Estimation of center of mass on sagittal plane during standing from force plate measurement (Mechanism of estimation error and evaluation of estimation accuracy)

Abstract

Center of pressure (COP) has been one of the most popular index for balance evaluation in quiet standing. However, the balance evaluation based on COP has been limited due to its accuracy. For accurate balance evaluation, an estimation method of the center of mass (COM) from simple measuring devices is desirable. The purpose of this study is to estimate COM on sagittal plane from force plate measurement. One of the most popular method suggested in past studies is to integrate the COM acceleration estimated from shearing force on support surface, however, this technique requires appropriate boundary conditions. Instead of the integration method, we adopted a COM estimation method via gravity term in equation of angular motion. While the method can estimate COM in real time, an estimation error arose from the assumption of a single rigid body model. To clarify the estimation error in theory, we introduced a modal coordinate (rigid body mode and zero shearing force mode) in the analysis of double-link model. As a result, we found that the estimation accuracy decreases due to the influence of acceleration of zero shearing force mode. From a practical point of view, if the amplitude of COP and the ratio of zero shearing force mode is relatively small, COM can be estimated with high accuracy. Validation of the present method was verified by comparing COM from force plate measurement with that from motion capture measurement in stationary standing test and horizontally sway test.

Keywords

• posture estimation
• force plate
• standing
• sagittal plane
• center of mass
• center of pressure