• Posted July 1, 2024

Prosthetic Legs Controlled by Person's Own Neural System Bring Natural Gait

"Smart"prosthetic legs can help amputees achieve a natural walking gait, but it's done through robotic sensors and algorithms that drive the limb forward at predetermined rates.

A better way would be to give people full control over the limb through their nervous system -- and that's just what an MIT research team says it's done.

An experimental surgical procedure combined with a cutting-edge robotic limb can restore a completely natural walking gait, fully driven by a person's own nervous system, researchers report in the July 1 issue of the journal Nature Medicine.

The procedure reconnects muscles in the residual limb, allowing patients to receive accurate, real-time feedback about the position of their prosthetic limb while walking, researchers explained.

Seven patients who had this surgery were able to walk faster, avoid obstacles and climb stairs much more naturally than people with a traditional amputation.

"No one has been able to show this level of brain control that produces a natural gait, where the human's nervous system is controlling the movement, not a robotic control algorithm,"said senior researcher Hugh Herr, co-director of the K. Lisa Yang Center for Bionics at MIT.

Most arm and leg movement is controlled by pairs of muscles that take turns stretching and contracting, researchers said in background notes.

A traditional below-the-knee amputation disrupts the interaction of these paired muscles, making it difficult for the nervous system to track and control movement.

As a result, people with that kind of amputation struggle to control a prosthetic leg because they can't accurately sense where the leg is in space. They must rely on robotic controllers and sensors to establish a walking gait and adjust to slopes and obstacles.

To help people achieve full neural control over their prosthetic legs, Herr and his colleagues developed what they call agonist-antagonist myoneural interface (AMI) surgery.

Instead of simply severing muscle pairs, AMI surgery instead connects the two ends of the muscles. That way they still dynamically communicate with each other inside what's left of the leg.

AMI surgery can be done during an amputation, or during a follow-up procedure after the initial amputation, researchers said.

A 2021 study by Herr's lab found that the muscles of a limb treated with AMI surgery produced electrical signals similar to those emitted by their intact limb.

As a next step, the researchers started figuring out a way for those electrical signals to generate commands to a prosthetic limb, and at the same time receive feedback from the limb about its position while walking.

That way, an AMI surgery amputee could both control a prosthetic leg and use the feedback to automatically adjust their gait as needed.

The new study shows that the sensory feedback does indeed translate into a smooth, near-natural ability to walk and navigate obstacles.

In the study, researchers compared seven AMI amputees with seven people who had traditional below-the-knee amputations.

All participants used the same type of bionic leg -- a prosthesis with a powered ankle, equipped with electrodes that can receive electrical signals from the major muscle groups of the leg.

These signals are fed into a robotic controller that helps the prosthesis calculate how much to bend the ankle, how much torque to apply and how much power to deliver.

Researchers tested all of the amputees with level-ground walking, walking up a slope, walking down a ramp, climbing up and down stairs and strolling on a level surface while avoiding obstacles.

Those who'd received the AMI amputation surgery were able to walk faster, at about the same rate as people without amputations.

They also navigated obstacles more easily and had more natural movement, results show. They were better able to coordinate the movement of their prosthetic limb to that of their natural limb, and could push off the ground with about the same amount of force as someone without an amputation.

"The cohort that didn't have the AMI, they were able to walk, but the prosthetic movements weren't natural, and their movements were generally slower,"Herr said. in an MIT news release.

Interestingly, the improved movement occurred even though the amount of sensory feedback provided by AMI is less than 20% of that normally received by people who still have their full leg, researchers noted.

"One of the main findings here is that a small increase in neural feedback from your amputated limb can restore significant bionic neural controllability, to a point where you allow people to directly neurally control the speed of walking, adapt to different terrain and avoid obstacles,"said lead researcher Hyungeun Song, a postdoctoral researcher in MIT's Media Lab.

Herr's goal is to ultimately "rebuild"human bodies by bonding prostheses to limbs in ways that make them feel natural and normal.

"The problem with that long-term approach is that the user would never feel embodied with their prosthesis. They would never view the prosthesis as part of their body, part of self,"Herr said. "The approach we're taking is trying to comprehensively connect the brain of the human to the electromechanics."

More information

Johns Hopkins Medicine has more about prosthetic legs.

SOURCE: MIT, news release, July 1, 2024