SPECIAL COMMUNICATIONS: Letters to the Editor-in-Chief
Byrne et al. (3) recently published observations of continuous core temperature monitoring to quantify physiological strain in runners competing in the 2003 Singapore half-marathon. In addition, the authors also gave careful consideration to the frequently observed differences between exercise thermoregulation in field and laboratory settings. We believe that this latter aspect of their paper deserves more attention.
That there are differences in ambient conditions between indoor and outdoor exercise is obvious (rain, wind, sun, terrain, race strategy), and that those differences can impact thermoregulation is undeniable. Further, inherent differences between observational (loose control, real circumstances) and experimental (tight control, artificial circumstances) research frequently make comparisons problematic. Researchers can study either indoor or outdoor exercise to answer basic and applied science questions, but the findings from both must be viewed in concert to fully understand the true nature of any phenomenon.
For example, Byrne et al. (3) reported that 12 of 18 runners achieved core temperatures > 40°C without medical consequence. This corroborates numerous historical observations of others (3-5) where core temperatures > 40°C were well tolerated in competitive athletes. It is also consistent with a critical thermal maximum > 42°C (2,5). These findings seem to conflict, however, with laboratory data supporting a lower (~40°C) critical core temperature for exercise fatigue under certain circumstances. The same 40°C temperature is also used in conjunction with other clinical observations as a threshold for heatstroke (1). However, laboratory findings actually show that the core temperature threshold for fatigue is contingent on many factors and must be understood as a continuum (5). Similarly, field observations lend support that both the etiology of serious heat injury and the triage of collapsed runners involve consideration of more than just a high body temperature (1).
Byrne et al. (3) and others (4) also report poor relationships between thermoregulatory variables like core temperature when plotted against running speed or the level of dehydration achieved racing outdoors. These poor relationships are sometimes considered a paradox when viewed against the profound and predictable influence of relative exercise intensity (8) and progressive dehydration (7) on body heat storage in the laboratory. The use of explained variance as a measure of the relationship between heat storage and hydration state or exercise intensity will be limited under outdoor conditions where other factors contribute to total variance, but this in no way obviates the fundamental laboratory findings. Byrne et al. (3) and others (4) recognize this fact and offer that when viewed thoughtfully and within situational context, the apparent differences observed in exercise thermoregulation between field and laboratory studies can be fully explained using basic biophysical principles of human exercise thermoregulation (4,6).
The work of Byrne et al. (3) adds to our knowledge of the temporal core body temperature response to competitive distance running in warm-hot environments and the tolerance of humans to exercise hyperthermia. Importantly, it also helps explain that harmony lies beneath what is often perceived as a paradox between field and laboratory findings.
Samuel N. Cheuvront
Robert W. Kenefick
Scott J. Montain
U.S. Army Research Institute of Environmental Medicine
1. Brennan, F. H., and F. G. O'Connor. Emergency triage of collapsed endurance athletes. Phys. Sports Med.
2. Bynum, G. D., K. B. Pandolf, W. H. Schuette, et al. Induced hyperthermia in sedated humans and the concept of a critical thermal maximum. Am. J. Physiol.
3. Byrne, C., J. K. Lee, S. A. Chew, C. L. Lim, and E. Y. Tan. Continuous thermoregulatory responses to mass-participation distance running in heat. Med. Sci. Sports Exerc.
4. Cheuvront, S. N., and E. M. Haymes. Thermoregulation and marathon running: biological and environmental influences. Sports Med.
5. Cheuvront, S. N., and M. N. Sawka. Physical exercise and exhaustion from heat strain. J. Korean Soc. Living Environ. Sys.
6. Kenefick, R. W., S. N. Cheuvront, and M. N. Sawka. Thermoregulatory function during the marathon. Sports Med.
7. Montain, S. J., and E. F. Coyle. Influence of graded dehydration on hyperthermia and cardiovascular drift during exercise. J. Appl. Physiol.
8. Saltin, B., and L. Hermansen. Esophageal, rectal, and muscle temperature during exercise. J. Appl. Physiol.