Introduction: Hyperthermia has been suggested to control central fatigue by a threshold temperature (critical limiting temperature, CLT) and/or selective brain cooling, having a neuroprotective effect against lethal heat stress. Recently, however, the role of these mechanisms has been questioned (Marino 2011). Previous studies in this field have been limited to small sample sizes due to technical difficulties and inconveniences associated with core body temperature measurements such as rectal or esophageal recordings. In addition, present temperature monitoring technologies might not accurately reflect brain temperature, and specifically temperature in the hypothalamus where the center of thermoregulation is located. We therefore introduced a new non-invasive heatflux technology for determining core body temperature at the forehead (Gunga et al. 2009; Stahn et al. 2011). The aim of the present study was therefore to investigate whether there is a consistent CLT determined close to the hypothalamus during intense exercise in a large sample of young men and women with varying degrees of maximal aerobic capacity. Methods A total of 64 young subjects (43 men, 21women) completed a graded maximal exercise until volitional exhaustion on a bicycle ergometer. In addition to oxygen uptake (breath-by-breath) core body temperature was continuously determined using a new non-invasive heatflux sensor (Double Sensor) positioned at the forehead. Results: Average CBT at the time of exhaustion was 40.2 °C with men displaying slightly, but significantly higher CBT than women (40.3 vs. 39.9 °C, P < 0.05). ANCOVA revealed that this difference could be attributed to higher maximal aerobic capacity in men. Comparing high and low fit subjects irrespective of gender demonstrated that CBT was significantly higher at submaximal (60% and 80% VO2max) and maximal exercise (40.6 vs. 39.7 °C, P < 0.001). In addition, CBT demonstrated a linear increase as a function of maximal aerobic capacity (r = 0.53; P < 0.001). Discussion: While volitional exhaustion was reached around 40 °C irrespective of initial CBT and its rate of increase, suggesting a consistent CLT there was considerable variation in CBT between subjects. In addition this variation was characterized by a positive linear relationship between CBT during maximal exercise and maximal aerobic capacity. These data suggest that either subjects may adapt to higher levels of heat stress by increasing CLT or that neuroprotection from heat stress might not be the primary cause per se for the discontinuation of vigorous physical activity (Marino 2011).
© Copyright 2012 17th Annual Congress of the European College of Sport Science (ECSS), Bruges, 4. -7. July 2012. Veröffentlicht von Vrije Universiteit Brussel. Alle Rechte vorbehalten.