There is evidence that brain temperature (Tbrain) 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 Tbrain 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. Tbrain was
continuously measured by a thermistor inserted through a brain guide cannula.
Exercise induced a speed-dependent increase in Tbrain, with the fastest
speed associated with a higher rate of Tbrain increase. Rats subjected to
constant exercise had similar Tbrain 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 Tbrain increase and performance
for all speeds that were studied. These results indicate that performance during
constant exercise is negatively associated with the increase in Tbrain,
particularly with its rate of increase. We then investigated how an incremental-speed
protocol affected the association between the increase in Tbrain and
performance. At volitional fatigue, Tbrain was lower during incremental
exercise compared with the Tbrain resulting from constant exercise
(39.3±0.3 vs 40.3±0.1°C; P<0.05), and no association between the
rate of Tbrain increase and performance was observed. These findings
suggest that the influence of Tbrain on performance under temperate
conditions is dependent on exercise protocol.