Review
Heat stress and strain in exercise and sport

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Summary

Heat stress arising from the thermal environment is of concern to sports medicine and to sports administration because of the perceived risk of heat casualties, in particular heat stroke. Many sports organizations recommend environmental indices such as the WBGT for assessing risk and setting environmental limits for training and competition. But the limits are not justified by evidence. This article describes the nature of heat stress in sport and how it may be assessed objectively. Heat stress and the principal human responses to exercise heat stress are reviewed briefly. Metabolic heat production and the thermal environment provoke separate and largely independent physiological strains. Metabolic heat production drives body core temperature, and the thermal environment drives skin temperature; the combined stresses are integrated to drive sweat rate. Control of core temperature depends on adequate sweat production and the capacity of the environment to evaporate the sweat. The nature of exercise heat stress is demonstrated by rational analysis of the physical heat exchanges between the body and the environment. The principles of this analysis are applied to critical review of current practice in the assessment of heat stress in sport. The article concludes with discussion of research to establish methods for objective sport-specific assessment of heat stress.

Introduction

Heat stress arising from the thermal environment is of concern to sports medicine and to sports administration because of the perceived risk of heat casualties. Guidelines intended to safeguard sports people from ill effects of heat stress often recommend environmental limits for sports participation, but do not provide objective evidence or justification for the limits. Such limits must be evidence-based; otherwise they could allow participation in potentially dangerous conditions. Alternatively, if they are too conservative, they could cause needless anxiety and restrict participation unnecessarily. Objective assessment of risk is essential in circumstances where commercial and organizational considerations restrict opportunities for rescheduling events to avoid adverse environmental conditions. Appropriate management of heat stress in sport therefore requires proper understanding of the stress and its likely impact on participants. The purpose of this article is to demonstrate the nature of heat stress in sport, and how it may be assessed objectively.

Heat stress and strain in sport are a complex mix of behaviour, physics and physiology. Nonetheless the thermal stresses arising from exercise and the environment can be analysed systematically to predict sweating requirements and determine environmental conditions in which body temperature cannot be controlled.

Behaviour underlies heat stress in sport because vigorous exercise imposes significant thermoregulatory stress which may be compounded by the thermal environment. Little can be done to change the environment, but in warm environments the athlete can adjust exercise heat production to achieve a tolerable overall stress.1 By contrast, in laboratory investigations behavioural regulation is excluded because exercise intensity is controlled by the investigator. Furthermore the laboratory environment usually does not reproduce the air movement over the body created by high intensity free moving exercise.2 Consequently, laboratory results may not always be appropriate to formulating guidelines for managing heat stress in the real world of sport.3

Section snippets

Heat production

Heat production (H) arising from muscular activity is usually the main component of sports heat stress. Table 1 shows examples of heat production and the potential for thermoregulatory strain in sports. In sport exercise heat production is a function of maximal aerobic power and relative exercise intensity. Thus in football heat production is the product of players’ moderate relative maximal aerobic power and exercise intensity, but large body mass. Whereas in endurance running it is a function

Physiological responses to exercise heat stress

Physiological temperature regulation must respond more or less precisely to the net heat load (Ereq) arising from the combined effects of exercise and the environment. For the purposes of describing and understanding the physical nature and effects of exercise heat stress the physiological responses of chief interest are skin temperature, sweat rate, and body core temperature.

Skin temperature (Tsk) is taken as the weighted average temperature of the body surface. It is important for its role in

Rational analysis of heat stress

The rational analysis of heat stress suggested by Belding and Hatch10 was employed to generate examples of exercise and environmental heat stress for laboratory exercise and for free running. Rational analysis applies fundamental physical principles to estimate each of the avenues of heat exchange between the body and the environment. The analysis predicts sweat requirements and determines whether balance between heat load and heat loss can be achieved.

Rational analysis

By means of rational and systematic analysis this article provides a basis for understanding exercise heat stress. Rational analysis of heat stress is based on fundamental physical principles determining exchanges of heat between the body and the environment. It evaluates the effects of infinite combinations of all the six factors that determine heat stress. Moreover, by doing so, it reveals the principal factors determining the stress. For this reason it must be the gold standard that informs

Conclusion

Heat stress and strain in sport are a complex mix of behaviour, physics and physiology. Heat stress arises from largely independent components that provoke specific thermoregulatory responses. (1) Exercise heat production drives body core temperature in relation to an athlete's maximal aerobic power. (2) Air temperature and thermal radiation drive skin temperature. The combined thermal stress arising from exercise and the environment are integrated to drive sweat production. (3) The evaporative

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