Is sample entropy based entropic half-life and de-trended fluctuation analysis correlated and do they reflect phase regularity of center of pressure measurements?

Abstract

Abstract The reshape scale (RS) method computes the transition of Sample Entropy ( SEn ) from low to large values as the scale is increased. At the largest scale, SEn asymptotically converges to the maximum SEn evaluated using the average of SEn for the fully randomly re-ordered realizations of the original signal. The entropic half-life ( EnHL ) characterizes such a transition. The EnHL is the scale representing the midpoint of the transition and yields a measure for the temporal degradation of the regularity in a signal. In postural balance studies, the EnHL of the center of pressure (COP) signal can be interpreted as the time elapsed before the old sensory states is no longer utilized by the postural control system to adjust the current COP position. Other equally sensitive measures of regularity, such as de-trended fluctuation analysis (DFA), can be interpreted in the same way; however, it results in a dimensionless measure of regularity and complexity. The primary objective of this study was to experimentally demonstrate the correlation between the scaling exponent α calculated using DFA and the inverse of EnHL . When the COP signal was studied in the Fourier domain, a non-random structure was observed in the phase of the COP signal, which might be related to the neuromuscular characteristics of the postural control system. The second objective of this paper was to demonstrate that the EnHL and α both retain information contained in the phase of the Fourier transformed signal. It is shown in this paper that the EnHL and α are both sensitive to non-random structures of the phase of the Fourier transformed signal. Contrary to α , which is a dimensionless number, the EnHL measures the regularity and complexity of the signal in units of time. Therefore, it was concluded that the EnHL provides a more physically interpretable and intuitively understandable measure of the properties of the control of the COP signal.