Powered Ankle-Foot Prosthesis

Toward the goal of creating realistic leg prostheses, we are developing actuator and control systems for an ankle-foot prosthesis capable of both joint impedance and motive force modulation. To this end, three research goals are being advanced in parallel. First, we are investigating various strategies for measuring myoelectric activity (EMG) within the amputee's residual limb, including both invasive and non-invasive approaches. Second, we are developing a virtual muscle-control scheme where the amputee's EMG signals are employed to control directly ankle position and impedance. The strategy uses both a dynamics model of the physical prosthesis and a model of virtual co-contracting, skeletal muscles that dictate the mechanical response of the prosthesis. Third, we are designing muscle-like actuator systems capable of mimicking the dynamic response of normal human muscle. In the advancement of biologically realistic prostheses, we feel muscle-like actuators, biomimetic control schemes, and distributed sensing are important areas of consideration.

A novel, motorized ankle-foot prosthesis, called MIT Powered Ankle-Foot Prosthesis is developed. Unlike conventional passive-elastic ankle-foot prostheses, this prosthesis can provide active mechanical power or net positive work during the stance period of walking. The basic architecture of the prosthesis is a unidirectional spring, configured in parallel with a force-controllable actuator with series elasticity. With this architecture, the anklefoot prosthesis matches the size and weight of the human ankle, and is also capable of delivering high mechanical power and torque observed in normal human walking. We also propose a biomimetic control scheme that allows the prosthesis to mimic the normal human ankle behavior during walking. To evaluate the performance of the prosthesis, we measured the rate of oxygen consumption of three unilateral transtibial amputees walking at self-selected speeds to estimate the metabolic walking economy. We find that the powered prosthesis improves amputee metabolic economy from 7% to 20% compared to the conventional passive-elastic prostheses (Flex-Foot Ceterus and Freedom Innovations Sierra), even though the powered system is twofold heavier than the conventional devices. This result highlights the benefit of performing net positive work at the ankle joint to amputee ambulation and also suggests a new direction for further advancement of an ankle-foot prosthesis.