Exercise in the heat is limited by a critical internal temperature

We examined whether fatigue during exertional heat stress occurred at a cri tical internal temperature independent of the initial temperature at the st art of exercise. Microwaves (2.1 GHz; 100 mW/cm(2)) were used to rapidly (3 -8 min) heat rats before treadmill exercise to exhaustion. In a repeated-me asures design, food-restricted male Sprague-Dawley rats (n = 11) were prehe ated to three levels (low, medium, and high). In addition, two sham exposur es, Sham 1 and Sham 2, were administered at the beginning and end of the st udy, respectively. At the initiation of exercise, hypothalamic (T-hyp) and rectal (T-rec) temperatures ranged from 39.0 degrees C to 42.8 degrees C (T -hyp) and 42.1 degrees C (T-rec). The treadmill speed was 17 m/min (8 degre es grade), and the ambient temperature during exercise was 35 degrees C. Ea ch treatment was separated by 3 wk. Run time to exhaustion was significantl y reduced after preheating. There was a significant negative correlation be tween run time and initial T-hyp and T-rec (r = 0.73 and 0.74, respectively ). The temperatures at exhaustion were not significantly different across t reatments, with a range of 41.9-42.2 degrees C (T-hyp) and 42.2-42.5 degree s C (T-rec). There were no significant differences in run time in the sham runs administered at the start and end of the investigation. No rats died a s a result of exposure to any of the treatments, and body weight the day af ter each treatment was unaffected. These results support the concept that a critical temperature exists that limits exercise in the heat.