Brain and abdominal temperatures at fatigue in rats exercising in the heat

Intracerebroventricular physostigmine facilitates heat loss mechanisms in running rats

The aim of this study was to evaluate the participation of central cholinergic transmission in the regulation of metabolic rate, core temperature, and heat storage in untrained rats submitted to exercise on a treadmill (20 m/min, 5% inclination) until fatigue. The animals were separated into eight experimental groups, and core temperature or metabolic rate was measured in the rats while they were exercising or while they were at rest after injection of 2 microl of 5 x 10(-3) M physostigmine (Phy) or 0.15 M NaCl solution (Sal) into the lateral cerebral ventricle. Metabolic rate was determined by the indirect calorimetry system, and colonic temperature was recorded as an index of core temperature. In resting animals, Phy induced only a small increase in metabolic rate compared with Sal injection, without having any effect on core temperature. During exercise, the Phy-treated animals showed a lower core heating rate (0.022 +/- 0.003 degrees C/min Phy vs. 0.033 +/- 0.003 degrees C/min Sal; P < 0.02), lower heat storage (285 +/- 37 cal Phy vs. 436 +/- 34 cal Sal; P < 0.02) and lower core temperature at fatigue point than the Sal-treated group (38.5 +/- 0.1 degrees C Phy vs. 39.0 +/- 0.1 degrees C Sal; P < 0.05). However, despite the lower core heating rate, heat storage, and core temperature at fatigue, the Phy-treated rats showed a similar running time compared with the Sal-treated group. We conclude that the activation of the central cholinergic system during exercise increases heat dissipation and attenuates the exercise-induced increase in core temperature without affecting running performance.

Passive hyperthermia reduces voluntary activation and isometric force production

It has been suggested that a critically high body core temperature may impair central neuromuscular activation and cause fatigue. We investigated the effects of passive hyperthermia on maximal isometric force production (MVC) and voluntary activation (VA) to determine the relative roles of skin (T(sk)) and body core temperature ( T(c)) on these factors. Twenty-two males [VO(2max)=64.2 (8.9) ml x kg(-1) min(-1), body fat=8.2 (3.9)%] were seated in a knee-extension myograph, then passively heated from 37.4 to 39.4 degrees C rectal temperature (T(re)) and then cooled back to 37.4(o)C using a liquid conditioning garment. Voluntary strength and VA (interpolated twitch) were examined during an isometric 10-s MVC at 0.5 degrees C intervals during both heating and cooling. Passive heating to a T(c) of 39.4(o)C reduced VA by 11 (11)% and MVC by 13 (18)% (P<0.05), but rapid skin cooling, with a concomitant reduction in cardiovascular strain [percentage heart rate reserve decreased from 64 (11)% to 29 (11)%] and psychophysical strain did not restore either of these measures to baseline. Only when cooling lowered T(c) back to normal did VA and MVC return to baseline (P<0.05). We conclude that an elevated T(c) reduces VA during isometric MVC, and neither T(sk) nor cardiovascular or psychophysical strain modulates this response. Results are given as mean (SD) unless otherwise stated.

Cerebral perturbations provoked by prolonged exercise

This review addresses cerebral metabolic and neurohumoral alterations during prolonged exercise in humans with special focus on associations with fatigue. Global energy turnover in the brain is unaltered by the transition from rest to moderately intense exercise, apparently because exercise-induced activation of some brain regions including cortical motor areas is compensated for by reduced activity in other regions of the brain. However, strenuous exercise is associated with cerebral metabolic and neurohumoral alterations that may relate to central fatigue. Fatigue should be acknowledged as a complex phenomenon influenced by both peripheral and central factors. However, failure to drive the motorneurons adequately as a consequence of neurophysiological alterations seems to play a dominant role under some circumstances. During exercise with hyperthermia excessive accumulation of heat in the brain due to impeded heat removal by the cerebral circulation may elevate the brain temperature to >40 degrees C and impair the ability to sustain maximal motor activation. Also, when prolonged exercise results in hypoglycaemia, perceived exertion increases at the same time as the cerebral glucose uptake becomes low, and centrally mediated fatigue appears to arise as the cerebral energy turnover becomes restricted by the availability of substrates for the brain. Changes in serotonergic activity, inhibitory feed-back from the exercising muscles, elevated ammonia levels, and alterations in regional dopaminergic activity may also contribute to the impaired voluntary activation of the motorneurons after prolonged and strenuous exercise. Furthermore, central fatigue may involve depletion of cerebral glycogen stores, as signified by the observation that following exhaustive exercise the cerebral glucose uptake increases out of proportion to that of oxygen. In summary, prolonged exercise may induce homeostatic disturbances within the central nervous system (CNS) that subsequently attenuates motor activation. Therefore, strenuous exercise is a challenge not only to the cardiorespiratory and locomotive systems but also to the brain.

