Robotic prosthetic ankles allow amputees more ‘natural’ and stable movement

Researchers at North Carolina State University (NC State) and the University of North Carolina at Chapel Hill built on previous work that found neural control of a powered prosthetic ankle can restore a range of abilities, including standing on challenging surfaces and squatting.

For this study, the researchers worked with five people who had amputations below the knee on one leg. Participants were fitted with a prototype robotic prosthetic ankle that responds to electromyographic (EMG) signals that are picked up by sensors on the leg.

Aaron Fleming, co-author of the study and recent Ph.D. graduate from NC State, said: “EMG sensors are placed over the muscles at the site of the amputation. When a study participant thinks about moving the amputated limb, this sends electrical signals through the residual muscle in the lower limb.

“The sensors pick these signals up through the skin and translate those signals into commands for the prosthetic device.”

Researchers tested participants’ responses to an ‘expected perturbation,’ that might throw off their balance. To ensure accurate replication, researchers developed a mechanical system designed to challenge the stability of participants.

Study participants were asked to respond to the expected perturbation under two conditions: using the prosthetic, ‘passive’ devices they normally used, and using the robotic prosthetic prototype.

It was concluded that participants were significantly more stable when using the robotic prototype, as they were less likely to stumble or fall.

Helen Huang, corresponding author of the study and professor in the Joint Department of Biomedical Engineering at NC State and UNC, said: “The robotic prototype allowed participants to change their postural control strategy. We found that the robotic ankle allows users to return to their instinctive response for maintaining stability.”

Researchers also asked participants to sway back and forth while using their normal prosthetic, compared to the robotic prosthetic, with sensors attached to measure muscle activity across the entire lower body.

Muscle activation patterns were found to be ‘very similar’ to the patterns seen in people who have full use of two intact lower limbs.

“This tells us that the prototype we developed mimics the body’s behaviour closely enough to allow people’s ‘normal’ neural patterns to return. This is important, because it suggests that the technology will be somewhat intuitive for users,” Huang said in a statement.

“We’re now conducting a larger trial with more people to both demonstrate the effects of the technology and identify which individuals may benefit most.”

The study was conducted with support from the US National Institutes of Health and the National Science Foundation.

A paper on the study has been published in Science Robotics and can be read in full here.