A, a typical example of the rapid rise in heart rate at the onset of wheel running. B, effect of chronic voluntary exercise training on the mean peak rate of heart rate increase at the onset of wheel exercise (means ± SEM, n = 6). This shows a significant rise in the peak gradient of heart rate onset (P < 0.01), with most of the increase occurring between days 7 and 15 of exercise. C, a typical example of the rapid fall in heart rate at the cessation of wheel running. D, effect of chronic voluntary exercise training on the mean peak rate of heart rate recovery at the cessation of wheel exercise (means ± SEM, n = 7). This shows a significant rise in the peak gradient of heart rate recovery (P < 0.01), reaching a peak at day 20 and plateauing thereafter.