Center of gravity estimation of upper body in sitting posture from force plates measurement

Abstract

Perturbations of the center of gravity (COG) in sitting posture provide significant information for motion sickness and low back pain. Although a motion capture system generally measures the trajectory of the COG, it is difficult to apply the method to measure a vehicle passenger. In a previous study, we proposed a method to estimate the COG in standing posture from a force platform using the single inverted pendulum model. To apply the method to the sitting posture, we adopted a new mechanical model considering that a subject contacts the floor at two points, the seat and under the feet. The purpose of this study is to estimate the upper body COG in sitting posture from force platform measurements and compare the estimation accuracy between the two models, with and without lower limbs. If the method considers the lower limbs, force platforms are needed under the feet in addition to the seat. Since the estimation accuracy may vary depending on the subject’s motion, we implemented a validation test under three conditions: rest sitting test, voluntary motion test, and seat sway test. We checked the estimation accuracy with the correlation coefficient and the root mean square error (RMSE) compared with a motion capture measurement. All experiments demonstrated high estimation accuracy in the lower-limbs-considering model. This method can be used to analyze the behavior of vehicle passengers and may help analyze motion sickness and low back pain.