To overcome the limitations of passive prosthetic feet, powered prostheses have been developed, that can provide the range of motion and power of their human counterparts. These devices can equalize spatio-temporal gait parameters and improve the metabolic effort compared to passive prostheses, but asymmetries and compensatory motions between the healthy and impaired leg remain. Unlike their human counter part, existing powered prosthetic feet are fully monoarticular actuating only the prosthetic ankle joint, whereas in the biological counter part, ankle and knee joint are additionally coupled by the biarticular gastrocnemius muscle.
The goal of this work is to investigate the benefits of a powered biarticular transtibial prosthesis comprising mono- and biarticular actuators similar to the human example.
The contributions of the present work are as follows: A biarticular prosthesis prototype is methodically designed to match the capabilities of the monoarticular muscles at the human ankle joint as well as the biarticular gastrocnemius muscle during level walking. The prototype consists of an existing powered monoarticular prosthetic foot, which is extended with a knee orthoses and a stationary biarticular Bowden cable actuator. Both actuators are modeled as serial elastic actuators (SEA) and the identification of the model parameters is conducted. A model based torque control utilizing the measurements commonly available in SEAs, an impedance control law based on human ankle reference trajectories, and a high level control to enable steady walking in the lab are introduced. The proposed hardware setup and control structure can provide sagittal plane angles and torques similar to the mono- and biarticular muscles at the human ankle, with proper torque tracking performance and a freely adjustable allocation of torque between the monoarticular and biarticular actuator.
The biarticular prosthesis is evaluated in the gait lab with three subjects with unilateral transtibial amputation utilizing a continuous sweep experimental protocol to investigate the metabolic effort and spatio-temporal gait parameters. All subjects show a tendency to reduced metabolic effort for medium activity of the artificial gastrocnemius, although noise level and time variation are large. In addition to the reduction in metabolic effort, the artificial gastrocnemius is able to influence spatio temporal gait parameters between the impaired and the intact side, but partially opposing effects are observed among the individual subjects.
In conclusion, this thesis describes the implementation of an artificial gastrocnemius following the human example and the systematic investigation of metabolic effort and spatio-temporal gait parameters. It is shown that the addition of the artificial gastrocnemius to a monoarticular prosthesis can positively affect the investigated parameters. The meaningfulness of the results should be improved by increased clinical effort in future work
