identical to that during recovery. Since the control and recovery thresholds were similar, hyperhydration (necessary in this protocol) appears not to have influenced sweating thresholds. Our results thus indicate that exercise per se reduces the sweating threshold sufficiently to explain reported differences in the sweating-to-shivering range.     

Thermoregulatory behavior of man during rest and exercise

Summary The thermoregulatory behavior of two highly trained racing cyclists and of five untrained male subjects was investigated. The subjects were totally immersed in a water bath. They could regulate the water temperature according to their sensation of thermal comfort. At rest, in the state of thermal comfort, no thermoregulatory events—as sweating—could be observed. During exercise, esophageal temperature rises and consequently the subjects lower the water temperature. The resulting skin and deep-body temperatures caused an increase in sweat rate and heat conductance. Thus, during exercise, thermoregulatory responses increase as a function of oxygen uptake although the subjects are at thermal comfort. The different function of the two systems regulating sweat rate and thermoregulatory behavior respectively may arise from different weighting factors of skin and deep-body temperature as input variables to both systems. In the correlation equation the ratioa ₁:a ₂ is about 12–15 in the case of sweat rate, and about 4 in the case of behavior     

Core temperature thresholds for hyperpnea during passive hyperthermia in humans

Humans have higher ventilation when they are hyperthermic but it is not known whether core temperature thresholds for ventilation exist, nor has a physiological rationale been presented for this response. To examine this question, ventilation was studied in relation to core temperatures in humans rendered hyperthermic in a warm bath. Seven subjects [mean (SE), 23.3 (1.4) years] wearing only shorts and a thick felt hat with ear flaps were immersed to the neck in a bath at 41 (0.5)°C for 25 min. Tympanic (T _ty), esophageal (T _es), thigh skin and forehead skin temperatures, heart rate, inspired minute ventilation (V _I at body temperature and pressure, saturated), ventilation frequency and oxygen consumption (VO₂ at standard temperature and pressure, dry) were recorded at 30-s intervals. At immersion V _I briefly increased to 18.6 (3.0)l·min^–1 returned to about the pre-immersion value,, and significantly increased to 19.3 (3.0) l·min^–1 by the end of immersion. VO₂ increased significantly from the pre-immersion value of 0.27 l·min^–1 to 0.67 l·min^–1 by the first 0.5 min of immersion, but then returned to its pre-immersion value. T _ty increased to 38.7 (0.2)°C and T _es increased to 39.0 (0.2)°C by the end of immersion. Core temperature thresholds for increases in V _I were evident at 38.1°C when expressed against T _ty and at 38.5°C when expressed against T _es. The results indicated that during body warming core temperature thresholds for V _I are reached and subsequently a hyperpnea was evident, despite VO₂ remaining at a resting value. This hyperpnea is seen as a thermoregulatory response likely to participate in selective brain cooling.     

Peripheral thermal responsivity to facial cooling during sleep

A recently developed technique for examining thermal sensitivity during sleep was used to assess whether skin and core temperature responses to thermal stimulation were altered by sleep state. The technique was designed to probe thermal responsivity without altering core body temperature or inducing awakening. Twenty-seven young men and women were studied during a sleep deprivation night and a sleep night three nights later. Cold water stimulation of the face alternated with an equal period of rewarming across a 40-min cycle throughout the night. Skin temperature from the finger and rectal temperature were continuously assessed. Sleep continuity and architecture were largely uninfluenced by the thermal stimulation. Finger skin temperature decreased during cold facial stimulation in both sleep and waking states. Skin temperature changes during sleep were approximately one-fifth the magnitude of those during waking. Core temperature was minimally influenced. REM sleep was associated with a greater amplitude decrease in finger temperature than was non-REM (NREM) sleep. The results support the utility of the technique as a probe of thermal responsivity during sleep and suggest a reduction of thermal responsivity during sleep and, more tentatively, an altered responsivity during REM versus NREM sleep.     

Effect of exercise on thermal comfort during hypothermia

Subjective measures of thermal comfort were made in 12 male subjects who exercised intermittently during reduction of core temperature. Cold discomfort was found to be inversely proportional to work rate in the range studied of up to 65% aerobic capacity. This relationship was independent of the effect of the exercise on body temperatures. It occurred while core temperature fell and mean skin temperature either fell, remained steady at low levels or rose again towards normal. Possible mechanisms are reviewed and it is concluded that this effect of voluntary exercise, like that of shivering, is psychological.     

