Researchers at North Carolina State University have developed a robotic prosthetic ankle that can provide stability for lower limb amputees. The ankle uses electromyographic sensors placed on the sites of muscles in the residual limb that then convey the intentions of the wearer with regard to movement. So far, the system has been shown to assist with postural control, which in this context refers to the many complex and unconscious movements that the muscles in our legs make to maintain balance and keep us upright, even when we are largely standing still. Previously, lower limb amputees have sometimes struggled to maintain postural control, even with robotic prostheses.
Sometimes, even standing in one place requires some effort, particularly for those with a lower limb amputation. You may be unaware of the activity of your muscles in constantly maintaining balance and posture, even while standing still. This has been hard to recreate in robotic prostheses, but this latest technology aims to assist through a robotic ankle that can monitor muscle activity in the residual limb and make adjustments accordingly to assist in maintaining balance.
“This work focused on ‘postural control,’ which is surprisingly complicated,” said Helen Huang, one of the developers of the new technology. “Basically, when we are standing still, our bodies are constantly making adjustments in order to keep us stable. For example, if someone bumps into us when we are standing in line, our legs make a wide range of movements that we are not even necessarily aware of in order to keep us upright. We work with people who have lower limb amputations, and they tell us that achieving this sort of stability with prosthetic devices is a significant challenge. And this study demonstrates that robotic prosthetic ankles which are controlled using electromyographic (EMG) signals are exceptionally good at allowing users to achieve this natural stability.”
The researchers tested their robotic device with five volunteers who had previously undergone a below-knee amputation on one leg. “Basically, the sensors are placed over the muscles at the site of the amputation,” said Aaron Fleming, another researcher involved in the study. “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.”
The volunteers were then subjected to a pre-agreed perturbation that could throw off their balance, to see how the robotic ankle helped them to maintain their balance. “Specifically, the robotic prototype allowed study participants to change their postural control strategy,” said Huang. “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.”
Study in journal Science Robotics: Neural prosthesis control restores near-normative neuromechanics in standing postural control