Age Effect on Static Balance: Center-of-Pressure Analysis in Chaotic and Frequency Domains

The aim of this study was to characterize prolonged standing and its effect on postural control in elderly individuals in comparison with adults. It is unknown how elderly individuals behave during standing and how demanding such a task is for them. We recorded the center of pressure (COP control signal) position of 12 elder participants and 12 young individuals while they performed standing for time duration of 30 seconds. The COP’s displacement of a subject can be used to quantify the postural stability; from this measure, we deduced the stabilogram which is the representation of the COP time series in anteroposterior and mediolateral direction. The stabilogram is known to be nonlinear and nonstationary signal. Some features are defined first and an analysis is then performed to find the most appropriate and discriminative features for elder and young subjects posture signals discrimination. Features are selected in frequency and time domains. In addition, largest Lyapunov exponents of the COP control signals are computed to show the impact of chaotic behavior in static balance characterization relative to the age. Implementations on the normal subjects demonstrate that working in frequency domain is preferred compared to the time domain analysis. Moreover largest Lyapunov exponent of the posture signals can be representatively used for COP signal discrimination between two groups. For an investigation and analysis of static balance for elders and unhealthy subjects, the signal of COP can be studied in chaotic domain beside frequency domain and characteristic features can be extracted from the signal in both chaotic and frequency domains.