Testing for nonlinearity in time series: the method of surrogate data
Conference proceedings on Interpretation of time series from nonlinear mechanical systems
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Background: Endurance exercise is associated with a decrease in post-exercise sympathetic nerve activity (SNA) and arterial pressure (AP) in humans and experimental animals. In this study, we measured pulsatile arterial pressure continuously for at least 5 min in rats before (n = 22) and after 8 weeks of daily exercise (n = 10) exercise. The purpose of this study was to determine if daily exercise produces post-exercise changes in measures of nonlinearity of the AP. The reason to analyze this signal before and after exercise is due to our previous findings of increased nonlinearity during sympathetic activation and/or vagal withdrawal and also our finding of a decreased low frequency power (LF: 0.19-0.61 Hz) after endurance exercise training, as the decrease in the LF band of systolic blood pressure (BP), mean BP and pulse pressures is mediated by the sympathetic nervous system.Methods: The AP data were sampled at 200 Hz and we obtained nonlinearity measures for the entire 256 s of AP series sampled at 200 Hz (51 200 points). We also obtained five single AP wave signals of 180 ms duration and averaged them before and after exercise to calculate the 1/f slopes and the R2 values.Results: There was a significant decrease in nonlinearity scores of AP signal after exercise. There was a significant decrease in the 1/f slopes of AP after exercise.Conclusions: These results suggest that endurance exercise decreases AP variability and non-linearity probably by decreasing SNA. The changes in the 1/f slopes need further investigation as to the physiological significance. The nonlinearity scores, especially the SnetGS appears to be mediated by SNA.