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

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 thermal sensations. Consequently, tennis players adjusted their behaviour (e.g. increase in duration of time between points) to minimise the deleterious effects of the heat [22]. This is not a novel idea, but it appears to have been forgotten by many contemporary thermal physiologists.

From a biophysical perspective, changes in core temperature are determined by the cumulative imbalance between metabolic heat production and net heat loss to the environment (i.e. body heat storage), body mass (i.e. internal heat sink), and body composition (i.e. specific heat capacity of body tissue) [23]. Previous studies have shown that differences in body mass led to diverse core temperature responses during exercise at the same absolute work rate or metabolic heat production [24]. More recently, body mass and body surface were clearly identified as important independent factors on thermoregulatory outcomes during exercise [23]. For example, athletes with larger body and muscle masses (e.g. American football or rugby players) generally showed greater metabolic heat production that must be dissipated to maintain a safe body temperature [25]. Conversely, smaller athletes such as distance runners had higher evaporative heat loss when environmental temperature increased [26]. Consequently, neglecting body composition led to an underestimation of the restricted evaporation stress. Effectively, among the environmental factors, the amount of water vapour in the air (absolute humidity) is a major influence because the water vapour pressure gradient between the skin and the environment drives evaporative cooling. Air temperature and speed (e.g. relative wind) are other contributors to overall heat exchange, in addition to radiant heat. Accordingly, clothing modifies the rate of heat exchange and is paramount for evaporative cooling. For example, the addition of a uniform (e.g. with or without pads) has been demonstrated to increase rectal and skin temperatures, and perceptual ratings at a given workload resulted in performance decreases [25]. This microenvironment leads to an increase in metabolic heat production in combination with reduced heat loss mechanisms efficiency [1].

Finally, to prevent the biases related to differences in metabolic heat production and body morphology [27], the prescription of an exercise intensity that elicits the same absolute heat balance may, however, lead to different local sweat rates between independent groups that are unmatched in body surface areas. Therefore, the choice of an intensity eliciting the same heat balance per unit of body surface area would reduce these inherent biases [28].

In conclusion, the responses of subjects exposed to an environment will depend on their activity (exercise type, duration and intensity), as well as other factors (mainly related to body composition, clothing or other weather-independent variables), all of which require a careful evaluation. These latter variables likely introduce large errors into any prediction of adverse weather effects [29]. Moreover, a satisfactory index of heat stress should evoke the same physiological and subjective responses for all combinations of its constituent elements (e.g. responses to a given level of the index should be the same in hot/humid and hot/dry conditions) [8]. It is particularly relevant that most indices have been developed for use within particular combinations and ranges of environmental factors and should not be used uncritically in environments and contexts for which they were not planned [30]. With this in mind, the idea of developing a universal thermal climate index (UTCI) based on the most advanced multi-node model of thermoregulation is emerging from both thermo-physiological and heat exchange theories [31]. Similarly, indices derived from heat budget models, such as the predicted mean vote and physiological equivalent temperature [10,32], demonstrated usefulness in evaluating thermal comfort in road cycling [33]. It is important to note that substantial inter-individual variability and variation in worldwide climatology might present barriers to developing an entirely perfect system for assessing heat stress. That being said, we anticipate that an integration of the combination of all relevant multidisciplinary data (i.e. thermal physiology, mathematical modelling, occupational medicine, bio-meteorology [31]) would generate better application in an ecological sport science setting with potential impact on heat stress guideline management. This represents the challenge that it is currently being addressed by our research group.