Balance response of human standing on a cart at constant frequencies (Interpretation of the steady-state response based on mechanical models)

Abstract

As a method for the balance assessment, a steady-state response of a subject standing on the support surface oscillating horizontally back and forth at a single frequency has been studied. While past studies on the method have focused on the kinematic analysis investigating the center of mass (COM) motion, there has no research based on mechanical models. The purpose of this study is to present mechanical models that concisely represent the steady-state response depending on the oscillation frequencies. This study implemented oscillation tests on six healthy subjects at 0.4, 0.6, 0.8, and 1.2 Hz. The steady-state response in the test tended to keep the head stationary in space and to suppress the amplitude of the center of pressure (COP). From the analysis based on a double inverted pendulum model, we found a boundary frequency around 0.7 Hz where the body keeps a straight-line and the head can be stationary without using ankle torque. Additionally, the analysis based on the double inverted pendulum model indicated that the steady-state response below the boundary frequency could be represented by a single inverted pendulum model. From comparing the theoretical feedback gain of the ankle torque for making the head stationary in the single inverted pendulum model, we found that the control strategy of some subjects changed significantly at 0.4 Hz. These findings were consistent with the results of the past studies and may be useful for understanding the steady-state response of the oscillation at constant frequencies.