Linear and nonlinear analysis of postural control in frailty syndrome

BACKGROUND Adaptive postural control can be impaired in the presence of frailty syndrome, given that this condition causes homeostatic dysregulation in physiological systems. OBJECTIVES To compare the center of pressure (CoP) displacements of non-frail, pre-frail, and frail elderly subjects in the standing position before and after postural transition of sitting and rising from a chair, using linear and nonlinear methods. METHODS Forty-two elderly subjects were divided into 3 groups: non-frail (n=15), pre-frail (n=15), and frail (n=12). The CoP displacements in the anteroposterior (AP) and mediolateral (ML) direction in the orthostatic position, 30s before and after sitting down and rising from a chair, were evaluated by means of linear measurements (root mean square (RMS), amplitude, and total average speed) and nonlinear measurements (corrected approximate entropy - CApEn), sample entropy (SampEn), and complexity index (CI) and its normalized versions. RESULTS After sitting and rising, there was an increase in RMS in the ML direction in all groups and in the AP direction in the non-frail and frail groups. The frail group showed no reduction in entropy values in either direction, and the pre-frail group showed no reduction in the ML direction. CONCLUSIONS The results of this study indicate that, in the presence of frailty syndrome, the organization of CoP displacements does not show less regularity after sitting and rising from a chair, reflecting a possible impairment of the integration of the systems involved in postural control.