Thermoregulatory aspects of performance

This issue of Experimental Physiology contains symposium papers from the conference The Biomedical Basis of Elite Performance held on the 19–21 March 2012 at The Queen Elizabeth II Conference Centre, London which covers the theme of Thermoregulatory aspects of performance. Elite athletes are capable of prodigious feats of exercise, but for every individual there is a limit to the power output that can be achieved at any given moment. It seems futile to look for a single factor that limits exercise performance in all individuals, in all types of exercise and in all environments. Nonetheless, there appear to be some common factors. Endurance exercise performance is impaired progressively as the ambient temperature increases (Galloway & Maughan, 1997) and, at least in a warm environment, is impaired progressively as the ambient humidity rises (Maughan et al. 2012). These observations, which are a matter of common experience among athletes and industrial workers, suggest that some aspect of thermoregulatory function is linked to the fatigue that accompanies prolonged exercise in hot, humid environments. They do not, however, identify how or where thermal stress is sensed, nor do they offer any clues concerning the physiological mechanisms responsible for the early onset of fatigue. The metabolic rate increases to meet the needs of exercise, and the marathon runner who wishes to break the men’s world best performance of 2 h 03 min 38 s must sustain a speed in excess of 20 km h−1. Assuming a body mass of 55 kg, this requires an energy expenditure of more than 1100 kcal h−1, with 20% or more of the energy appearing as heat. Most of this heat is generated in a relatively small muscle mass, and González-Alonso (2012) has considered the implications for the cardiovascular system of the need to dissipate this heat. Some is lost by conduction to the skin overlying the active limbs and thence to the environment, but most must be transferred by convection via blood flow from the active musculature to the body core and thence to the skin. A high flow rate to the muscle and to the vascular bed of the skin requires a cardiac output that may exceed the pumping capacity of the heart, especially if the blood volume is reduced by the high rates of sweating that are necessary to maintain evaporative heat loss. In order to maintain the rate of evaporative heat loss, there is a need to maintain a high skin temperature and this in turn requires a high skin blood flow. Sawka et al. (2012) have argued that the skin temperature, rather than a high core temperature, plays a key role in the aetiology of fatigue. The problem with a high skin temperature is that it narrows the temperature gradient from the body core to the skin, thus increasing the blood flow requirement for convection of heat from the core to the skin. They provide compelling evidence that high skin temperatures will lead to impairment of maximal aerobic power as well as of endurance performance, and argue against the widely held view that a high core (brain) temperature is in itself the primary limitation to exercise. The idea that the brain – rather than the heart, lungs or muscles – acts to limit performance is not new. Indeed, this concept was the prevailing one at the end of the 19th century (Lagrange, 1889). Only in recent years, however, has it been possible to investigate events taking place within the central nervous system in exercising man. Nybo and colleagues have made an extensive series of measurements of heat exchange across the brain during exercise and have shown that brain temperature is closely related to body temperature (Nybo, 2012). Transcranial magnetic stimulation and other techniques have been used to show that both passive and exercise-induced hyperthermia impair voluntary muscle activation during intense muscle activity. In contrast to the stance of Sawka et al. (2012), Nybo’s conclusion is that brain temperature plays a key role in performance in the heat. This may be at least partly because a high skin temperature will be