Center of pressure preprocessing methods to analyze complexity/performance relation in quiet standing

Abstract

 

Objective. The aim of this study is to analyze the relationship between the complexity and postural balance in a standing balance task. In this way, this study assesses the effect of the sample frequency, the stationarity of center of pressure (COP) and the used of different analysis.

Method. Twenty-one volunteers were asked to stand still for 30 s on a stable surface. Displacement of COP was measured. The performance was assessed through the standard deviation (SD) of COP. Complexity of COP was measured through Sample Entropy (SampEn) and Detrended Fluctuation Analysis (DFA). All variables were analyzed on three different frequencies (20,100 and 1000 Hz) and on two signal treatment (the original and detrended signal).

Results. The sample frequency influences the values of SampEn and DFA. However, these values keep the trend shown between experimental conditions. SD values are negatively related with SampEn of COP data in both axes. Nevertheless, the relationship between SD and DFA varies by axis; this relationship is negative in the AP axis and positive in the ML axis.

Conclusion. The sample frequency has a significant influence in the results, but frequencies close to the dynamics of COP enhance the differences. On the other hand, we suggest to detrend the signal to improve the analysis of dynamics of COP. The results support the direct relationship between performance and complexity of COP.