Response of the respiratory network of mice to hyperthermia

Most mammals modulate respiratory frequency (RF) to dissipate heat (i.e., panting) and avoid heat stroke during hyperthermic conditions. During hyperthermia, the RF of intact mammals increases and then declines or ceases (apnea). It has been proposed that this RF modulation depends on the presence of higher brain structures such as the hypothalamus. However, the direct effects of hyperthermia on the respiratory neural network have not been examined. To address this issue, the respiratory neural network [i.e., ventral respiratory group (VRG)] was isolated in a brain stem preparation taken from the medulla of mice (P0 -P6). Integrated population activity, predominated by inspiratory neurons, was recorded extracellularly from VRG neurons. The bath temperature was then heated from 30 to 40 degrees C, resulting in a biphasic frequency response in VRG activity. Following an initial six- to sevenfold increase and subsequent decline, fictive RF was maintained at a frequency that was higher than baseline frequency; at 40 degrees C, the RF was maintained at about two to four times that at 30 degrees C. The inspiratory burst amplitude and duration were significantly reduced during hyperthermic conditions. An increase in RF and decrease in VRG burst amplitude and duration also occurred when heating from 37 to 40 degrees C. Fictive apnea typically occurred during cooling to the control temperature. Furthermore, changes in hypoglossal motor nucleus activity paralleled those of the VRG, suggesting that temperature modulation of the VRG is likely to have a behaviorally relevant impact on respiration. We conclude that the VRG activity itself is modulated during hyperthermia and the respiratory network is particularly sensitive to temperature changes.

Endurance training and performance in runners: research limitations and unanswered questions

The purpose of this review is to discuss several limitations common to research concerning running and, secondly, to identify selected areas where additional research appears needed. Hopefully, this review will provide guidance for future research in terms of topics, as well as design and methodology. Limitations in the research include: lack of longitudinal studies, inadequate description of training status of individuals, lack of confirmation of state of rest, nourishment and hydration, infrequent use of allometric scaling to express oxygen uptake, relative neglect of anaerobic power and physical structure as determinants of performance, neglect of the central nervous system, and reliance on laboratory data. Further research in a number of areas is needed to enhance our knowledge of running performance. This includes: body mass as a performance determinant, evaluation of methods used to measure economy of running, assessing the link between strength and running performance, and further examination of training methods. While the amount of research on distance running is voluminous, the present state of knowledge is somewhat restricted by the limitations in research design and methodology identified here.

