Ankle kinematics describing gait agility: Considerations in the design of an agile ankle-foot prosthesis

The designs of available lower extremity powered prostheses are focused on a single degree of freedom (DOF) in sagittal plane, allowing the control of their ankle joints in dorsiflexion and plantarflexion. The human gait however, shows that the ankle movements in both sagittal and frontal planes are significant even during walking on a straight path. Additionally, there is a significant change in the ankle movements during straight walking compared to turning and cutting, especially in frontal plane. A better understanding of the ankle characteristics in both sagittal and frontal planes may result in the design of significantly more effective lower extremity prostheses that mimic the ankle function and improve the agility of gait. In this paper, the ankle rotations are estimated during step turn and cutting to provide evidence for necessity of a multi-axis design while providing the preliminary design parameters for a prototype multi-axis powered ankle-foot prosthesis. It is shown that the proposed cable-driven prototype is capable of closely mimicking the ankle movements in both sagittal and frontal planes during turning and walking on a straight path.