Cooling and pre-cooling during exercise under heat stress: insights into the debate "Skin vs. Core temperature"

Human temperature regulation is challenged during exercise in the heat (Cheuvront et al. 2010). The increase in the metabolic heat production (resulting from exercising muscles), and the decrease in the gradient for heat loss to the environment (resulting from high ambient temperatures and humidity), translate into an increased rate of body heat storage (Tikuisis, McLellan, & Selkirk, 2002; Gleeson, 1998). This results into a quicker obtainment of the “critical” (~40°C) core temperature (T_c) (Cheung & Sleivert 2004), suggested as the main limit to aerobic performance in the heat (González-Alonso & Teller 1999). Elevated T_c can result in a decreased neural drive to contraction (Nybo & Nielsen 2001), as well as in cellular perturbations, which could disrupt metabolic and contractile processes within skeletal muscle (Febbraio 2000).

Pre-cooling strategies (i.e. cold water immersion, cooling garments, ice/fluids ingestion) have been developed to counterbalance the effects of exercising under heat stress (Wegmann et al. 2012; Siegel 2012; Quod et al. 2006). These methods have primary focused on reducing T_c before exercise, in order to increase the margin for metabolic heat production, and thus the time to reach the critical temperature (F. Marino 2002).

However, emerging evidence suggests that the role of hot (>35°C) skin temperature  (T_sk) is also critical in impairing aerobic performance under heat stress (Kenefick et al. 2010; Sawka et al. 2012; Cheuvront et al. 2010). Elevated T_sk narrows the skin to core temperature gradient, thus increasing the skin blood flow requirements, and eventually resulting in an increased level of cardio-vascular strain (Sawka et al. 2012). This is exacerbated by the competition for the available cardiac output between the blood flow required by the exercising muscles to meet the oxygen demands, and the blood flow required by the skin to meet the demands of temperature regulation (i.e. heat dissipation to the environment) (González-Alonso et al. 2008). Also, heat-induced changes in T_sk influence perceptual and cutaneous-sensory feedbacks such as thermal sensation, comfort and “sensation of fatigue”, (Cheung 2010; Noakes 2012), which have been proposed as critical determinants of pacing strategies during performance under heat stress (Tucker & Noakes 2009; Marcora et al. 2009)

Therefore, in order to preserve performance under heat stress, keeping the skin cool during the exercise, might be as important as a pre-exercise reduction in T_c (Schlader et al. 2011).

Cooling methods, such as air and water cooled systems (Stephenson et al. 2007; Cadarette et al. 2006; Cadarette et al. 2002), garments made of phase change materials (House et al. 2013; Bergmann Tiest et al. 2012), as well as the use of menthol (Gillis et al. 2010; Lee et al. 2012), have been developed and shown to be potentially effective in preserving performance in the heat, due to their effects on skin and whole body temperature (Smolander & Dugue 2012; Hasegawa & Takatori 2005; Lopez et al. 2008). However, due to some specific disadvantages, such as weight of the systems, wearability of the garments or side effects of menthol application (i.e. skin irritation), these methods still present numerous practical limitations (Marino 2002; Quod et al. 2006). 

In this respect, evaporative cooling garments (e.g. Personal Cooling System, Unico, Switzerland; Omni Freeze-Zero, Columbia USA; KoolSorb, Technical Absorbent, UK) have recently received attention, as they could represent a potentially effective alternative to more traditional cooling methods (Bogerd et al. 2010; Webster et al. 2005; Sakoi 2011; Kocjan & Rothmaier 2007). These light weight garments induce mild-cooling via the process of water evaporation. These are made of particular hydrophilic fabrics, which, if wetted, allow improved water evaporation, thereby cooling the shirt and underlying skin.

However, although using the concept of evaporative cooling translates into the possibility to design cooling garments which are light weight and practical, the limited empirical evidence on their physiological as well as perceptual (i.e. thermal comfort) effects, made any conclusion on these methods difficult to drawn. Nevertheless, there is a clear need to develop cooling systems which would result effective in counteracting the thermal strain, as well as practical and thermally comfortable  (Flouris & Cheung 2006; Cadarette et al. 2006; Minniti et al. 2011; Marino 2002).

This has important practical applications, not only for elite performance under heat stress (Cheung 2010), but also, in the context of amateur and recreational exercise. Indeed, individuals who enjoy outdoor sporting activities, such as running or cycling, encounter a variety of environmental conditions, some of which (e.g. heat) can significantly decrease their thermal comfort (Vanos et al. 2010; Brotherhood 2008). As the type and amount of physical activity performed has been shown to be influenced by the level of comfort achievable with the surrounding environment (Vanos et al. 2012), developing a practical cooling method, able to reduce the thermal discomfort experienced while exercising in the heat, might have a positive impact on the activity levels of healthy individuals.

Davide Filingeri 

PhD Reseracher

Environemntal Ergonomics Research Centre

Loughborough Univeristy

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