|  | Current Issues | No.53 May 2008 |
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Athletic Performance in the Heat Ross Tucker |
The 29th Olympic Games in Beijing in August this year promises to provide athletes in the endurance disciplines in particular with two enormous environmental challenges: High temperatures and humidity, and high levels of pollution. The success (or failure) of athletes in Beijing will depend on their response to these two environmental factors. The present article reviews the impacts of pollution and heat on exercise performance, with particular reference on the heat. Pollution During the months leading up to the August Games, media attention has been focused on the potential for Beijing’s air to harm both the performances and the health of athletes participating in the endurance events. The withdrawal of Haile Gebrselassie, the Marathon world record holder, on the grounds that the pollution would be harmful to his health, turned the spotlight on the quality of the air in Beijing. And while some research exists to suggest that high levels of pollution will negatively impact on performance, the unanswered question is whether Olympic level athletes, taking part in competitions, will show the same negative effects are athletes who are usually at best moderately trained and participating in non-competitive laboratory trials. Heat and humidity – more serious problems to consider? While many countries have initiated plans to combat this potential problem, including the use of gas masks for athletes to wear when not in competition, and the appointment of asthma specialists in medical teams, many nations are suggesting that it is not the pollution, but the heat and humidity of Beijing that will pose a far greater problem. By virtue of the position of the Summer Olympics on the sporting calendar, the issue of exercise performance in the heat is a common topic of discussion during the Summer. During the build-up to the 1996 Olympic Games in Atlanta, Nielsen suggested that the conditions in Atlanta would be so severe that events should either be rescheduled for cooler times, or possibly even cancelled (Nielsen, 1996). Based on calculations of heat loss and heat gain given the expected environmental temperatures, Nielsen calculated that a 67 kg male, running a 2:10-marathon pace, would increase body temperature by 1oC every 8.7 minutes, and increase which would result in the attainment of a limiting core temperature within about 30 minutes of the start of the event (Nielsen, 1996), and which would force the athlete to slow down for reasons described subsequently. The impact of the heat in Beijing is expected to be similar, since both the temperature and humidity are likely to be similar to those experienced in Beijing. The following section describes the physiological responses to exposure to hot and humid environments. The physiology of impaired exercise performance in the heat The well known impairment of exercise performance in the heat provides an interesting study into the mechanisms of fatigue and performance regulation during exercise. With regards to hot and humid conditions, laboratory research has established that a critical core temperature is limiting to performance in both animals and humans (Walters et al., 2000;Fuller et al., 1998) (Taylor & Rowntree, 1973;Young et al., 1959;Caputa et al., 1986). Thus, volitional fatigue in exercising humans occurs at a core temperature of approximately 40oC (Nielsen et al., 2001;Nybo & Nielsen, 2001c;Nybo & Nielsen, 2001a;Galloway & Maughan, 1997), irrespective of the rate of heat storage, the pre-exercise core temperature (Gonzalez-Alonso et al., 1999) or the extent of prior heat acclimatization (Nielsen et al., 1997;Nielsen et al., 1993). The observation that subjects began to display symptoms such as dizziness, loss of cohesion, balance and co-ordination led to the suggestion that the high body (and hence brain) temperatures act directly on higher centres and motor cortex to reduce the central drive for exercise (Gonzalez-Alonso et al., 1999). Since the central nervous system was implicated as a cause of fatigue during exercise in the heat, numerous subsequent studies have examined the effects of hyperthermia on various aspects of central nervous system functioning (Nielsen et al., 2001;Nybo & Nielsen, 2001a;Nybo & Nielsen, 2001c;Nybo & Nielsen, 2001b;Nybo et al., 2002a;Nybo et al., 2002b). In 2001, Nybo and Nielsen showed that force production, EMG activity and voluntary activation percentage were lower during a sustained isometric maximal voluntary contraction (MVC) following exercise in hot (40oC, sufficient to raise body temperature to 40oC) than in temperate (18oC, final core temperature 38oC) conditions. They concluded that hyperthermia, caused by failure to maintain body temperatures below the limiting level, resulted in “central fatigue”, causing reduced central activation of the exercised muscles by the motor cortex leading to a lower force production. Recent research has looked at brain function, and provided evidence that brain waves are altered at the point of volitional fatigue, co-inciding with the attainment of the previously described