Knee exoskeletons to combat workplace fatigue and injury

 This device, they found, helps users maintain optimal lifting posture even when tired, an essential factor in reducing workplace injuries.

"Rather than directly bracing the back and giving up on proper lifting form, we strengthen the legs to maintain it," said Robert Gregg, U-M professor of robotics and lead author of the study published in Science Robotics. "This differs from what’s more commonly done in industry."

In professions where workers perform frequent lifting, such as in construction and manufacturing, back braces and back exoskeletons, equipped with springs or motors, are already in use. However, these devices support the back, which assumes an improper lifting posture, or stooping. Back exoskeletons can also be cumbersome, requiring deactivation for movements outside of the lifting task, Gregg noted. Instead, the Michigan team's knee exoskeleton focuses on supporting the quadriceps, the primary muscle group involved in safe squat lifting, providing a less intrusive way to reduce back injuries. During testing, study participants lifted and carried a 20lb kettlebell, both on flat ground and while ascending or descending inclines and stairs. Results showed that, while fatigued, the participants maintained better posture and lifted faster with the exoskeleton – just 1% slower than their pre-fatigued speed, compared to 44% slower without the exoskeleton.

"This is especially important when a worker has to keep up with a conveyor belt. Usually, when a worker is fatigued, they’ll keep up with that rate, but with a compromised posture. They’ll bend their back more, and that’s when injuries are most likely," explained Nikhil Divekar, postdoctoral research fellow at U-M and first author of the study.

Feedback from participants indicated overall satisfaction with the exoskeleton, with the exception of walking on flat ground, where they felt only moderately assisted. Gregg explained that minimal support is needed for the quadriceps in such conditions, as the task is relatively easy.

Two factors were instrumental in making the exoskeleton comfortable to wear: motor design and software. The motors are geared to allow free knee movement for a natural gait, while the software anticipates user needs by tracking the knee joint angle, leg orientation, and forces measured in a sensor embedded in the user’s shoe. This sensor gathers data 150 times per second, allowing the exoskeleton to seamlessly adapt to different activities without the delays found in typical exo controllers, which use fixed patterns and can lag when switching tasks. "If your exo is trying to walk upstairs, and you’re trying to walk downstairs, that could be a problem, right?" Gregg noted.

The new controller combines physics models with machine learning to prevent erratic movements if the user performs an unfamiliar action. With lab prototypes costing about $4,000, Gregg estimated that scaled production could reduce the cost to roughly $2,000 per pair. In the study, 10 participants – five men and five women – performed tasks both fresh and fatigued, achieving fatigue through repetitive squat lifts with the kettlebell. All participants were familiar with proper squat lifting techniques.

The study received funding from the National Institutes of Health. The team, supported by U-M Innovation Partnerships, has applied for a patent and is actively seeking partners to commercialise the technology.

Image depicts Emily Keller, a doctoral student in robotics at the Locomotor Control Systems Lab, University of Michigan, demonstrating the knee exoskeletons on the steps outside of the Ford Robotics Building at U-M. (Image credit: José Montes-Pérez, Robotics, University of Michigan)