Cerebral changes during exercise in the heat

This review focuses on cerebral changes during combined exercise and heat stress, and their relation to fatigue. Dynamic exercise can elevate the core temperature rapidly and high internal body temperatures seem to be an independent cause of fatigue during exercise in hot environments. Thus, in laboratory settings, trained participants become exhausted when they reach a core temperature of approximately 40 degrees C. The observation that exercise-induced hyperthermia reduces the central activation percentage during maximal isometric muscle contractions supports the idea that central fatigue is involved in the aetiology of hyperthermia-induced fatigue. Thus, hyperthermia does not impair the ability of the muscles to generate force, but sustained force production is lowered as a consequence of a reduced neural drive from the CNS. During ongoing dynamic exercise in hot environments, there is a gradual slowing of the electroencephalogram (EEG) whereas hyperthermia does not affect the electromyogram. The frequency shift of the EEG is highly correlated with the participants' perception of exertion, which furthermore may indicate that alterations in cerebral activity, rather than peripheral fatigue, are associated with the hyperthermia-induced development of fatigue. Cerebral blood flow is reduced by approximately 20% during exercise with hyperthermia due to hyperventilation, which causes a lowering of the arterial CO(2) pressure. However, in spite of the reduced blood flow, cerebral glucose and oxygen uptake does not seem to be impaired. Removal of heat from the brain is also an important function of the cerebral blood flow and the lowered perfusion of the brain during exercise and heat stress appears to reduce heat removal by the venous blood. Heat is consequently stored in the brain. The causal relationship between the circulatory changes, the EEG changes and the hyperthermia-induced central fatigue is at the present not well understood and future studies should focus on this aspect.

Variability in brain and arterial blood temperatures in free-ranging ostriches in their natural habitat

We used implanted miniature data loggers to measure brain (in or near the hypothalamus) and carotid arterial blood temperatures at 5 min intervals in six free-ranging ostriches Struthio camelus in their natural habitat, for a period of up to 14 days. Carotid blood temperature exhibited a large amplitude (3.0-4.6 degrees C) circadian rhythm, and was positively correlated with air temperature. During the day, brain temperature exceeded carotid blood temperature by approx. 0.4 degrees C, but there were episodes when brain temperature was lowered below blood temperature. Selective brain cooling, however, was not present in all ostriches, and was not tightly coupled to the prevailing body temperature. Brain temperature was maintained within narrow daily limits of approx. 2 degrees C, and varied significantly less than blood temperature at short time scales of 5 to 20 min. At night, brain temperature exceeded blood temperature by as much as 3 degrees C. We attribute the elevated brain temperatures to warming of cerebral arterial blood, by reduced heat exchange in the ophthalmic rete or possibly heat gain from cranial structures, before supplying the hypothalamus. Further studies are necessary to elucidate the significance of such variations in brain temperature and the importance of selective brain cooling in free-living birds.

Inadequate heat release from the human brain during prolonged exercise with hyperthermia

Brain temperature appears to be an important factor affecting motor activity, but it is not known to what extent brain temperature increases during prolonged exercise in humans. Cerebral heat exchange was therefore evaluated in seven males during exercise with and without hyperthermia. Middle cerebral artery mean blood velocity (MCA V(mean)) was continuously monitored while global cerebral blood flow (CBF) and cerebral energy turnover were determined at the end of the two exercise trials in three subjects. The arterial to venous temperature difference across the brain (v-aD(temp)) was determined via thermocouples placed in the internal jugular vein and in the aorta. The jugular venous blood temperature was always higher than that of the arterial blood, demonstrating that heat was released via the CBF during the normothermic as well as the hyperthermic exercise condition. However, heat removal via the jugular venous blood was 30 +/- 6 % lower during hyperthermia compared to the control trial. The reduced heat removal from the brain was mainly a result of a 20 +/- 6 % lower CBF (22 +/- 9 % reduction in MCA V(mean)), because the v-aD(temp) was not significantly different in the hyperthermic (0.20 +/- 0.05 degrees C) compared to the control trial (0.22 +/- 0.05 degrees C). During hyperthermia, the impaired heat removal via the blood was combined with a 7 +/- 2 % higher heat production in the brain and heat was consequently stored in the brain at a rate of 0.20 +/- 0.06 J g(-1) min(-1). The present results indicate that the average brain temperature is at least 0.2 degrees C higher than that of the body core during exercise with or without hyperthermia.

