Evaluation of personal balance characteristics based on a model during standing on a sway board

Abstract

Balance evaluation during standing is an important issue for diagnosis of disease and determination of training effect. The purpose of this study is to evaluate personal balance characteristics based on a model from the response of human body when the support surface moves horizontally as an external stimulus. The response of human body was defined as posture angle based on a single-link model and the angle was estimated from two kinds of measurement system. One is a motion capture system and the other is force plates mounted on the support surface. We applied the frequency analysis to the response and identified personal balance characteristics in an assumed balance control model. The balance control model was defined as a multiple feedback loop system which is composed of the delayed state feedback control as an inner-loop and target posture definition according to the velocity of the support surface as an outer-loop. To evaluate personal characteristics from the identified parameters, equivalent damping ratio and natural angular frequency were defined from a pair of conjugate complex roots of the system. As a result, we found that inner-loop characteristics were uniform when the posture angle was estimated from force plates. Outer-loop characteristics contributed to reduce the body sway in the frequency range from 0.1 Hz to 0.2 Hz in theory. The result of seven subjects implied that a subject who has less damping effect tends to use outer-loop system actively.

Keywords

• balance modelling
• frontal plane
• inverted pendulum
• single link
• force plate
• cascade control system