Detection of the critical point interval of postural control strategy using wavelet transform analysis

Abstract : Postural balance is often studied in order to understand the effect of sensory degradation with age. The aim of this study was to develop a new method of detecting the critical point interval (CPI) at which sensory feedback is used as part of a closed-loop postural control strategy. Postural balance was evaluated using centre of pressure (COP) displacements measured using a force plate for 17 control subjects and 10 elderly subjects under control (eyes open) and experimental (eyes closed, vibration) conditions. A modified local-maximum-modulus wavelet transform analysis using the power spectrum of COP signals was used to calculate the critical point when closed-loop control occurs. Lower values of CPI are associated with increased closed-loop postural control, indicating a quicker response to sensory input. This strategy of postural control will require greater energy expenditure due to the repeated muscular interventions in order to remain stable. The CPI for elderly subjects occurred significantly quic er than for control subjects, indicating that posture was more closely controlled. Similar results were observed for eyes closed and vibration conditions. The CPI parameter offers a new method of detecting differences in postural control between different experimental conditions or changes due to ageing.