Time-varying human ankle impedance in the sagittal and frontal planes during stance phase of walking

This paper, for the first time, describes the estimation of the time-varying impedance of the human ankle in the sagittal (SP) and frontal (FP) planes during the stance phase of walking. The result of this work is aimed to provide design parameters for the development of 2-DOF powered ankle-foot prostheses capable of mimicking the time-varying impedance of the human ankle. Sixteen axes of rotations combining different amounts of SP and FP rotations were studied. For each axis, positive and negative rotations were considered separately. Four unimpaired male subjects walked on an instrumented vibrating platform that applied combined torque perturbations in the SP and FP simultaneously, while the ankle angles and torques were recorded. Based on the recorded data, the ankle impedance was estimated with a time resolution of 20 ms from 7% to 93% of the stance length (SL). The ankle stiffness and damping showed great variability through the SL and across axes of rotation. The maximum stiffness was 4.7±0.5 Nm/rad/kg at 0.21 s of the SL when the ankle rotated at an axis 22.5° from the SP combining Dorsiflexion (D) and Inversion (I). The minimum stiffness was 1.4±0.6 Nm/rad/kg at 0.05 s of the SL at an axis 45° from the SP combining D and Eversion (E). The maximum damping was 0.09±0.02 Nms/rad/kg at 0.21 s of the SL combining D and I at an axis 25° from the SP. The minimum was 0.02±0.01 Nms/rad/kg at 0.05 s of the SL combining P and I at an axis 45° from the SP.