Effects of exercise intensity and environmental stress on indices of oxidative stress and iron homeostasis during exercise in the horse

The effects of prolonged variable-intensity and short-term high-intensity exercise on indices of oxidative stress and iron homeostasis were compared in six fit horses under cool [20°C, 40% relative humidity (RH)] or hot/humid (30°C, 80% RH) environmental conditions. The exercise protocols were designed to simulate equine competition, including racing (intense exercise) or the speed and endurance phase of a 3-day event (prolonged exercise). Increased plasma concentrations of lipid hydroperoxides and haemolysate concentrations of oxidised glutathione (GSSG) were measured within 30 min of the completion of exercise, indicating production of reactive oxygen species (ROS) and lipid membrane peroxidation. The horses were unable to complete the prolonged exercise protocol at high temperature and humidity. This coincided with higher maximal values of lipid hydroperoxides [138.2 (17.7) M and GSSG [110.6 (18.2) M], compared to high-intensity [105.2 (14.9) M and 63.6 (8.6) M, respectively] or prolonged [100.7 (18.7) M and 86.2 (9.1) M, respectively] exercise performed under cooler environmental conditions. Significant correlations were found between the duration of the final stage of exercise during hot/humid environmental conditions and increased levels of lipid hydroperoxides (r = 0.85), GSSG (r = 0.94), xanthine (r = 0.92) and uric acid (r = 0.96). Excerise also decreased the iron (Fe)-binding antioxidant activity of the plasma and increased the total plasma Fe levels, although this was only significant for prolonged exercise in ambient conditions. There was no detectable free Fe in the plasma at any stage of exercise. Other changes in biochemical parameters had returned to pre-exercise levels within 24 h after exercise. The results show that exercise can induce changes in biochemical parameters that are indicative of oxidative stress in the fit horse and that this was, exacerbated during exercise at high temperature and humidity.     

Subjective time runs faster under the influence of bright rather than dim light conditions during the forenoon

The study investigated if 6 h morning bright light exposure, compared with dim light exposure, could influence time sense (range: 5-15 s). Eight women served as participants. The participant entered a bioclimatic chamber at 10:00 h on the day before the test day, where an ambient temperature and relative humidity were controlled at 25 degrees C and 60%RH. She sat quietly in a sofa in 50 lx until 22:00 h, retired at 22:00 h and then slept in total darkness. She rose at 07:00 h the following morning and again sat quietly in a sofa till 13:00 h, either in bright (2500 lx) or dim light (50 lx), the order of light intensities between the two occasions being randomized. The time-estimation test was performed from 13:00 to 13:10 h in 200 lx. The participant estimated the time that had elapsed between two buzzers, ranging over 5-15 s, and inputting the estimate into a computer. The test was carried out separately upon each individual. Results showed that the participants estimated higher durations of the given time intervals after previous exposure to 6 h of bright rather than dim light. The finding is discussed in terms of different load errors (difference between the actual core temperature and its thermoregulatory set-point) following 6-h exposure to bright or dim light in the morning.     

Thermoregulatory effects of three different types of head cooling in humans during a mild hyperthermia

Seven healthy young men participated in six trials with three different types of local cooling [cool air breathing (CAB), face skin cooling (FaC), and combined cooling (CoC)] in a warm environment for 90 min while either resting (operative temperature: T ₀ = 40°C, dew point temperature: T _dp = 15°C, air velocity: v _a = 0.3 m·s⁻¹) or exercising on a cycle ergometer with an external work load of 90 W (T ₀ = 36°C, T _dp = 15°C, v _a = 0.3 m·s⁻¹). Cool air (10°C) arrived at the entry point of the hood and/or the mask at a ventilation rate of 12 m · s⁻¹. Oesophageal temperature was not affected by any kind of cooling, while tympanic temperature was decreased at rest by both FaC and CoC [respectively −0.15 (0.06) and −0.09 (0.03)°C, P ≤ 0.05]. Mean skin temperature was decreased by FaC and CoC at rest [respectively −0.31 (0.07) and −0.27 (0.09)°C, P ≤ 0.05] and during exercise [respectively −0.64 (0.15) and −1.04 (0.22)°C, P ≤ 0.01]. CAB had no effect on skin temperatures. CoC and FaC reduced head skin temperature during both rest and work (P < 0.001) with no effect on the skin temperature of the rest of the body, except under CoC with exercise (P < 0.05). CAB did not influence local sweating. FaC, however, decreased the more profuse sweat rates (P ≤ 0.05) at rest, while CoC decreased all sweating rates at rest (P ≤ 0.05) and only the back, head and leg sweating rates during exercise (P ≤ 0.05). These results suggest that head skin cooling causes a reduction in heat strain, while CAB does not. This beneficial influence does not, however, appear to be the result of selective brain cooling. Tympanic temperature seems to be a good index of the core thermal inputs to the hypothalamic regulatory system, since variations in that parameter were associated with similarly directed variations in the sweating outputs. Accepted: 12 April 1999     

