Nonlinear Analysis of Electromyography Time Series of Low Back Muscles

We have calculated the mean-square displacement $\Delta (t)$ from the EMG time series and have found that increases diffusively, $\Delta ( t) \sim t$, for short times $t < 10 \mbox{ms}$, and shows a plateau-like behavior, $\Delta (t) \sim t^{0}$, for interintermediate times $10 \, \mbox{ms} < t < 0.5 , \mbox{s}$. The plateau-like behavior implies the presence of correlations in the signal. We characterize these correlations and relate them to properties of the power spectrum of the signal.