Brain activity and fatigue during prolonged exercise in the heat

Abstract

We hypothesized that fatigue due to hyperthermia during prolonged exercise in the heat is in part related to alterations in frontal cortical brain activity. The electroencephalographic activity (EEG) of the frontal cortex of the brain was measured in seven cyclists [maximal O2 uptake ([(V)\dot]O2maxVO2max ) 4.8-0.1 (SE) l min-1] cycling at 60% [(V)\dot]O2maxVO2max in a hot (H, 42°C) and a cool (C, 19°C) environment. Fast Fourier transformation of the EEG was used to obtain power spectrum areas in the ! (8-13 Hz) and # (13-30 Hz) frequencies. The ratio !/# was calculated as an index of arousal level; an elevated !/# index reflects suppressed arousal. In H, subjects fatigued after 34.4-1.4 min coinciding with an oesophageal temperature (Toes) of 39.8-0.1°C, an almost maximal heart rate (HR 192-3 beats·min-1), a rating of perceived exertion (RPE) of 19.0-0.8 and significantly elevated !/# index (188-71% of the value after 2 min of exercise; P<0.05). In C, subjects cycled for a similar period while Toes was below 38°C, HR and RPE were low, and the !/# index was not significantly elevated (59-27% of 2 min value; P=NS). Increases in the !/# index were strongly correlated to increases in Toes (r2=0.98; P=0.0001).