Neurally Controlled Prosthetic Ankle Allows for Balance Correction

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Funding from the National Institute of Biomedical Imaging and Bioengineering (NIBIB) is allowing researchers to develop an ankle prosthesis that relies on users' residual muscles to help with their postural control. The prototype of the prosthesis is controlled by directly mapping the electrical activity generated from the user's muscles in the residual limb without the need for external sensors or complex automation. The neurally controlled, powered prosthetic device relies on training the user's residual muscles to create continuous control of posture and balance.

"This study is a culmination of work demonstrating that a prosthetic user can be trained to provide continuous neural control during various postural tasks," said Grace Peng, PhD, director of the NIBIB program in Mathematical Modeling, Simulation and Analysis. "This case study is an important first step in implementing this unique control framework for future ‘user-driven' prosthetic devices."

To use a joint, either consciously or subconsciously, our brain sends an electrical signal to the necessary muscle groups, resulting in the contraction of muscle fibers and enabling movement. After an amputation, the muscles in the residual limb can still receive these electrical signals, said Helen Huang, PhD, a professor in biomedical engineering at North Carolina (NC) State University and University of North Carolina (UNC) at Chapel Hill. Her research group, hoping to harness that signaling ability, is working to develop a lower-limb prosthetic device operated solely by the body's own electrical signals, a concept known as direct electromyographic  (dEMG) control. Huang and her team recently reported their case study evaluating this dEMG-controlled prosthetic, operated by an individual with a transtibial amputation, in Wearable Technologies.

When users contract their residual muscles, with the intention to flex their foot, for example, electrodes on their residual limbs collect and process the associated EMG activity. This EMG signal drives pneumatic artificial muscles, which function using pressurized air to contract or extend, allowing them to continuously control the movement of the artificial limbs.

"Often times, if part of our limb is removed, we start to use the muscles in the residual limb a little bit differently," said Huang. The training in this case study, which lasted three weeks and with the aid of a physical therapist, included daily life activities that require postural control, such as transitioning from sitting to standing, picking up a heavy object, or reaching forward.

"This regimen helps the user to train their muscles—and their brain—to better control the prosthetic ankle," said Aaron Fleming, a graduate student in the Huang lab and first author of this case study.

After the training, Huang and colleagues evaluated the participant's stability while he was performing specific tasks, either with his usual passive prosthetic ankle, or with the new dEMG-controlled device. His stability was markedly improved when wearing the dEMG-controlled device, even when standing on a foam surface, which requires additional postural control, or with his eyes closed, which tests balance. The researchers also looked at the synchronization between his intact limb and his artificial limb and found that it was far higher when he was wearing the dEMG-controlled prosthetic compared with his usual passive device, both when he was standing or when he was picking up a heavy load.

"Postural stability is something that many able-bodied people take for granted," Huang said. "Being able to stand in a crowded space without being afraid that you'll fall if someone bumps into you—this is one of the many challenges that people with lower-limb loss face," she said. "While this research captures just one case study, it demonstrates the feasibility of developing a device that allows the user to intuitively adjust their posture, which could greatly increase their quality of life."

In the image, courtesy of Neuromuscular Rehabilitation Engineering Lab at UNC/NC State, a schematic represents how the neurally controlled prosthetic ankle works.