Inertial Sensor Based Estimation of the Center of Pressure during Unperturbed Upright Stance

The unperturbed upright stance is considered one of the most important task for evaluating active and passive control of balance. In this field, the analysis of the center of pressure (COP) displacement is a valuable source of information for extracting and evaluating postural features in healthy and pathological subjects. COP is commonly measured by dynamometric force plates (DFP), which represent the gold-standard for measuring dynamic quantities. However, the availability of DFP is often limited by their cost and by the need for instrumented environments. In the past few years, inertial measurement units (IMU) have been proposed for estimating ground reaction forces and torques during selected motor tasks and under specific experimental conditions. Therefore, the aim of this study was to propose a novel method for deriving COP displacement during unperturbed posture by using a single sacrum-worn IMU. Results showed a remarkable correspondence between estimated COP time-series and those measured simultaneously by a DFP (root mean square error not beyond 1.5 mm, with a correlation not under 0.98). Further, the COP mean distance presented biases which are below 2 mm for both anterior-posterior and medial-lateral COP components, with the absence of any tendency towards proportional errors. Outcomes of this study seem to suggest the reliability of a single IMU based method for estimating COP displacement during balance maintenance task, providing a low-cost and unobtrusive procedure suitable for those contexts where classical motion analysis instrumentation is hardly or totally unavailable.