The independent roles of temperature and thermal perception in the control of human thermoregulatory behavior

The present study independently evaluated temperature and thermal perception as controllers of thermoregulatory behavior in humans. This was accomplished using a self-paced exercise and heat stress model in which twelve physically active male subjects exercised at a constant subjective rating of perceived exertion (16, ‘hard - very hard’) while their face was thermally and non-thermally cooled, heated, or left alone (control trial). Thermal cooling and heating were achieved via forced convection, while non-thermal cooling and heating were accomplished via the topical application of menthol and capsaicin solutions. Evidence for thermoregulatory behavior was defined in terms of self-selected exercise intensity, and thus exercise work output. The results indicate that, in the absence of changes in temperature, non-thermal cooling and warming elicited thermal sensory and discomfort sensations similar to those observed during thermal cooling and warming. Furthermore, the perception of effort was maintained throughout exercise in all trials, while the initial and final exercise intensities were also similar. Thermal and non-thermal cooling resulted in the highest work output, while thermal warming the lowest. Non-thermal warming and control trials were similar. Heart rate, mean skin and core (rectal) temperatures, and whole body and local (neck) sweat rates were similar between all trials. These data indicate that changes in temperature are not a requirement for the initiation of thermoregulatory behavior in humans. Rather, thermal sensation and thermal discomfort are capable behavioral controllers.     

Facilitation of cortically evoked potentials with motor imagery during post-exercise depression of corticospinal excitability

This study examined whether muscle fatigue alters the facilitatory effect of motor imagery on corticospinal excitability. We aimed to determine if post-exercise depression of potentials evoked magnetically from the motor cortex is associated with alterations in internally generated movement plans. In experiment 1, motor-evoked potentials (MEPs) were recorded from two right hand and two right forearm muscles, at rest and during motor imagery of a maximal handgrip contraction, in eight neurologically normal subjects, before and after a 2-min maximal voluntary handgrip contraction. Resting MEP amplitude was facilitated by motor imagery in three of the four muscles, but consistently only in two. Motor imagery also reduced the trial-to-trial variability of resting MEPs. Following the exercise, resting MEP amplitude was depressed reliably in only one muscle engaged in the task, although two other muscles exhibited some depression. Motor imagery MEPs were smaller after exercise, but the degree of facilitation compared to the rest MEP was unchanged. In experiment 2, TMS intensity was increased after exercise-induced MEP depression so that the MEP amplitude matched the pre-exercise baseline. The amplitude of the MEP facilitated with motor imagery was not altered by MEP depression, nor was it increased when the TMS intensity was increased. These results suggest, at least with a simple motor task, that while post-exercise depression reduces corticospinal excitability, it does not appear to significantly affect the strength of the input to the motor cortex from those areas of the brain responsible for the storage and generation of internal representations of movement.     

