Analysis of static equilibrium in healthy subjects by the calculation of sample entropy

Abstract

Purpose. The aim of this study was to determine the sample entropy values (SampEn) during the analysis of static equilibrium in healthy people.

Methods. We have studied 12 healthy subjects (5 males and 7 females) 18 to 25 years old. The gravity center projection over a dynamometric platform was recorded with both open and closed eyes while in a relaxed and static bipedal position. The test duration was 60 seconds with a sampling frequency of 60 Hz, 3,600 points being obtained for each recording. We analyzed total displacement, dispersion and displacement of the signal on X and Y axis, total swept area and maximal force in each axis. SampEn was calculated from the original X and Y series.

Results. All the linear parameters are higher when they are recorded with closed eyes, although the difference is significant only for the total swept area and the values related to Y axis. SampEn values for X axis are 1.53 ± 0.39 (open eyes) and 1.53 ± 0.53 (closed eyes). The values are 1.37 ± 0.42 and 1.06 ± 0.57, respectively, for the Y axis.

Conclusion. The gravity center performance over Y axis is more predictable than over the X one, this being even greater with closed eyes. The entropy value in X axis indicates that the system maintains high complexity and that it remains constant regardless of whether the eyes are open or closed.