Brain activity and fatigue during prolonged exercise in the heat

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 O-2 uptake (V-O2max) 4.8 +/-0.1 (SE)1 min(-1)] cycling at 60% V-O2max in a hot (H, 42 degreesC) and a cool (C, 19 degreesC) environmental. Fast Fourier transformation of the EEG was used to obtain power spectrum areas in the alpha (8-13 Hz) and beta (13-30 Hz) frequencies. The ratio alpha/beta was calculated as an index of arousal level; an elevated alpha/beta index reflects suppressed arousal. In H, subjects fatigued after 34.4 +/-1.4 min coinciding with an oesophageal temperature (T-oes) of 39.8 +/-0.1 degreesC, 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 alpha/beta index (188-+/- 71% of the value after 2 min of exercise; P <0.05). In C, subjects cycled for a similar period while T-oes was below 38 degreesC, HR and RPE were low and the alpha/beta index was not significantly elevated (59 +/- 27% of 2 min value; P=NS). Increases in the odp index were strongly correlated to increases in T-oes (r(2)=0.98; P=0.0001).