By Drs. Zachary J. Schlader & Toby Mündel
We read with great interest the study by Barwood and colleagues published recently within BJSM . In this study, the authors aimed to explore the relationships between body temperature(s), thermal perception, and the voluntary control of exercise intensity in the heat. For this they should be commended for although this area is quite topical, our understanding of these relationships remains virtually unknown. By chemically activating peripheral cold thermo-sensors with menthol, Barwood et al.  have demonstrated that improvements in thermal comfort and reductions in sensations of warmth, independent of changes in skin temperature, both prior to, and during, the initial stages of self-paced exercise in the heat did not influence the selection of exercise intensity. Although the conclusions drawn appear appropriate, we would like to further discuss their results in the context of others to explore this topic and perhaps allow a better understanding of our current base of knowledge.
As part of the rationale for conducting their study, Barwood et al.  state “It is presently unknown whether altered pacing strategy is regulated as part of behavioral thermoregulation driven by a conscious awareness of thermal state or if a central and subconscious homeostatic mechanism is activated when skin temperature rises above a threshold rate”. Furthermore, Barwood et al.  conclude that “This study addresses an as-yet unanswered question of whether the fatiguing mechanisms during exercise in the heat are primarily consciously or subconsciously mediated”. Firstly, we would like to draw the authors’ attention to our recent work testing the hypothesis that voluntary reductions in exercise intensity in the heat are thermoregulatory behaviors . Our results demonstrated that the observed reductions in power output during exercise in ~40°C compared to ~20°C were, at least in part, due to a conscious action that was inversely related to total heat body storage and thermal discomfort, and improved heat exchange . Secondly, it is unfortunate that it was not possible to discuss two of our recent studies demonstrating that skin temperature and/or the perceptions of this temperature play a large role in the initial selection of  or sustained decrease in  exercise intensity. Perhaps in “addressing [only] two current viewpoints on how exercise pacing is driven in hot conditions” Barwood et al. have unintentionally overlooked this recent evidence?
This notwithstanding, the results put forward by Barwood and colleagues  appear to be in stark contrast to those we obtained utilizing a similar methodology whereby menthol and skin cooling was used to independently alter thermal perception and skin temperature during exercise at a fixed rating of perceived exertion (RPE) . These results showed that an improved thermal comfort and reduced sensations of warmth with menthol enhanced the capacity to maintain exercise intensity. Thus, we concluded that thermal perception is a capable modulator of exercise intensity independent of any change in skin temperature ; so why, then, such opposing views?
As supported by Barwood et al. , it is becoming increasingly clear that RPE is perhaps the most dependable criterion dictating the voluntary selection of exercise intensity . In such circumstances, it appears as though, independent of perturbation (e.g. hypoxia, heating, cooling etc.), the exerciser compares how they feel to how they expect themselves to feel at that moment in time and adjust their exercise intensity accordingly . Thus, although the RPE response during self-paced exercise appears to be tightly controlled, the effect of a given perturbation is found in changes in the selection of exercise intensity (or pacing strategy). Therefore, the sole manner in which pacing strategy can be altered is if the perturbation is large enough in magnitude to alter RPE. Herein lies the difference between our studies. It is unlikely that the cooling modalities (either skin cooling or menthol) utilized by Barwood et al.  were sufficiently sustained or large enough in magnitude to alter RPE. In contrast, by utilizing a significantly different experimental design to address the same question, we were successful in altering RPE.
The reason for this is likely four-fold, but certainly other rationale cannot be discounted. Firstly, in contrast to the entire skin surface we chose to manipulate the skin of the face, an area that is both of high thermal sensitivity during heat stress  and an area that has been directly demonstrated to modulate exercise duration . Secondarily, we used a greater concentration of menthol (8% vs. 0.05%) which, together with the facial manipulation, likely elicited a larger change in thermal perception. Thirdly, we chose to use fit but untrained subjects, as trained individuals have an altered perception of their physiological thermal strain during exercise . Thus, our subjects were likely more sensitive to changes in thermal perception. The fourth, and perhaps final reason for the observed differences between these two studies likely stems from the exercise protocols used, i.e. fixed-RPE vs. time trial. For instance, anecdotal observations from our laboratory suggest that the fixed-RPE protocol may be more sensitive to a given thermal stimulus than a time trial; although to our knowledge there is no formal data suggesting this arrangement. Other rationale that should probably also be considered include the heat stress compensability and modality, exercise duration, and suitable subject blinding to the experimental conditions, amongst others.
In conclusion, we would like to commend Barwood and colleagues for their study and the data it adds to the literature. However, we would urge caution before readers draw conclusions based on this study alone. As it currently stands, the relationships between temperature, thermal perception, and exercise intensity remain uncertain and further research is required before conclusions can and should be drawn. The differences between our study  and that of Barwood et al.  further highlight that the choice of experimental methodology may greatly influence a study’s outcome(s). Issues pertaining to methodology are not specific to perception and exercise. For instance, this journal recently highlighted another (equally debated) area, i.e. exercise and fluid replacement, which suffers from similar methodological concerns [10, 11]. Nevertheless, these studies [1, 5] endorse (and encourage) the use of menthol and other chemicals capable of affecting thermal perception without changing skin temperature in providing a useful paradigm to study the interactions between thermal perception and the voluntary control of exercise intensity. Finally, as is the case with nearly all areas of research, we would encourage further studies in this area to ensure a better understanding and therefore, perhaps, a resolution to this interesting and topical area.
1. Barwood MJ, Corbett J, White D, et al. Early change in thermal perception is not a driver of anticipatory exercise pacing in the heat. Br J Sports Med 2011.
2. Schlader ZJ, Stannard SR, Mundel T. Evidence for thermoregulatory behavior during self-paced exercise in the heat. J Therm Biol 2011;36:390-6.
3. Schlader ZJ, Simmons SE, Stannard SR, et al. Skin temperature as a thermal controller of exercise intensity. Eur J Appl Physiol 2011;11:1631-9.
4. Schlader ZJ, Stannard SR, Mundel T. Is peak oxygen uptake a determinant of moderate-duration self-paced exercise performance in the heat? Appl Physiol Nutr Metab 2011;36:863-72.
5. Schlader ZJ, Simmons SE, Stannard SR, et al. The independent roles of temperature and thermal perception in the control of human thermoregulatory behavior. Physiol Behav 2011;103:217-24.
6. Schlader ZJ, Stannard SR, Mundel T. Human thermoregulatory behavior during rest and exercise – a prospective review. Physiol Behav 2010;99:269-75.
7. Cotter JD, Taylor NA. The distribution of cutaneous sudomotor and alliesthesial thermosensitivity in mildly heat-stressed humans: an open-loop approach. J Physiol 2005;565:335-45.
8. Ansley L, Marvin G, Sharma A, et al. The effects of head cooling on endurance and neuroendocrine reponses to exericse in warm conditions. Physiol Res 2008;57:863-72.
9. Tikuisis P, McLellan TM, Selkirk G. Perceptual versus physiological heat strain during exercise-heat stress. Med Sci Sports Exerc 2002;34:1454-61.
10. Mundel T. To drink or not to drink? Explaining “contradictory findings” in fluid replacement and exercise performance: evidence from a more valid model for real-life competition. Br J Sports Med 2011;45:2.
11. Goulet ED. Effect of exercise-induced dehydration on time-trial exercise performance: a meta-analysis. Br J Sports Med 2011;45:1149-56.
Zachary J. Schlader, PhD, Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital, Dallas, TX, USA
Toby Mündel, PhD, School of Sport and Exercise, Massey University, Palmerston North, New Zealand