Adaptive heterothermy and selective brain cooling in arid-zone mammals

Adaptive heterothermy and selective brain cooling are regarded as important thermal adaptations of large arid-zone mammals. Adaptive heterothermy, a process which reduces evaporation by storing body heat, ought to be enhanced by ambient heat load and by water deficit, but most mammals studied fail to show at least one of those attributes. Selective brain cooling, the reduction of brain temperature below arterial blood temperature, is most evident in artiodactyls, which possess a carotid rete, and traditionally has been considered to protect the brain during hyperthermia. The development of miniature ambulatory data loggers for recording body temperature allows the temperatures of free-living wild mammals to be measured in their natural habitats. All the African ungulates studied so far, in their natural habitats, do not exhibit adaptive heterothermy. They have low-amplitude nychthemeral rhythms of temperature, with mean body temperature over the night exceeding that over the day. Those with carotid retes (black wildebeest, springbok, eland) employ selective brain cooling but zebra, without a rete, do not. None of the rete ungulates, however, seems to employ selective brain cooling to prevent the brain overheating during exertional hyperthermia. Rather, they use it at rest, under moderate heat load, we believe in order to switch body heat loss from evaporative to non-evaporative routes.

Perceived exertion is associated with an altered brain activity during exercise with progressive hyperthermia

The present study tested the hypothesis that perceived exertion during prolonged exercise in hot environments is associated with changes in cerebral electrical activity rather than changes in the electromyogram (EMG) of the exercising muscles. Therefore, electroencephalogram (EEG) in three positions (frontal, central, and occipital cortex), EMG, rating of perceived exertion (RPE), and core temperature were measured in 14 subjects during submaximal exercise in normal (18 degrees C, control) and hot (40 degrees C, hyperthermia) environments. RPE increased from 11 +/- 1 units at 5 min to 20 +/- 0 units at exhaustion (50 +/- 3 min) in the trial with progressive hyperthermia, whereas exercise in the control trial was maintained with a stable core temperature for 1 h without exhausting the subjects. Altered EEG activity was observed in all electrode positions, and stepwise forward-regression analysis identified core temperature and a frequency index of the EEG over the frontal cortex as the best predictors of RPE. In contrast, there were no significant correlations between RPE and any of the measured EMG parameters (median spectral frequency, root mean square, or amplitude), and the EMG parameters were not different in hyperthermia compared with control. Thus hyperthermia does not seem to affect the activation pattern of the muscles. Rather, the linear correlation among core temperature, EEG frequency index, and RPE indicates that alterations in cerebral activity may be associated with the hyperthermia-induced development of fatigue during prolonged exercise in hot environments.

Hyperthermia and central fatigue during prolonged exercise in humans

The present study investigated the effects of hyperthermia on the contributions of central and peripheral factors to the development of neuromuscular fatigue. Fourteen men exercised at 60% maximal oxygen consumption on a cycle ergometer in hot (40 degrees C; hyperthermia) and thermoneutral (18 degrees C; control) environments. In hyperthermia, the core temperature increased throughout the exercise period and reached a peak value of 40.0 +/- 0.1 degrees C (mean +/- SE) at exhaustion after 50 +/- 3 min of exercise. In control, core temperature stabilized at approximately 38.0 +/- 0.1 degrees C, and exercise was maintained for 1 h without exhausting the subjects. Immediately after the cycle trials, subjects performed 2 min of sustained maximal voluntary contraction (MVC) either with the exercised legs (knee extension) or with a "nonexercised" muscle group (handgrip). The degree of voluntary activation during sustained maximal knee extensions was assessed by superimposing electrical stimulation (EL) to nervus femoralis. Voluntary knee extensor force was similar during the first 5 s of contraction in hyperthermia and control. Thereafter, force declined in both trials, but the reduction in maximal voluntary force was more pronounced in the hyperthermic trial, and, from 30 to 120 s, the force was significantly lower in hyperthermia compared with control. Calculation of the voluntary activation percentage (MVC/MVC + EL) revealed that the degree of central activation was significantly lower in hyperthermia (54 +/- 7%) compared with control (82 +/- 6%). In contrast, total force of the knee extensors (MVC + force from EL) was not different in the two trials. Force development during handgrip contraction followed the same pattern of response as was observed for the knee extensors. In conclusion, these data demonstrate that the ability to generate force during a prolonged MVC is attenuated with hyperthermia, and the impaired performance is associated with a reduction in the voluntary activation percentage.