limiting core temperature. These altered brain waves suggested reduced levels of arousal and motivation, and were postulated to represent the mechanism by which the exercising athlete could no longer sustain the required work output during the laboratory trial. These studies (Nielsen et al., 2001;Nybo & Nielsen, 2001a;Nybo & Nielsen, 2001c;Nybo & Nielsen, 2001b;Nybo et al., 2002a) provide evidence that above body temperatures of 40oC, exercise is limited by a direct effect of hyperthermia on central function. However, these studies are artificial in that they fail to provide insight into what may occur during self-paced exercise when the exercising athlete is able to reduce the exercise work rate in response to physiological changes before limiting values are achieved. That is, athletes in the Olympic Games are not required to exercise at a fixed, pre-determined work rate until they find that it becomes impossible to maintain a level of muscle activation to sustain that work load. Instead, they are able to decrease (or increase) work rate in response to a number of different cues, including physiological changes such as body temperature. When viewed in this way, the ability to alter work rate represents an important component of a regulatory system, for it enables the athlete to alter heat production, and hence heat storage and body temperature. During exercise in the heat, then, changes in work rate may be mediated specifically to regulate the rate of heat storage, ensuring that the core temperature does not reach the identified “limit” of 40oC. There is evidence for such regulation from laboratory studies which have allowed athletes to self-select their work rate. For example, Tatterson et al. (Tatterson et al., 2000) found that power output during a 30-minute time-trial in the heat was reduced within the first half of the trial, despite body temperatures that were similar to those measured during a trial in cool conditions. Subsequently, Marino et al. examined the pacing strategies of African runners and Caucasian runners during 8km time-trials in hot and cool conditions (Marino et al., 2004). They found that the African runners out-performed the Caucasian runners in hot, but not cool conditions. This ability was related to altered pacing strategy: The Caucasian runners began the 8km time-trial significantly slower than the African runners, even though the body temperatures between the two groups were not different, following a 30-minute sub-maximal run. It was suggested that the slower running speed at the start of the 8km trial was the result of an anticipatory system that regulated the running speed in order to regulate the rates of heat production and heat storage, and hence rise in body temperature (Marino et al., 2004). Under this anticipatory hypothesis, the Caucasian runners, by virtue of their larger body size would have had greater rates of heat production which are not offset completely by the greater potential for heat loss, leading to an overall higher rate of heat storage than the African runners at the same running speed. Thus, in order to regulate the rate of heat storage, they would have reduced the running speed, allowing them to finish the trial below the limiting core temperature. Subequently, Tucker et al. (Tucker et al., 2004) found that power output and skeletal muscle activation were reduced during the first 6km of a 20km cycling time-trial in hot (35oC) compared to cool (15oC) conditions. The rectal temperatures, heart rates and RPE values recorded at this stage, when pacing strategy began to differ, were similar between the two conditions. As a result of the progressive decrease in power output, the body temperatures in the hot condition remained similar to those measured in the cool condition until the very final kilometer of the trial. In the final kilometer, power output and iEMG activity increased in both conditions. Significantly, the rectal temperature had increased to 39.2oC, but subjects were still able to increase both skeletal muscle activation and power output in this characteristic “end-spurt”. The authors concluded that in the heat, the activation of skeletal muscle motor units and hence power output were reduced as part of an anticipatory regulation of performance, the function of which was to prevent excessive heat storage and increased in body temperature (Tucker et al., 2004). These studies suggest that in the heat, it is the pacing strategy that will be altered, resulting in slower performances from the outset of the endurance events – this is intuitively true, and will be borne out in the competition of the Beijing Olympics. Physiological predictors for success in the heat Given the hot and humid conditions, what factors might account for success in Beijing? There are two key factors that may determine which athletes are successful in hot conditions – small body size and appropriate adaptation to the heat. The advantage of small body size in hot and humid conditions was described previously with reference to the studies of Marino et al (Marino et al., 2004;Marino et al., 2000), which found that smaller African runners outperform larger Caucasian runners in hot, but not cool