Multi-axis Capability for Powered Ankle-Foot Prostheses

The ankle joint of lower extremity powered prostheses are generally designed to be capable of controlling a single degree of freedom (DOF) in the sagittal plane, allowing a focus on improved mobility in straight walking. However, the single DOF ankle movements are rare in normal lower limb actions such as walking on a straight path or turning when the ankle movements in both sagittal and frontal planes are significant. Therefore, the effectiveness of next-generation lower extremity prostheses may be significantly enhanced by improved understanding of the ankle dynamics in both sagittal and frontal planes during different maneuvers and by implementing strategies to account for these intricacies in prosthesis design. In this chapter, the concept of a multi-axis powered ankle-foot prosthesis is introduced. The feasibility of this concept, to the extent allowed, by a proof of concept prototype is shown. Further, the design kinematics and its mechanical impedance in non-load bearing conditions are evaluated and discussed. It is shown that the proposed cable-driven mechanism for the multi-axis powered ankle-foot prosthesis is capable of closely mimicking the ankle movements in both sagittal and frontal planes during step turn and walking on straight path.