Heat challenges multiple physiological systems, and its effects are heavily felt by endurance athletes due to the duration and intensity that must be sustained in competition and training sessions. Runners may demonstrate impaired thermoregulatory responses or opportunities due to lower rates of convective cooling and fewer opportunities to provide cooling interventions during exercise than other endurance athletes e.g. cyclists. Cooling strategies may be employed before or during exercise to minimise the effects of heat exposure, and their effects have been studied for at least three Olympic cycles. Hence, the optimisation of timing and method of delivery of cooling provision, with the addition of any novel strategies, would be of benefit to the contemporary sport and exercise science practitioner.
Initially this thesis sought to better understand the effects of cooling strategies upon time trial performance in endurance sports with a systematic review and meta-analysis. The efficacy of strategies was assessed with respect to intervention timing (pre or per-cooling or both) and method of delivery (oral or topical or both). Cooling strategies were found to provide small but ecologically relevant improvements in time trial performance, especially when administered during the exercise bout to the oral cavity; the addition of menthol was seen to possibly enhance ergogenic effects. Hence, a second systematic review regarding external or internal application of menthol was conducted and found that menthol demonstrated improvements in performance when applied internally, most likely due to altered thermal and ventilatory responses.
A range in menthol concentrations and dilution methods was noted in the literature, establishing a clear need for a randomised trial to ascertain menthol concentration preference. Following appropriate dilution, 0.1% menthol was determined to be preferred; colour preference was also established to maximise the perceptual cooling effect of menthol solution. This solution was then used (without colour to ensure blindness) in subsequent investigations. At rest this solution was shown to improve perceptions of thermal comfort, thermal sensation and thirst, when compared to carbohydrate and water swilling.
Two exercise trials were conducted: the first examined the effects of menthol mouth swilling upon physiological and perceptual markers over four minute intervals at a range of pertinent running speeds (14-20km.h-1), and following 1km time trial performance. Effects on time trial performance were unclear, as were the effects in physiological parameters. Thermal comfort however was improved, with menthol mouth swilling counterintuitively increasing thermal sensation and thirst in the heat (35oC), but
i
ameliorating these factors in the cold (15oC). Secondly, at a fixed rating of perceived exertion, corresponding to 2mmol.L-1 blood lactate, runners demonstrated a lower oxygen consumption following menthol mouth swilling for the latter two thirds of a 30-minute training session than compared to no swill or ice swilling. No changes in ventilation were shown, and the perceptual responses at a group level were unclear – suggesting that whilst menthol may improve the oxygen cost of running at a fixed rating of perceived exertion, this does not correspond to improvements in thermoregulatory perception in this sample. Qualitative responses regarding the swill from the athletes involved in the exercise studies were collated and menthol was considered an enjoyable and useful strategy by the athletes. Further research is required to assess if these hedonic and utilitarian perspectives are rated as highly in more ecologically valid environments; the athletes indicated this would be the case.
The findings presented in this thesis demonstrate that a light blue or light green 0.1% menthol mouth rinse is preferred and can alleviate thermal sensation and thirst, and improve thermal comfort at rest in the heat. During exercise in a small sample of trained distance runners, menthol mouth swilling may alleviate perceptual symptoms of heat exposure without necessarily improving performance, dependent upon the running speeds chosen. Furthermore, menthol mouth swilling is considered a pleasant and potentially ergogenic strategy by athletes who have used it, suggesting that even in the absence of performance or physiological enhancements that exceed the typical coefficient of variation in performance, menthol mouth swilling is a viable nutritional support strategy for trained distance runners, when exercising in the heat.