A leading cause of disability in the United States is hemiparesis, a condition where impaired motor control, muscle weakness, and spasticity affect one side of the body. Occurring in 80% of stroke survivors, reduced mobility and decreased quality of life are challenges that impact millions of individuals.
Walking may seem like a simple activity, but it relies on complicated biomechanics. The impact of even a small loss of strength on one side of the body is multiplied as muscles on the other side engage to compensate. As a result, those with hemiparesis expend 60% more energy walking than those with a healthy gait. This means slower walking speeds, lower endurance, more pain, and a greater risk of falls.
Researchers at the University of Utah’s John and Marcia Price College of Engineering are now piloting a device that can rebalance this equation. In a paper recently published in the journal Nature Communications, the team has shown that their portable, lightweight hip exoskeleton can reduce the energy required to walk by nearly 20% in individuals with hemiparesis after stroke.
The five-and-a-half-pound device is worn around the hips and straps to the user’s thighs. Battery-powered motors help move the user’s legs with every step, leading to a more efficient gait. The exact level of motor assistance on each side is custom-tuned for each user, and an intelligent control system synchronizes with them in real time, providing a boost exactly when the hip needs to lift or push off.
“Improving quality of life after a stroke is one of the biggest unmet challenges in healthcare today,” says Tommaso Lenzi, associate professor in the Department of Mechanical Engineering and the senior researcher on the study. “We’re now showing that robotics can make a measurable impact here.”
Kai Pruyn, a graduate student in the Lenzi Lab, led the study; lab members Rosemarie Murray, a postdoctoral scholar, and Lukas Gabert, a research scientist, contributed to it. The researchers collaborated with Bo Foreman, professor of Physical Therapy & Athletic Training in the College of Health.
Other types of powered exoskeletons have been explored in earlier attempts at solving this problem. Identifying foot drop and impaired ankle propulsion as key contributors to this problem, these prototypes were built to assist with ankle mobility.
“Portable ankle exoskeletons have failed to reduce the energy required for stroke patients to walk, so we proposed a different approach,” Pruyn says. “Patients with ankle weakness often compensate with their hip joints, which requires extra energy. Our goal was to develop a powerful and fully portable hip exoskeleton. Hip exoskeletons can also be extremely lightweight because they are worn closer to the user’s center of mass and have lower torque requirements compared to ankle exoskeletons. We found that the hip assistance effectively compensated for reduced ankle propulsion.”
Other research groups have also experimented with hip exoskeletons, generally showing that they could increase gait efficiency in healthy populations, but Lenzi’s group is the first to demonstrate their effectiveness in patients with hemiparesis.
The study implemented precise motion-capture techniques to analyze the gait of seven patients with hemiparesis as they walked on an instrumented treadmill, both with and without the exoskeleton. The participants also wore equipment that estimated their caloric expenditure. From that data, the researchers calculated the metabolic cost of walking under both conditions.
The results were substantial: using the exoskeleton offloaded nearly 30% of the work from the hip joints, translating into an 18% decrease in the overall metabolic cost of walking.
“For a person with a healthy gait, this would be like taking off a 30-pound backpack,” says Foreman. “For someone with hemiparesis, that’s a life-changing difference.”
“In the beginning, I couldn’t move my leg,” said Lidia, one of the study participants.”But with the device, it’s much better now.”
“In a way, the exoskeleton was doing some of that movement for her,” added Marcellus, her husband. “The more we used it, the better she was when she wasn’t using it.”
Next steps for the research team include making sure the hip exoskeleton is safe and effective at home and in daily life. This will require improvements to the device’s mechanics and controls to support activities beyond walking. The lab is partnering with leaders in prosthetics and orthotics to translate this technology into a product that anyone can use.
“Our goal is to ensure that a stroke doesn’t define the limits of where a person can go or how they can live,” Lenzi says.