Time sense for short intervals during the follicular and luteal phases of the menstrual cycle in humans

The present study investigated if the time sense (using short intervals) was influenced by the phase of the menstrual cycle. Fourteen women (age: 21.8+/-1.3 years, height: 158.1+/-4.6 cm, body weight: 50.9+/-3.2 kg, mean+/-S.D.) were examined. Their menstrual phase was checked by the participants themselves prior to the study, by measuring oral temperatures after waking but while still in bed over the course of 1-2 months. Participants entered a climatic chamber (26 degrees C, 60% RH, 200 lx) at 09:00 h during either the follicular or luteal phase, and sat quietly on a sofa for 4 h before performing the time sense tests. These used intervals ranging from 1 to 60 s, and consisted of time estimation and time production. In the time estimation test, the participant was required to estimate the interval between two signals provided by buzzers; 10 random intervals were given. For the time production test, the participant was required to produce the interval displayed on a screen by pushing the space key of a computer twice; 10 random intervals were given. The order of testing during the follicular and luteal phases, and of performing the time estimation and time production tests, was counterbalanced. The results indicate that the time sense ran significantly faster during the luteal phases. The findings are discussed in terms of a higher load error between the actual core temperature and its thermoregulatory set-point during the luteal phase of the menstrual cycle.     

Changes in concentrations of tissue free radical marker and serum creatine kinase during the post-exercise period in rats

Changes in the concentrations of thiobarbituric acid-reactive substances (TBARS), an index of lipid peroxidation in liver, heart and soleus muscle, were studied in trained (T) and untrained (U) rats throughout a period of 48–72 h following running until exhaustion. Creatine kinase (CK) concentration in serum was also determined. The running time till exhaustion in group T was significantly longer than in group U [174.5 (SEM 9.8) vs 92.7 (SEM 8.3) min,P < 0.01]. In group U TBARS concentration in investigated tissues increased significantly (P < 0.01) after exercise with the peak values observed 3 h after running. The post-exercise increase in the TBARS concentration persisted longer in the soleus muscle (48 h) than in the liver or heart (3 h). A postexercise increase of TBARS was observed in group T only in the liver. The influence of training on the TBARS content depended on the kind of tissue. The TBARS concentrations in the liver at rest and immediately after the exercise were lower in group U than in group T. In contrast, TBARS concentrations in the heart and soleus muscle were higher in group U than in group T. The exercise resulted, in both groups, in a rise of serum CK concentration, peak values being observed 3 h following the exercise. Postexercise concentrations of CK were considerably lower in group T than in group U [3 h postexercise: 1740 (SEM 170) vs 2750 (SEM 231) U · 1⁻¹ P < 0.01]. A positive correlation (r = 0.66,P < 0.05) between TBARS content in muscle and serum CK concentration was found only in group U. The results obtained indicated that the generation of lipid peroxidation products in the soleus muscle was intensified for a relatively long time after the exercise. Endurance training decreased the susceptibility of tissues to the action of free radicals. However, this influence of training was more pronounced in the heart and soleus muscle than in the liver.     

Effect of a personal ambient ventilation system on physiological strain during heat stress wearing a ballistic vest

The present study was conducted in order to evaluate whether physiological strain is alleviated by a new personal cooling system (CS) consisting of a layered vest and integrated blower that generate a flow of air. Twelve male volunteers were exposed to climatic conditions of 40 degrees C, 40%RH (40/40), and 35 degrees C, 60%RH (35/60) during a 115 min exercise routine, followed by 70 min resting recovery, while wearing a battle dress uniform (BDU) and a ballistic vest, with (COOL) or without (NOCOOL) CS. The CS was able to attenuate the physiological strain levels during exercise, when compared to identical exposures without the CS. Temperature elevation, (DeltaT (re)) after 100 min of exercise, was lower by 0.26 +/- 0.20 and 0.34 +/- 0.21 degrees C in 40/40 and 35/60, respectively, (p < 0.05). Mean skin temperature [Formula: see text] was lower by 0.9 +/- 0.4 and 0.6 +/- 0.5 degrees C for 40/40 and 35/60, respectively. Heart rate (HR) was not significantly different for COOL versus NOCOOL for 40/40. At 35/60, HR was lower by 10 beats per min (bpm) (p < 0.05). Physiological strain index (PSI) was 9 and 21% lower for the 40/40 and 35/60, respectively, for COOL versus NOCOOL (p < 0.05). Heat storage (S) rates were 19 and 24% lower and sweat rates were 21 and 25% lower for the 40/40 and 35/60, respectively, for COOL versus NOCOOL (p < 0.05). However, the CS was not effective in alleviating physiological strain during resting recovery with no difference in T (re) cooling rate, S, or HR drop rate between groups over resting recovery periods. The CS tested in this study was found to be an effective tool for lowering physiological strain while exercising but not during resting recovery. Therefore, the CS should be further developed in order to achieve greater attenuation of the thermal strain during exercise and improve effectiveness during rest. Overall, it has the potential to be useful for both military and sports personnel.     

