Using a new type of surgical intervention and neuroprosthetic interface, researchers at Massachusetts Institute of Technology (MIT), in collaboration with colleagues from Brigham and Women’s Hospital, have shown that a natural walking gait can be achievable using a prosthetic leg fully driven by the body’s own nervous system. The surgical amputation procedure reconnects muscles in the residual limb, which allows patients to receive proprioceptive feedback about where their prosthetic limb is in space.
The MIT team found that the seven patients who had the procedure were able to walk faster, avoid obstacles, and climb stairs more naturally than people with traditional amputations.
“This is the first prosthetic study in history that shows a leg prosthesis under full neural modulation, where a biomimetic gait emerges. 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 Hugh Herr, PhD, a professor of media arts and sciences, codirector of the K. Lisa Yang Center for Bionics at MIT, an associate member of MIT’s McGovern Institute for Brain Research, and the senior author of the study.
Patients also experienced less pain and less muscle atrophy following this surgery, which is known as the agonist-antagonist myoneural interface (AMI). To date, about 60 patients worldwide have received this type of surgery, which can also be done for people with upper-limb amputations.
To try to help people achieve a natural gait under full nervous system control, Herr and his colleagues began developing the AMI surgery several years ago. Instead of severing natural agonist-antagonist muscle interactions, they connect the two ends of the muscles so that they still communicate with each other within the residual limb. This surgery can be done during a primary amputation, or the muscles can be reconnected after the initial amputation as part of a revision procedure.
“With the AMI amputation procedure, to the greatest extent possible, we attempt to connect native agonists to native antagonists in a physiological way so that after amputation, a person can move their full phantom limb with physiologic levels of proprioception and range of movement,” Herr said.
In a 2021 study, Herr’s lab found that patients who had this surgery were able to more precisely control the muscles of their residual limb, and that those muscles produced electrical signals similar to those from their intact limb.
The researchers then explored whether those electrical signals could generate commands for a prosthetic limb and at the same time give the user feedback about the limb’s position in space. The person wearing the prosthesis could then use that proprioceptive feedback to volitionally adjust their gait as needed.
“Because of the AMI neuroprosthetic interface, we were able to boost that neural signaling, preserving as much as we could. This was able to restore a person’s neural capability to continuously and directly control the full gait, across different walking speeds, stairs, slopes, even going over obstacles,” said Hyungeun Song, PhD, a post-doctorate researcher in MIT’s Media Lab and lead author of the paper.
For the study, the researchers compared seven people who had the AMI surgery with seven who had traditional transtibial amputations. All of the subjects used the same type of bionic limb: a prosthesis with a powered ankle as well as electrodes that can sense EMG signals from the tibialis anterior and the gastrocnemius muscles. These signals were fed into a robotic controller that helps the prosthesis calculate how much to bend the ankle, how much torque to apply, or how much power to deliver.
The researchers tested the subjects in several different situations: level-ground walking across a ten-meter pathway, walking up a slope, walking down a ramp, walking up and down stairs, and walking on a level surface while avoiding obstacles.
In all the tasks, the researchers found that the people with the AMI neuroprosthetic interface were able to walk faster—at about the same rate as people without amputations—and navigate around obstacles more easily. They also showed more natural movements, such as pointing the toes of the prosthesis upward while going up stairs or stepping over an obstacle, and they were better able to coordinate the movements of their prosthetic limb and their intact limb. They were also able to push off the ground with the same amount of force as someone without an amputation.
“With the AMI cohort, we saw natural biomimetic behaviors emerge,” Herr said. “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.”
These natural behaviors emerged even though the amount of sensory feedback provided by the AMI was less than 20 percent of what would normally be received in people without an amputation.
“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,” Song said.
Editor’s note: This story was adapted from materials provided by MIT.
To watch a video of the device in action, visit the MIT YouTube page.