environments. This confirmed previous research which had examined the extent to which a reduction in body mass might allow an athlete to run at faster speeds in hot conditions before thermal balance could no longer be maintained. Dennis and Noakes (Dennis & Noakes, 1999) calculated, for example, that a 75-kg man could run at a speed of 12.2 km/hour whereas a 45-kg man could run at 19.1 km/hour in hot and humid conditions (35oC, 60% RH) before thermal balance is lost and high rates of heat storage drive body temperatures up towards the limiting value of 40oC. These calculations, as well as those of Nielsen (Nielsen, 1996) show that large body size is a disadvantage during exercise in the heat, and it is thus expected that successful athletes in Beijing’s endurance events will be smaller individuals. A second factor which is critical for success in the heat is the degree of acclimatization to hot and humid conditions. Studies (Nielsen et al., 1997;Nielsen et al., 1993;Young et al., 1985;King et al., 1985;Kirwan et al., 1987)have found that a 4 to 10 day period of training in the heat is effective in restoring exercise ability to near-normal levels. These studies typically use a design where athletes exercise to exhaustion at a sub-maximal work rate for 5 to 10 consecutive days, with performance tests consisting of trials to exhaustion on either side of the training period. The validity of such testing may again be questionable, since athletes are unlikely to sacrifice a week of training to perform sub-maximal exercise to volitional fatigue, but these studies have all provided evidence that time to exhaustion is significantly improved by approximately seven days of training in the heat (Nielsen et al., 1993;King et al., 1985;Kirwan et al., 1987;Febbraio et al., 1994;Young et al., 1985;Nielsen et al., 1997). The primary mechanism for the performance improvement is a reduced rate of increase in body temperature after the acclimatization period, as a result of increased sweat rate and sweat sensitivity (Nielsen et al., 1997;Nielsen et al., 1993;King et al., 1985;Young et al., 1985), as well as cardiovascular adaptations which reduce heart rate and thermal strain. The net result of these physiological adaptations is that the rate of heat storage is reduced, and the athlete can exercise for longer before reaching a limiting internal temperature. During a self-paced model of exercise, this would allow a greater work rate to be maintained before the rate of heat storage does induce a regulatory adjustment in muscle activation and work rate (Tucker et al., 2004). A key consideration with regards to the period of heat adaptation is that performance ability is likely to be compromised for up to eight days on arrival in a hot and humid environment. It is therefore crucial for all aspirant Olympians to expose themselves to hot conditions before their events in Beijing. However, equally crucial is the requirement to balance the heat adaptation period with regular training, for it is evident that during the heat adaptation period, the level of training possible will decline. It is for this reason, as well as the logistical difficulties surrounding heat adaptation, that athletes who are habituated to hot and humid conditions as a result of living in those climates, may be favoured to succeed in Beijing. 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J Appl Physiol 91, 1055-1060. Nybo, L. & Nielsen, B. (2001b). Middle cerebral artery blood velocity is reduced with hyperthermia during prolonged exercise in humans. J Physiol 534, 279-286. Nybo, L. & Nielsen, B. (2001c). Perceived exertion is associated with an altered brain activity during exercise with progressive hyperthermia. J.Appl.Physiol 91, 2017-2023. Nybo, L., Secher, N. H., & Nielsen, B. (2002b). Inadequate heat release from the human brain during prolonged exercise with hyperthermia. J Physiol 545, 697-704. Tatterson, A. J., Hahn, A. G., Martin, D. T., & Febbraio, M. A. (2000). Effects of heat stress on physiological responses and exercise performance in elite cyclists. J Sci Med Sport 3, 186-193. Taylor, C. R. & Rowntree, V. J. (1973). Temperature regulation and heat balance in running cheetahs: a strategy for sprinters? Am J Physiol 224, 848-851. Tucker, R., Rauch, L., Harley, Y. X. R., & Noakes, T. D. (2004). Impaired exercise performance in the heat is associated with an anticipatory reduction in skeletal muscle recruitment. Pflugers Arch. 448, 422-430. Walters, T. J., Ryan, K. L., Tate, L. M., & Mason, P. A. (2000). Exercise in the heat is limited by a critical internal temperature. J Appl.Physiol 89, 799-806. Young, A. J., Sawka, M. N., Levine, L., Cadarette, B. S., & Pandolf, K. B. (1985). Skeletal muscle metabolism during exercise is influenced by heat acclimation. J.Appl.Physiol. 59, 1929-1935. Young, D. R., Mosher, R., Erve, P., & Spector, H. (1959). Body temperature and heat exchange during treadmiill running in dogs. J Appl Physiol 14, 839-843. Contact: Ross Tucker University of Cape Town and Scientific Editor of Runners World SA Cape Town, South Africa Email: ross.tucker@mweb.co.za http://www.icsspe.org/portal/index.php?w=1&z=5 Athletic Performance in the Heat Ross Tucker