Quantitative Evaluation for Standing Stability during Anterior-Posterior Body Tilt Movement Using Inverted Pendulum Model(Mechanical Systems)

Abstract

The purpose of this study is to obtain the quantitative indices of evaluation for standing stability during anterior-posterior tilt movement. We assumed human body in standing position which is modeled as a inverted pendulum, and two parameters based on control engineering, namely, natural frequency and damping ratio were estimated from the model for the quantitative indices. Twelve healthy volunteers participated in the experiment. Subjects inclined forward as large as possible around axis of the ankle joint mainly from upright standing, and recovered to the initial standing posture again. This task was executed in four conditions on the assumption that the standing stability decreased respectively as follows; both legs standing with eyes open, both legs standing with eyes closed, one leg standing using a tiltable step that prevents lateral sway, one leg standing without the step. The angular data at hip and ankle joint while the subject performed tilt movement followed by recovery were measured, and natural frequency and damping ratio were estimated using theoretical model about all conditions. The estimated parameters were compared among four conditions. As the results natural frequency was more increased and damping ratio was more decreased according to the decrease of standing stability if the subject could execute the movement task by ankle joint mainly. On the other hand, when the larger movement at hip joint was measured, estimated indices suggested more unstable situation. It is confirmed these two parameters are useful as the quantitative indices of evaluation for standing stability.