Brain-controlled ankle prosthesis promotes natural movement, stability

Neural control of a robotic ankle prosthesis appears to help its users to employ mechanisms that the body naturally uses to maintain balance and stability, including ankle movements, for better postural control, as shown in a study.

Called direct electromyographic (dEMG) control, the neural control strategy leverages electrical signals from the remaining muscles, which previously controlled the amputated joint, to modify the movements of an artificial joint directly and continuously. [J Neural Eng 2021;18:10.1088/1741-2552/ac1176]

“This control paradigm follows the biological process for limb movement production, restores the biomechanical influence of residual muscle activation, and enables individuals to continuously operate their prosthetic joint based on the neural control commands in humans, just like control of an intact joint,” the investigators explained in their paper. [Sci Robot 2023;doi:10.1126/scirobotics.adf5758; IEEE Trans Neural Syst Rehabil Eng 2016;24:573-581; IEEE/ASME Trans Mechatron 2012;18:1191-1200; IEEE Int Conf Rehabil Robot 2013:2013:6650391]

“Basically, the sensors are placed over the muscles at the site of the amputation,” said lead study author Dr Aaron Fleming of North Carolina State University, Raleigh, North Carolina, US, in a statement. “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.”

Fleming and colleagues were confident that dEMG could assist with anticipatory and compensatory postural control, which is crucial for maintaining balance when individuals know a perturbation is impending versus when they are blinded to the timing as well as after a perturbation has already occurred—something that current lower-limb prostheses are not able to help with. [J Electromyogr Kinesiol 2010;20:398-340; J Electromyogr Kinesiol 2015;25:400-405]

Device testing

Five male participants with transtibial amputation (mean age 43.4 years; TT group) underwent training with the dEMG-controlled device. These participants were evaluated under an expected perturbation, a condition that could potentially disrupt the participants’ balance and stability. Response to the expected perturbation was examined under two conditions, such as while using the dEMG-controlled device and while using the daily, passive device that the participants had currently.

“We found that study participants were significantly more stable when using the robotic prototype. They were less likely to stumble or fall,” Fleming said.

“Specifically, the robotic prototype allowed study participants to change their postural control strategy," added study co-author Prof Helen Huang of North Carolina State University, Raleigh, North Carolina, US.

“For people who have their intact lower limb, postural stability starts at the ankle. For people who have lost their lower limb, they normally have to compensate for lacking control of the ankle. We found that using the robotic ankle that responds to EMG signals allows users to return to their instinctive response for maintaining stability,” Huang said.

In a separate experiment, the participants underwent a postural sway task (ie, sway back and forth) while using the dEMG ankle prosthesis and while using the normal prosthesis. Sensors were used to capture and measure the participants’ muscle activity across the entire lower body.

“We found that muscle activity patterns in the lower body were very different when people used the two different prostheses,” Huang noted.

“[M]uscle activation patterns when using the prototype prosthetic were very similar to the patterns we see in people who have full use of two intact lower limbs. That 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,” she added.

Huang shared that they are currently conducting a larger-scale trial with more participants both to demonstrate the effects of the dEMG-controlled device and identify which individuals may benefit most.