Though critical for walking, ankles play a larger role in human movement, allowing people to stand up comfortably, balance on uneven surfaces, or squat in place – functions that have so far not been the focus for prosthetics.

Researchers at North Carolina State University (NC State) and the University of North Carolina (UNC) at Chapel Hill are hoping to change that with training and neural control of a powered prosthetic ankle to restore a wide range of abilities.

“This case study shows that it’s possible to use these neural control technologies, in which devices respond to electrical signals from a patient’s muscles, to help patients using robotic prosthetic ankles move more naturally and intuitively,” says Helen Huang, corresponding author of the study.

“This work demonstrates that these technologies can give patients the ability to do more than we previously thought possible,” says Aaron Fleming, first author of the study and a Ph.D. candidate in the joint biomedical engineering department.

When the participant used his daily prosthesis (left) he had a limited range of motion and had to bend his back significantly to lift objects off the ground. When using a neural controlled prosthetic ankle (right) the participant was able to voluntarily control his prosthesis ankle joint force and angle and keep a healthy posture while lifting weight from the ground.
PHOTO: AARON FLEMING

Most existing research on robotic prosthetic ankles has focused solely on walking using autonomous control – the person wearing the prosthesis decides whether to walk or stand still, the fine movements happen automatically rather than because of anything the wearer is doing.

Huang, Fleming, and their collaborators wanted to know what would happen if an amputee, working with a physical therapist, trained with a neurally controlled powered prosthetic ankle to complete activities that challenge typical prostheses.

The powered prosthesis reads electrical signals from two residual calf muscles that are responsible for controlling ankle motion. The technology’s control paradigm converts electrical signals from those muscles into commands that control prosthesis movement.

The study participant, who had lost one leg about halfway between the knee and the ankle, was fitted with the powered prosthetic ankle and did an initial evaluation. The participant then had five training sessions with a physical therapist throughout two and a half weeks. After training, the study participant was able to sit or stand without external assistance or squat to pick something up off the ground without compensating for other body movements.

One of the most pronounced differences was the study participant’s stability, whether standing or moving. This was reflected in both empirical evaluations – such as testing the patient’s stability when standing on foam – and in the patient’s level of confidence in his own stability.

“The concept of mimicking natural control of the ankle is very straightforward,” Huang says. “But implementation of this concept is more complicated. It requires training people to use residual muscles to drive new prosthetic technologies. The results in this case study were dramatic. This is just one study, but it shows us what is feasible.” The researchers are already having more people go through the training paradigm and are expanding testing to assess results.

NC State University
https://www.ncsu.edu

The University of North Carolina at Chapel Hill
https://www.unc.edu