Influence of body temperature on the development of fatigue during prolonged exercise in the heat

We investigated whether fatigue during prolonged exercise in uncompensable hot environments occurred at the same critical level of hyperthermia when t he initial value and the rate of increase in body temperature are altered. To examine the effect of initial body temperature [esophageal temperature ( T-es) = 35.9 +/- 0.2, 37.4 +/- 0.1, or 38.2 +/- 0.1 (SE) degrees C induced by 30 min of water immersion], seven cyclists (maximal O-2 uptake = 5.1 +/- 0.1 1/min) performed three randomly assigned bouts of cycle ergometer exer cise (60% maximal O-2 uptake) in the heat (40 degrees C) until volitional e xhaustion. To determine the influence of rate of heat storage (0.10 vs. 0.0 5 degrees C/min induced by a water-perfused jacket), four cyclists performe d two additional exercise bouts, starting with T-es of 37.0 degrees C. Desp ite different initial temperatures, all subjects fatigued at an identical l evel of hyperthermia (T-es = 40.1-40.2 degrees C, muscle temperature = 40.7 -40.9 degrees C, skin temperature = 37.0-37.2 degrees C) and cardiovascular strain (heart rate = 196-198 beats/min, cardiac output = 19.9-20.8 1/min). Time to exhaustion was inversely related to the initial body temperature: 63 +/- 3, 46 +/- 3, and 28 +/- 2 min with initial T-es of similar to 36, 37 , and 38 degrees C, respectively (all P < 0.05). Similarly, with different rates of heat storage, all subjects reached exhaustion at similar T-es and muscle temperature (40.1-40.3 and 40.7-40.9 degrees C, respectively), but w ith significantly different skin temperature (38.4 +/- 0.4 vs. 35.6 +/- 0.2 degrees C during high vs. low rate of heat storage, respectively, P < 0.05 ). Time to exhaustion was significantly shorter at the high than at the low er rate of heat storage (31 +/- 4 vs. 56 +/- 11 min, respectively, P < 0.05 ). Increases in heart rate and reductions in stroke volume paralleled the r ise in core temperature (36-40 degrees C), with skin blood flow plateauing at T-es of similar to 38 degrees C. These results demonstrate that high int ernal body temperature per se causes fatigue in trained subjects during pro longed exercise in uncompensable hot environments. Furthermore, time to exh austion in hot environments is inversely related to the initial temperature and directly related to the rate of heat storage.