Center of mass estimation of upper body in sitting posture using force platforms

Abstract

Perturbations in sitting posture, represented by the center of mass (COM) of the upper body, provide critical information for analyzing balance control. While the COM is generally measured by an optical motion capture system, it is not practical. In a previous study, we proposed a method to estimate the COM during standing from a force platform under the feet based on inverted pendulum models. However, it is unclear whether the technique can be applied to the sitting posture. Furthermore, it is also unclear whether two force platforms under the buttocks and the feet would be required if it could be applied. The purpose of this study is to estimate the upper body COM of a subject sitting on a backless seat from one or two force platforms and to validate the estimation accuracy by comparing the results to those from an optical motion capture system. For considering forces acting from the lower limbs, it is necessary to adopt a mechanical model that includes the lower limbs and to use a force platform under the feet in addition to those on the seat. We compared estimation accuracy between the models with and without lower limbs in three conditions: rest sitting, voluntary sway, and seat sway with an electric cart. The result indicated a clear difference between the two models, and the model considering lower limbs had high accuracy. Since the present method is a practical and highly accurate method of sitting posture, it may be applied in the analysis of low back pain and motion sickness.

Keywords

• sitting
• center of mass
• force platform
• sagittal plane
• seat sway