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

Abstract : Dear Editor, In recent decades, the effects of hot/dry and hot/humid environmental conditions on exercise capacity have been extensively investigated in laboratory settings [1]. Despite a better understanding of the thermoregulatory mechanisms, the transfer to 'outdoor' applications is rather limited, as many currently omitted factors cause us to question the practical relevance of these laboratory-based studies. Thus, if a combination of field-and laboratory-based experiments is warranted to provide insight into the effects of heat stress on exercise capacity and on cognitive function [2], a multidisciplinary approach might strengthen the impact of the results. The influence of meteorological parameters on field-exercise performance has been demonstrated previously. For example, in road cycling, weather conditions and thermal comfort are important for tailoring a training plan, nutritional advice and race strategy [3,4]. However, certain meteorological factors (e.g. atmospheric pressure, wind and precipitation) are often disregarded when assessing exercise capacity in an ecological test setting, although the efficient management of weather forecast undoubtedly contributes to improved sports performance [5]. There are four basic weather elements (air temperature, mean radiant temperature, absolute humidity, and air movement) that can be measured by using simple and inexpensive instruments. Their combined effects on heat load and evaporative restriction are calculated using long-established procedures [6,7]. There is a historical demand to synthesise these isolated factors into a single 'heat stress index' to express their combined effect on health, comfort and performance, and ultimately to use this index as a regulatory standard and guideline [8]. This has led to the development of various predictive models to attempt to describe thermal comfort and the resultant thermal stress [9,10]. Although more than 40 indices have been proposed, the wet-bulb globe temperature (WBGT) (ISO certification (ISO/DIS 7933 1984)), originally developed by the US Navy [11], is commonly used to quantify environmental heat stress during industrial, military, occupational and sport applications , and its use has been recommended in many guidelines (e.g. from the American College of Sports Medicine [12], the International Olympic Committee [13] and FIFA (Fédération Internationale de Football Association) [14,15]). Moreover, given that not all facilities have the equipment required to measure WBGT, approximations of the WBGT formula [16] and direct indices (e.g. the discomfort index [17] and the modified discomfort index [18]) relying on temperature and humidity have been proposed; however, their use is valid only for full sunshine and light wind because they do not take cloud cover (which influences the intensity of solar radiation) and wind speed [8,14] into consideration. The superior validity of WBGT over dry air temperature and humidity alone has recently been challenged [8,10]. For example, in South Australia, dry air temperature was found to be more appropriate and more robust than WBGT as a measure of extreme heat related to sports participation [19]. Although these authors confirmed the need for on-site, specific interpretations of heat participation guidelines to ensure sports safety in hot weather [15], they stated that only dry air temperature can be readily measured irrespective of the geographical location. This highlights the need to test critically the efficacy of establishing an evidence-based, sport-specific threshold for on-site athletes' health and welfare interventions [15]. Similarly, the dry air temperature and WBGT approximation were found to be of comparable relevance to predict football match outcomes [20]. Budd [8] reinforced the fact that WBGT can only provide 'a general guide to the likelihood of adverse effects of heat.' (p.30) Thus, measuring independent elements of the thermal environment would provide a better assessment. In this context, based on the fundamental physical principles determining heat exchange [21], it is widely acknowledged that it is the metabolic rate that determines exercise-induced heat strain in sports, regardless of the environmental conditions [7]. For example, when playing tennis in hot conditions (37 C, 36% relative humidity, 34 C WBGT and 0.5 m/s wind velocity), players' rectal and thigh skin temperatures increased to 39.4 and 37.5 C, respectively, leading to exacerbation of the perception of effort and
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Soumis le : vendredi 14 avril 2017 - 16:28:23
Dernière modification le : lundi 9 juillet 2018 - 16:36:20

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Franck Brocherie, Olivier Girard, Grégoire P. Millet, Alessandro Pezzoli. Outdoor exercise performance in ambient heat: Time to overcome challenging factors?. International Journal of Hyperthermia, Taylor & Francis, 2014, 34 (8), pp.433 - 549. ⟨10.1080/00140138308963365⟩. ⟨hal-01497947⟩

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