State-of-the-Art Prosthetic Limbs: Achieving Natural Walking Gaits

Modern prosthetic limbs have significantly advanced, enabling people with amputations to walk with a natural gait. However, these prosthetics do not yet allow full neural control over the limb. Instead, they rely on robotic sensors and controllers that operate the limb using predefined gait algorithms.

2G6GGMY London (UK), 6 July 2021: A man with a prosthetic leg negates an escalator in central London shopping centre.

Breakthrough in Prosthetic Technology

Researchers at MIT, in collaboration with colleagues from Brigham and Women’s Hospital, have developed a revolutionary surgical intervention and neuroprosthetic interface. This advancement demonstrates that a natural walking gait is achievable with a prosthetic leg entirely driven by the body’s own nervous system. The new surgical amputation procedure reconnects muscles in the residual limb, providing patients with “proprioceptive” feedback regarding the position of their prosthetic limb.

Enhancing Mobility and Reducing Pain

In a study involving seven patients who underwent this surgery, the MIT team found that these individuals could walk faster, avoid obstacles, and climb stairs more naturally than those with traditional amputations. According to Hugh Herr, a professor of media arts and sciences, co-director of the K. Lisa Yang Center for Bionics at MIT, and the senior author of the study, “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.”

Patients who had this surgery, known as the agonist-antagonist myoneural interface (AMI), also experienced less pain and muscle atrophy. So far, approximately 60 patients worldwide have undergone this type of surgery, which is also applicable for arm amputations. The lead author of the paper, which will appear in Nature Medicine, is Hyungeun Song, a postdoc at MIT’s Media Lab.

The Role of Sensory Feedback

Most limb movements are controlled by pairs of muscles that alternately stretch and contract. Traditional below-the-knee amputations disrupt these paired muscle interactions, making it difficult for the nervous system to sense muscle position and contraction speed—critical sensory information for the brain to determine limb movement.

Individuals with such amputations struggle to control their prosthetic limb due to the lack of accurate spatial awareness. Instead, they depend on robotic controllers built into the prosthetic limb, which includes sensors to detect and adjust to slopes and obstacles.

Development of the AMI Surgery

To enable a natural gait under full nervous system control, Herr and his team developed the AMI surgery. Instead of severing natural agonist-antagonist muscle interactions, the ends of the muscles are connected, allowing them to dynamically communicate within the residual limb. This surgery can be performed during the primary amputation or as part of a revision procedure after the initial amputation.

Male amputee with prosthesis in hospital. Man sitting on chair with nurse touching prosthetic limb. Female nurse working with male patient. Rehabilitation, recovery, improvement.

Herr explains, “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.”

Testing and Results

In a 2021 study, Herr’s lab found that patients who underwent this surgery could more precisely control the muscles of their amputated limb, producing electrical signals similar to those from their intact limb. Encouraged by these results, the researchers explored whether these electrical signals could generate commands for a prosthetic limb and simultaneously provide users with feedback about the limb’s position in space. The new study in Nature Medicine confirmed that this sensory feedback translates into a smooth, near-natural ability to walk and navigate obstacles.

“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,” says Song.

Achieving a Natural Gait

The study compared seven individuals with the AMI surgery to seven with traditional below-the-knee amputations. All subjects used the same type of bionic limb: a prosthesis with a powered ankle and electrodes to sense electromyography (EMG) signals from the tibialis anterior and gastrocnemius muscles. These signals feed into a robotic controller that calculates the necessary movements.

The researchers tested the subjects in various scenarios, including level-ground walking, slope walking, stair climbing, and navigating obstacles. The AMI group walked faster, navigated obstacles more easily, and displayed more natural movements. They could coordinate their prosthetic and intact limbs better and push off the ground with similar force as non-amputees.

Herr notes, “With the AMI cohort, we saw natural biomimetic behaviors emerge. The cohort that didn’t have the AMI could walk, but their prosthetic movements weren’t natural, and their movements were generally slower.”

Implications for Future Prosthetics

These natural behaviors emerged even though the sensory feedback provided by the AMI was less than 20 percent of what non-amputees typically receive. “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 explains.

Matthew Carty, a surgeon at Brigham and Women’s Hospital and associate professor at Harvard Medical School, who also contributed to the paper, emphasizes the collaborative nature of this breakthrough, stating, “This work represents yet another step in us demonstrating what is possible in terms of restoring function in patients who suffer from severe limb injury.”

Rebuilding Human Bodies

Herr’s lab aims to “rebuild human bodies” rather than relying on increasingly sophisticated robotic controllers and sensors. “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 says. “The approach we’re taking is trying to comprehensively connect the brain of the human to the electromechanics.”

The research was funded by the MIT K. Lisa Yang Center for Bionics, the National Institute of Neurological Disorders and Stroke, a Neurosurgery Research Education Foundation Medical Research Fellowship, and the Eunice Kennedy Shriver National Institute of Child Health and Human Development.

A nerve-controlled prosthesis enables amputees to walk naturally