A case study from North Carolina State University and the University of North Carolina at Chapel Hill demonstrated that neural control of a powered prosthetic ankle can restore a wide range of abilities, including standing on challenging surfaces and squatting. The researchers are currently working with a larger group of study participants to see how broadly applicable the findings may be.
“This case study shows that it is 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,” said Helen Huang, PhD, corresponding author of the study. Huang is the Jackson Family Distinguished Professor in the Joint Department of Biomedical Engineering at NC State and UNC.
“This work demonstrates that these technologies can give patients the ability to do more than we previously thought possible,” said Aaron Fleming, first author of the study and a PhD candidate in the joint biomedical engineering department.
Most of the existing research on robotic prosthetic ankles has focused solely on walking using autonomous control. Autonomous control, in this context, means that while the person wearing the prosthesis decides whether to walk or stand still, the fine movements involved in those 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 on activities that are challenging with typical prostheses. Would it be possible for people with amputations to regain a fuller range of control in the many daily motions that people make with their ankles in addition to walking?
The powered prosthesis in this study reads electrical signals from two residual calf muscles that are responsible for controlling ankle motion. The prosthetic technology uses a control paradigm developed by the researchers to convert electrical signals from those muscles into commands that control the movement of the prosthesis.
The participant, who has a transtibial amputation, was fitted with the powered prosthetic ankle and did an initial evaluation. He then had five training sessions with a physical therapist, each lasting about two hours, over the course of two-and a half weeks. After the training was completed, the participant did a second evaluation.
After training, the study participant was able to do a variety of tasks that had been difficult before, such as going from sitting to standing without any external assistance or squatting to pick something up off the ground without compensating for the movement with other body parts. But 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 said. “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.”
“There is also a profound emotional impact when people use powered prosthetic devices that are controlled by reading the electrical signals that their bodies are making,” Fleming said. “It is much more similar to the way people move intuitively, and that can make a big difference in how people respond to using a prosthesis at all.”
The researchers are already having more people go through the training paradigm and are expanding their testing to assess the results of that training.
“Powered prostheses that exist now are very expensive and are not covered by insurance,” Fleming said. “So, there are issues related to access to these technologies. By attempting to restore normal control of these type of activities, this technology stands to really improve quality of life and community participation for individuals with amputation. This would make these expensive devices more likely to be covered by insurance in the future if it means improving the overall health of the individual.”
The paper, “Direct Continuous EMG Control of a Powered Prosthetic Ankle for Improved Postural Control after Guided Physical Training: A Case Study,” was published in the journal Wearable Technologies.
Editor’s note: This story was adapted from materials provided by North Carolina State University.