Supine and standing sympathovagal balance in athletes and controls

Differences in autonomic nerve activity between athletes and controls during supine rest and standing were investigated by recording the cardiac rhythm in 18 professional cyclists and 11 controls. We computed four indexes of autonomic control: the standard deviation (SD) of the interbeat intervals, the coefficient of variance (CV) of the interbeat intervals, the percentage of successive intervals differing by more than 50 ms (pNN50), and the fraction low-frequency (0.07-0.14 Hz) spectral power (LF), and we also measured the mean interbeat interval (MI). Significant differences (Student's t-test, P < 0.005) between the athletes and the controls in the supine position were found for pNN50 [mean 52.6 (SEM 2.5) vs 37.1 (SEM 3.4)%], LF [mean 32.2 (SEM 1.6) vs 40.7 (SEM 2.1) normalized units], and MI [mean 1241 (SEM 20) vs 1021 (SEM 25) ms]. A significant difference between the athletes and the controls in the standing position was found for MI [mean 888 (SEM 13) vs 801 (SEM 23) ms]. These results would suggest that there is a parasympathetic predominance in athletes in the supine, but not in the standing position. The finding that pNN50 and LF, but not SD and CV, differed between the athletes and the controls, would seem to demonstrate that the differences in autonomic control between the athletes and the controls are reflected in the quality (balance between slow and fast heart rate fluctuations) rather than in the quantity of heart rate variability.