A University of Michigan project to advance lower-limb prosthetic devices received renewed support from the National Institutes of Health with a grant of $3 million. Researchers are working to create a control model that moves seamlessly between activities like sit to stand, stand to walk, and going up and down stairs and inclines. The grant will also enable testing of the system on a commercially available robotic prosthetic leg.
Principal investigator Robert Gregg, PhD, an associate professor of robotics at the university, has been working on the system since 2013, and has seen success in controlling the position of the knee and ankle joints through a model that continuously represents all stages of the gait cycle. Previously, robotic prosthetic legs used separate controllers for each stage in the gait cycle like the heel strike, push-off, and swing. As a result, the control parameters for each model and rules for switching from one model to another had to be optimized for each patient.
“Every person has different parameters because every person walks differently. And that resulted in very, very cumbersome clinical deployment,” said Gregg, who is also an associate professor of electrical and computer engineering and mechanical engineering.
Using the motion of the thigh to predict joint position with a continuous model turned out to be a good way of creating a natural gait. With the initial grant in 2018, study participants could do inclines, stairs, sit to stand, and stand to walk with more typical biomechanics using the robotic leg than they could with passive prosthetics. However, pinning the control algorithm to joint angles leads to a more rigid experience when trying to change activities.
“The robot has very strong motors, and so, if you’re controlling the position and it’s somehow incompatible with the environment, it can feel very rigid and jarring,” Gregg said, which can cause pain where the limb meets the socket.
So the team is now looking at controlling joint position indirectly—mimicking biomechanical impedance instead—using their continuous modeling framework. With an impedance approach, there’s an equilibrium position, and the forces are set to gently pull the joint back into that position if it’s disturbed.
Gregg compared it to suspension on a car. “You have the spring, and then you have a shock-absorbing mechanism. You want to hit the pothole and have enough bounce to soften the jolt, but you don’t want to oscillate forever.”
This should enable the leg to offer the same ability to seamlessly move from one activity to another while also providing a smoother ride.
The programming of the leg is based on biomechanical measurements of people with two biological legs to replicate the motion that the hips and back evolved for. Users of passive prosthetic legs often experience pain in the hips, back, and organic knee due to the way they need to compensate for the dead weight of the prosthetic leg.
To learn how the leg should behave across activities, Gregg united with collaborator Elliott Rouse, PhD, who studies the mechanical properties of healthy human gait. Jeff Wensman, CPO, clinical collaborator from the University of Michigan Orthotics and Prosthetics Center, is also a co-investigator of the project.
“We obtain the measurements for determining the biomechanical properties of the leg using an exoskeleton,” said Rouse, associate professor of robotics and mechanical engineering, and co-investigator on the project. “The exoskeleton mostly provides no assistance but occasionally applies a quick perturbation that displaces the limb. From these measurements, we can determine the mechanical impedance, including properties like stiffness, viscosity, and inertia.”
The control programs will first be tested on the robotic leg that Gregg’s team built in-house, with motors to power both the ankle and knee. Then, to see if the new algorithms are ready to start helping people now, the team will test them on Össur’s Power Knee prosthetic leg. In addition to measuring the biomechanics of study participants as they walk with the prosthetic legs, the team will collect formal feedback to quantify increases in comfort and reductions in pain.
The Power Knee’s passive ankle has a smaller range of motion, but it is also lighter than the lab leg. Gregg’s team is confident that they can modify their control model to work on it, perhaps improving on the Össur models.
Editor’s note: This story was adapted from materials provided by the University of Michigan.
To watch a video of the device in action, visit the university’s website.