Local thermal unpleasantness and discomfort prediction in the vicinity of thermoneutrality

This work emphasizes a better understanding of the origin of human thermal discomfort under heterogeneous but steady environments, in subjects in the vicinity of physiological and sensory thermoneutrality. The knowledge of skin temperatures allows a psychophysiological study aiming at linking the body thermal state (local and global) to thermal sensation (perceptive and affective judgements). By using two driving simulators, 345 subjects were exposed to different thermal environments, modulated by factors such as the air distribution in the automotive cockpit or the clothing insulation (winter or summer). This work shows that consideration of the local thermal state is essential for the evaluation of thermal comfort in the case of non-uniform environments. Our experimental conditions point out that the overall sensation of discomfort is quantitative, with local unpleasantness needing to be felt for a certain number of body surfaces. A local origin is suggested for cold discomfort, in opposition to the global characteristics of warm discomfort.     

Cardiovascular changes during whole body hyperthermia treatment of advanced malignancy

Summary Cardiovascular studies were carried out on patients subjected to whole body hyperthermia treatment for advanced malignancy in order to assess the magnitude of the changes occurring and the degree of strain imposed on the system. The subjects, who were anaesthetised with a nitrous oxide/oxygen and relaxant sequence, were heated in a modified Siemens hyperthermia cabin and maintained at a body temperature of 41.8 C for 2 h. The results of 30 treatments are presented. Large increases in cardiac output and heart rate were accompanied by large decreases in peripheral resistance in both the systemic and pulmonary vascular beds. The pulmonary arterial pressure rose whereas that in the systemic circulation fell. This caused right ventricular work to increase proportionately more than left ventricular work. Care should be exercised when subjecting patients with limited right ventricular function to this treatment.This work was supported by K. W. F. Grant: EUR 77-4     

Modulation, probably presynaptic in origin, of monosynaptic Ia excitation during human gait

Modulation of presynaptic inhibition of Ia afferents projecting monosynaptically to soleus motoneurones was investigated during human gait. Changes in presynaptic inhibition of Ia afferents were deduced from alterations in the amount of heteronymous soleus H-reflex facilitation evoked by a constant femoral nerve stimulation. It has been shown that this facilitation is mediated through a monosynaptic Ia pathway and that during its first 0.5 ms it is still uncontaminated by any polysynaptic effect and can be used to assess ongoing presynaptic inhibition of Ia terminals to soleus motoneurones. During gait, heteronymous facilitation was reduced with respect to its control value (rest during sitting) and modulated during the step cycle: it reached its maximum at mid-stance and decreased to near zero by the end of stance. At the same time the H-reflex amplitude was to some extent similarly modulated. It is argued that this decrease in heteronymous Ia facilitation and in H-reflex amplitude reflects an increased, ongoing presynaptic inhibition of Ia terminals projecting onto soleus motoneurones, which could be from central and/or peripheral origin. D1 inhibition, i.e. the late and long-lasting inhibition of the soleus H-reflex evoked by a train of stimuli to the common peroneal nerve, was used as another method to assess presynaptic inhibition. This D1 inhibition was decreased during gait, and it is argued that this decrease might reflect an occlusion in presynaptic pathways or increased presynaptic inhibition of pathways mediating the conditioning volley.     

The effects of activity and thermal environment on creatinine clearance

Summary Rates of creatinine clearance (C_Cr) were measured in 3 healthy young men while standing at rest, walking at 5.6 km/hr, or jogging at 9.6 km/hr for 1 hr in environments set at water vapor tensions equal to about 10 mm Hg and dry bulb temperatures of 10, 25, or 40 ° C. The purpose was to study the differential effects of activity and thermal environment on glomerular filtration rate (GFR). C_Cr was always greater for walking than for standing in each environmental situation considered. For jogging, C_Cr decreased below standing values in the hot and temperate environments, but in the cold C_Cr for jogging did not differ from that for standing. Furthermore, during the jogging experiments C_Cr decreased in a linear fashion for each subject as heat stress (sweat rate) increased. We conclude that in comparison with rest the effects of mild and moderate exercise on GFR can be directionally different. With increasing heat stress, these differences in GFR can be accentuated.