Effect of pre-cooling, with and without thigh cooling, on strain and endurance exercise performance in the heat

Body cooling before exercise (i.e. pre-cooling) reduces physiological strain in humans during endurance exercise in temperate and warm environments, usually improving performance. This study examined the effectiveness of pre-cooling humans by ice-vest and cold (3 degrees C) air, with (LC) and without (LW) leg cooling, in reducing heat strain and improving endurance performance in the heat (35 degrees C, 60% RH). Nine habitually-active males completed three trials, involving pre-cooling (LC and LW) or no pre-cooling (CON: 34 degrees C air) before 35-min cycle exercise: 20 min at approximately 65% VO2peak then a 15-min work-performance trial. At exercise onset, mean core (Tc, from oesophagus and rectum) and skin temperatures, forearm blood flow (FBF), heart rate (HR), and ratings of exertion, body temperature and thermal discomfort were lower in LW and LC than CON (P<0.05). They remained lower at 20 min [e.g. Tc: CON 38.4+/-0.2 (+/-S.E.), LW 37.9+/-0.1, and LC 37.8+/-0.1 degrees C; HR: 177+/-3, 163+/-3 and 167+/-3 b.p.m.), except that FBF was equivalent (P=0.10) between CON (15.5+/-1.6) and LW (13.6+/-1.0 ml.100 ml tissue(-1) x min(-1)). Subsequent power output was higher in LW (2.95+/-0.24) and LC (2.91+/-0.25) than in CON (2.52+/-0.28 W kg(-1), P=0.00, N=8), yet final Tc remained lower. Pre-cooling by ice-vest and cold air effectively reduced physiological and psychophysical strain and improved endurance performance in the heat, irrespective of whether thighs were warmed or cooled.

Alteration in dopamine metabolism in the thermoregulatory center of exercising rats

To examine the role of monoamines and amino acids in thermoregulation, we measured their concentrations in the preoptic area and anterior hypothalamus (PO/AH) in exercising rats, using an in vivo microdialysis technique. Body temperature (Tb) was monitored using a telemetry system. Tb increase by about 1.0 degrees C in the first 15 min of treadmill exercise (10 m/min; for 60 min), and was maintained thereafter at a steady high level possibly due to activation of the heat loss system. The levels of dopamine metabolites (3,4-dihydroxyphenylacetic acid and homovanillic acid) in the PO/AH significantly increased during exercise. However, exercise did not induce an increase in the level of either serotonergic substances (5-hydroxytryptamine and 5-hydroxyindoleacetic acid) or amino acids (aspartate and glutamate). Our data indicate that dopamine breakdown processes in the PO/AH are activated during exercise. Dopamine in the PO/AH may be involved in the heat loss mechanisms for thermoregulation when Tb rises during exercise.

Exercise in the heat is limited by a critical internal temperature

We examined whether fatigue during exertional heat stress occurred at a critical internal temperature independent of the initial temperature at the start of exercise. Microwaves (2.1 GHz; 100 mW/cm(2)) were used to rapidly (3-8 min) heat rats before treadmill exercise to exhaustion. In a repeated-measures design, food-restricted male Sprague-Dawley rats (n = 11) were preheated to three levels (low, medium, and high). In addition, two sham exposures, Sham 1 and Sham 2, were administered at the beginning and end of the study, respectively. At the initiation of exercise, hypothalamic (T(hyp)) and rectal (T(rec)) temperatures ranged from 39.0 degrees C to 42.8 degrees C (T(hyp)) and 42.1 degrees C (T(rec)). The treadmill speed was 17 m/min (8 degrees grade), and the ambient temperature during exercise was 35 degrees C. Each treatment was separated by 3 wk. Run time to exhaustion was significantly reduced after preheating. There was a significant negative correlation between run time and initial T(hyp) and T(rec) (r = 0.73 and 0.74, respectively). The temperatures at exhaustion were not significantly different across treatments, with a range of 41.9-42.2 degrees C (T(hyp)) and 42.2-42.5 degrees C (T(rec)). There were no significant differences in run time in the sham runs administered at the start and end of the investigation. No rats died as a result of exposure to any of the treatments, and body weight the day after each treatment was unaffected. These results support the concept that a critical temperature exists that limits exercise in the heat.

