The human ankle plays a major role in locomotion as it the first major joint to transfer the ground reaction torques to the rest of the body while providing power for locomotion and stability. One of the main causes of the ankle impedance modulation is muscle activation [1, 2], which can tune the ankle’s stiffness and damping during the stance phase of gait. The ankle’s time-varying impedance is also task dependent, meaning that different activities such as walking at different speeds, turning, and climbing/descending stairs would impose different profiles of time-varying impedance modulation.
The impedance control is commonly used in the control of powered ankle-foot prostheses; however, the information on time-varying impedance of the ankle during the stance phase is limited in the literature. The only previous study during the stance phase, to the best of the authors knowledge, reported the human ankle impedance at four points of the stance phase in dorsiflexion-plantarflexion (DP)  during walking. To expand previous work and estimate the impedance in inversion-eversion (IE), a vibrating platform was fabricated (Fig. 1) . The platform allows the ankle impedance to be estimated at 250 Hz in both DP and IE, including combined rotations in both degrees of freedom (DOF) simultaneously. The results can be used in a 2-DOF powered ankle-foot prosthesis developed by the authors, which is capable of mimicking the ankle kinetics and kinematics in the frontal and sagittal planes . The vibrating platform can also be used to tune the prosthesis to assure it properly mimics the human ankle dynamics. This paper describes the results of the preliminary experiments using the vibrating platform on 4 male subjects. For the first time, the time-varying impedance of the human ankle in both DP and IE during walking in a straight line are reported.