Today, commercially available ankle-foot prostheses are completely passive, and consequently, their mechanical properties remain fixed with walking speed and terrain. Conversely, normal human ankle stiffness varies within each gait cycle and also with walking speed. Furthermore, some studies have indicated that one of the main functions of the human ankle is to provide adequate energy for forward progression of the body. Thus, an ideal ankle-foot prosthesis should be able to actively control joint impedance, motive power, and joint position. Understanding the dynamic inter-action between an amputee and an active prosthesis is critical to developing truly functional leg pros-theses. For this reason, we have developed a novel robotic ankle-foot emulator. The emulator is capable of changing mechanical impedance and providing sufficient mechanical energy for forward propulsion. In this paper, we present pilot data on the actual interaction between an amputee and the ankle emulator during walking. These data support the hypothesis that actively controlling ankle joint impedance may provide a more natural gait than a conventional passive prosthesis. Furthermore, adding additional mechanical energy beyond that a passive spring ankle can provide during powered plantar flexion can dramatically increase the self-selected walking speed of an amputee.

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