Physiological determinants of endurance exercise performance

Performance in endurance events is typically evaluated by the power or velocity that can be maintained for durations of 30 min. to four hours. The two main by-products of intense and prolonged oxidative metabolism that can limit performance are the accumulation of hydrogen ion (i.e. lactic acidosis) and heat (i.e. hyperthermia). A model for endurance performance is presented that revolves around identification of the lactate threshold velocity which is presented as a function of numerous morphological components as well as gross mechanical efficiency. When cycling at 80 RPM, gross mechanical efficiency is positively related to Type I muscle fiber composition, which has great potential to improve endurance performance. Endurance performance can also be influenced by altering the availability of oxygen and blood glucose during exercise. The latter need forms the basis for ingesting carbohydrate at 30-60 grams per hour during exercise. In laboratory simulations of performance, athletes fatigue due to hyperthermia when esophageal is approximately 40 degrees C, in association with near maximal heart rate and perceived exertion. It is likely that the central nervous system is involved in the aetiology of fatigue from hyperthermia. Dehydration during exercise promotes hyperthermia by reducing skin blood flow, sweating rate and thus heat dissipation. The combination of dehydration and hyperthermia during exercise causes large reductions in cardiac output and blood flow to the exercising musculature, and thus has a large potential to impair endurance performance. Endurance performance is optimized when training is aimed specifically at developing individual components of the model presented and nutritional supplementation prevents hypoglycemia and attenuates dehydration and hyperthermia. Indeed, the challenge at the transition to a new millennium is to synergistically integrate these physiological factors in training and competition.

Influence of body temperature on the development of fatigue during prolonged exercise in the heat

We investigated whether fatigue during prolonged exercise in uncompensable hot environments occurred at the same critical level of hyperthermia when the initial value and the rate of increase in body temperature are altered. To examine the effect of initial body temperature [esophageal temperature (Tes) = 35.9 +/- 0.2, 37.4 +/- 0. 1, or 38.2 +/- 0.1 (SE) degrees C induced by 30 min of water immersion], seven cyclists (maximal O2 uptake = 5.1 +/- 0.1 l/min) performed three randomly assigned bouts of cycle ergometer exercise (60% maximal O2 uptake) in the heat (40 degrees C) until volitional exhaustion. To determine the influence of rate of heat storage (0.10 vs. 0.05 degrees C/min induced by a water-perfused jacket), four cyclists performed two additional exercise bouts, starting with Tes of 37.0 degrees C. Despite different initial temperatures, all subjects fatigued at an identical level of hyperthermia (Tes = 40. 1-40.2 degrees C, muscle temperature = 40.7-40.9 degrees C, skin temperature = 37.0-37.2 degrees C) and cardiovascular strain (heart rate = 196-198 beats/min, cardiac output = 19.9-20.8 l/min). Time to exhaustion was inversely related to the initial body temperature: 63 +/- 3, 46 +/- 3, and 28 +/- 2 min with initial Tes of approximately 36, 37, and 38 degrees C, respectively (all P < 0.05). Similarly, with different rates of heat storage, all subjects reached exhaustion at similar Tes and muscle temperature (40.1-40.3 and 40. 7-40.9 degrees C, respectively), but with significantly different skin temperature (38.4 +/- 0.4 vs. 35.6 +/- 0.2 degrees C during high vs. low rate of heat storage, respectively, P < 0.05). Time to exhaustion was significantly shorter at the high than at the lower rate of heat storage (31 +/- 4 vs. 56 +/- 11 min, respectively, P < 0.05). Increases in heart rate and reductions in stroke volume paralleled the rise in core temperature (36-40 degrees C), with skin blood flow plateauing at Tes of approximately 38 degrees C. These results demonstrate that high internal body temperature per se causes fatigue in trained subjects during prolonged exercise in uncompensable hot environments. Furthermore, time to exhaustion in hot environments is inversely related to the initial temperature and directly related to the rate of heat storage.