Development of three-dimensional motion measuring device for the human ankle joint by using parallel link mechanism

This paper presents a novel ankle motion measuring device that can measure three-dimensional motions without a motion capture system (MCS). We adapted a parallel link mechanism for the device using six wire-type displacement sensors to measure the ankle joint motions in six degrees of freedom (six-DOF). We define the motions of a foot plate which is attached to a foot sole as ankle joint motions. A posture of the foot plate, i.e., the three-dimensional position (x, y, z) and rotation angle (θ, Φ, ψ), is numerically calculated by solving the forward kinematics of the developed device. We conducted performance verification experiments of the developed device by comparing these results with those of the MCS. The experimental results show that the maximum root mean square error of the three-dimensional position and rotation angle measured by the developed device are 2.6 mm and 1.5°, respectively. This measuring performance of the developed device indicates that the ankle motion measuring device is accurate and valid. Moreover, this device enables physical therapists to easily measure ankle motions with an accuracy as high as that of an MCS.