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Prosthetic Limb Design and Control based on Gait Templates

Final Activity Report Summary - PRO-LEG (Prosthetic Limb Design and Control based on Gait Templates)

Estimates show that one out of ten European Union citizens will be affected by a disability in the future. Echoing this development, rehabilitation engineering is one of the fastest growing fields in industry. In particular, technological advances in lightweight actuators and high density batteries have made it possible to envision powered leg prostheses that restore or replace lost limb functionality in everyday activities of disabled citizens. Despite this opportunity, however, commercial leg prostheses remain passive devices without actuation. One reason for this discrepancy is that relatively little is known about how powered leg prostheses can be controlled to provide the functionality observed in the human leg.

Within the project 'Lower Limb Prostheses Design and Control based on Gait Templates' (PRO-LEG) we followed a specific approach to advance the science and technology of powered leg prostheses. The approach was based on principles of legged mechanics which predicted that a substantial part of human leg control happened at the level of muscle reflexes in an autonomous and self-organising manner. The main goals of PROLEG were to identify such muscle reflexes and demonstrate their potential for improving the functionality of powered leg prostheses.

We firstly developed a computer simulation model of human gait to accomplish these goals. The model control was based on known principles of legged mechanics, which were integrated as muscle reflexes in a step-by-step way from conceptual to more detailed reflexes relevant to human physiology. With this neuromuscular model we found that human walking and its control could organise itself from the dynamic interplay of reflex-driven legs with the ground. For instance, we found that the model automatically adapted to stair ascent and decent while predicting individual muscle activities known from human experiments in steady-state walking.

In a second step, we transferred the model muscle-reflex control to the control of a powered ankle prosthesis. The ankle prosthesis was powered by a motor that flexed and extended a prosthetic foot and could measure the ankle angle and torque. The controller of the prosthesis mimicked the muscle-reflex control of the human model in the background, outputting the desired ankle torque based on the measured state of the prosthesis. Clinical trials with a transtibial amputee walking on level ground and on ramps revealed an automatic adaptation of the prosthetic ankle behaviour, in a manner comparable to intact subjects, without the difficulties of explicit terrain sensing.

The results of PROLEG highlighted the importance of principles of legged mechanics for understanding human motor control, as well as of neuromuscular controllers for improving the functionality of powered leg prostheses. Eventually, these results might change how we understood human locomotion and engineered artificial legs, improving the quality of life of disabled people.