The effects of heat stress on neuromuscular activity during endurance exercise.

This study analysed the effect of hot (35 degrees C) and cold (15 degrees C) environments on electromyographic (EMG) signal characteristics, skin and rectal temperatures and heart rate during progressive endurance exercise. Eight healthy subjects performed three successive 15-min rides at 30%, 50% and 70% of their peak sustained power output and then cycled at increasing (15 W/min) work rates to exhaustion in both 35 degrees C and 15 degrees C environments. Skin and rectal temperatures, heart rate and EMG data were measured during the trials. The skin temperatures were higher and the subjects felt more uncomfortable in the hot conditions (Bedford scale) ( P<0.01). Rectal temperature was slightly, but not significantly, higher under hot conditions. Heart rate was significantly higher in the hot group (between condition P<0.05). Peak power output (267.4+/-67.7 W vs. 250.1+/-61.5 W) and time-to-exhaustion (55.7+/-16.7 min vs. 54.5+/-17.1 min) (COLD vs. HOT) were not different between conditions. There were no differences in integrated EMG (IEMG) or mean power frequency spectrum between conditions. Rating of perceived exertion increased similarly in both conditions over time. Although the hot conditions increased heart rate and skin temperature, there were no differences in muscle recruitment or maximal performance, which suggests that the thermal stress of 35 degrees C, in combination with exercise, did not impair maximal performance in this study.