Association between the increase in brain temperature and physical performance at different exercise intensities and protocols in a temperate environment

There is evidence that brain temperature (T(brain)) provides a more sensitive index than other core body temperatures in determining physical performance. However, no study has addressed whether the association between performance and increases in T(brain) in a temperate environment is dependent upon exercise intensity, and this was the primary aim of the present study. Adult male Wistar rats were subjected to constant exercise at three different speeds (18, 21, and 24 m/min) until the onset of volitional fatigue. T(brain) was continuously measured by a thermistor inserted through a brain guide cannula. Exercise induced a speed-dependent increase in T(brain), with the fastest speed associated with a higher rate of T(brain) increase. Rats subjected to constant exercise had similar T(brain) values at the time of fatigue, although a pronounced individual variability was observed (38.7-41.7°C). There were negative correlations between the rate of T(brain) increase and performance for all speeds that were studied. These results indicate that performance during constant exercise is negatively associated with the increase in T(brain), particularly with its rate of increase. We then investigated how an incremental-speed protocol affected the association between the increase in T(brain) and performance. At volitional fatigue, T(brain) was lower during incremental exercise compared with the T(brain) resulting from constant exercise (39.3±0.3 vs 40.3±0.1°C; P<0.05), and no association between the rate of T(brain) increase and performance was observed. These findings suggest that the influence of T(brain) on performance under temperate conditions is dependent on exercise protocol.