Outdoor exercise performance in ambient heat: Time to overcome challenging factors?

Dear Editor,In recent decades, the effects of hot/dry and hot/humidenvironmental conditions on exercise capacity have beenextensively investigated in laboratory settings [1]. Despite abetter understanding of the thermoregulatory mechanisms, thetransfer to ‘outdoor’ applications is rather limited, as manycurrently omitted factors cause us to question the practicalrelevance of these laboratory-based studies. Thus, if acombination of field- and laboratory-based experiments iswarranted to provide insight into the effects of heat stress onexercise capacity and on cognitive function [2], a multidis-ciplinary approach might strengthen the impact of the results.The influence of meteorological parameters on field-exercise performance has been demonstrated previously. Forexample, in road cycling, weather conditions and thermalcomfort are important for tailoring a training plan, nutritionaladvice and race strategy [3,4]. However, certain meteoro-logical factors (e.g. atmospheric pressure, wind and precipi-tation) are often disregarded when assessing exercise capacityin an ecological test setting, although the efficient manage-ment of weather forecast undoubtedly contributes to improvedsports performance [5]. There are four basic weather elements (air temperature,mean radiant temperature, absolute humidity, and air move-ment) that can be measured by using simple and inexpensiveinstruments. Their combined effects on heat load andevaporative restriction are calculated using long-establishedprocedures [6,7]. There is a historical demand to synthesisethese isolated factors into a single ‘heat stress index’ toexpress their combined effect on health, comfort andperformance, and ultimately to use this index as a regulatorystandard and guideline [8]. This has led to the development ofvarious predictive models to attempt to describe thermalcomfort and the resultant thermal stress [9,10]. Althoughmore than 40 indices have been proposed, the wet-bulbglobe temperature (WBGT) (ISO certification (ISO/DIS7933 1984)), originally developed by the US Navy [11],is commonly used to quantify environmental heat stressduring industrial, military, occupational and sport applica-tions, and its use has been recommended in many guidelines(e.g. from the American College of Sports Medicine [12], theInternational Olympic Committee [13] and FIFA (Fe´de´rationInternationale de Football Association) [14,15]). Moreover,given that not all facilities have the equipment required tomeasure WBGT, approximations of the WBGT formula [16]and direct indices (e.g. the discomfort index [17] and themodified discomfort index [18]) relying on temperature andhumidity have been proposed; however, their use is valid onlyfor full sunshine and light wind because they do not takecloud cover (which influences the intensity of solar radiation)and wind speed [8,14] into consideration.The superior validity of WBGT over dry air temperatureand humidity alone has recently been challenged [8,10]. Forexample, in South Australia, dry air temperature was foundto be more appropriate and more robust than WBGT as ameasure of extreme heat related to sports participation [19]. Although these authors confirmed the need for on-site,specific interpretations of heat participation guidelines toensure sports safety in hot weather [15], they stated that onlydry air temperature can be readily measured irrespective ofthe geographical location. This highlights the need to testcritically the efficacy of establishing an evidence-based,sport-specific threshold for on-site athletes’ health andwelfare interventions [15]. Similarly, the dry air temperatureand WBGT approximation were found to be of comparablerelevance to predict football match outcomes [20]. Budd [8]reinforced the fact that WBGT can only provide ‘a generalguide to the likelihood of adverse effects of heat. ’ (p.30) Thus,measuring independent elements of the thermal environmentwould provide a better assessment. In this context, basedon the fundamental physical principles determining heatexchange [21], it is widely acknowledged that it is themetabolic rate that determines exercise-induced heat strainin sports, regardless of the environmental conditions [7]. For example, when playing tennis in hot conditions ( 37