AbstractWhen exercise is undertaken in warm, humid conditions, the thermal gradient between the skin and environment, and the capacity for evaporative heat loss, are reduced. These factors, along with an increase in metabolic heat production, lower work capacity and exercise performance. Thermoreceptors located within the skin and deep in the body convey information on this accumulation of thermal energy to higher brain structures and, if mean body temperature rises uncontrollably, the cumulative neuronal input is thought to produce inhibitory signals that lower work capacity, such that metabolic heat production decreases to protect the organism from heat injury. Lessening these inhibitory signals may enhance or help to maintain exercise performance in the heat. The inhibitory signals might be lessened by cooling the skin and deep body temperature prior to or during exercise, or perhaps by applying menthol on the skin, or some combination of these.
Menthol is a chemical compound that activates cold receptors (TRPM8) in the skin to elicit cool sensations. These receptors are not otherwise activated unless cooled below 27 °C. Hence, menthol, when applied to the skin of heat stressed humans, may provide a “cool’’ neuronal input to higher brain structures in addition to the neuronal signals arising from warm thermo receptors located within the body. But menthol may also induce a heat storage (cold defence) response that would then heighten the activity of warm receptors deep in the body. Therefore, it is not clear whether menthol might reduce, enhance or help to maintain exercise performance in heat stressed humans. Moreover, no studies have assessed the perceptual and thermoregulatory response to menthol during rest or exercise, or the consequence of its repeated use. Before it is recommended as a possible ergogenic aid, these studies should be undertaken. The early work presented in this thesis tested the hypotheses that a water-based spray, containing ethanol and/or menthol, would enhance evaporative cooling when sprayed on the skin, thereby lowering heat storage and improving thermal perception compared to an unsprayed Control condition; but menthol would also improve thermal perception independent of temperature by directly stimulating cold receptors, during rest and exercise in warm, humid conditions. The hypothesis that menthol-mediated cool sensations would not undergo any habituation after repeated exposures was also tested.
The general approach to testing these hypotheses involved presenting human participants with a thermal challenge that would induce warm sensations and increase thermal discomfort, whilst encouraging a level of heat storage that could be compensated for by increasing heat loss through vasodilation and sweating. This was achieved by manipulating metabolic heat production through a combination of rest and fixed intensity exercise in warm (30 °C) and humid (70 %) conditions. The influence of a menthol solution spray was tested against the backdrop of this thermal challenge.
The results supported the general hypothesis that a water-based upper-body spray containing menthol can increase sensations of cool than compared to no spraying or water only spraying during rest and exercise in warm, humid conditions, but menthol also influences body temperature regulation. The effect that menthol exerts over perception and thermoregulation differs by dose and fades with time. Specifically, 0.2 % menthol spraying encourages heat storage by enhancing vasoconstriction, and there is no habituation in these responses. 0.05 % menthol spraying did not encourage any additional heat storage compared to a Control spray. Menthol also influenced perception, with a 0.2 % menthol spray promoting cooler sensations and greater irritation than 0.05 % menthol and Control spraying. Compared to a Control spray, 0.2 % menthol reduced thermal comfort during rest and improved it during exercise, while 0.05 % menthol did not alter thermal comfort during rest, and may have improved it during exercise. Neither menthol spray influenced perceived exertion during exercise. Menthol-mediated cool sensations lasted 15 to 30minutes. Both 0.2 % and 0.05 % menthol sprays underwent an habituation compared to the Control spray, with cool sensations diminishing after repeated daily exposures.
It is concluded that a 0.05 % menthol spray, which induces cool sensations without a significant heat storage response, could be considered as a perceptual cooling intervention with some capacity to enhance evaporative heat loss when sprayed on the skin during rest and moderate fixed-intensity exercise in the heat. A 0.2 % menthol spray might be deployed later in exercise, but may increase heat storage and irritation. Further testing is required to identify whether menthol spraying improves maximal exercise performance.
|Date of Award||Nov 2011|
|Supervisor||James R. House (Supervisor)|