The ankle exoskeleton is made from carbon fiber and weighs about one pound per leg. The 7 percent energy reduction it offers is about equivalent to removing a ten-pound backpack. Photograph courtesy of the National Science Foundation.
According to an article published in Nature on April 1, a team of researchers from Carnegie Mellon University (CMU) and North Carolina State University (NC State) have developed a lightweight, unpowered ankle exoskeleton to increase walking efficiency, dubbed the walking assist clutch. This wearable boot-like apparatus, when attached to the foot and ankle, reduces the energy expended in walking by about 7 percent. The device has implications for use in elderly individuals, those with mobility impairments, patients in need of rehabilitation, and workers who are on their feet all day.
The device is the result of eight years of work, first mapped out on a whiteboard by CMU assistant professor of mechanical engineering Steven Collins, PhD, and NC State assistant professor of biomedical engineering Greg Sawicki, PhD, when they were graduate students at the University of Michigan in 2007. The research was based upon work supported by the National Science Foundation.
Collins, Sawicki, and co-author M. Bruce Wiggin, PhD, a mechanical engineer at TransEnterix, Morrisville, North Carolina, analyzed the biomechanics of human walking and then designed a device that relieved the calf muscle of its efforts when it wasn’t doing any productive work.
“Studies show that the calf muscles are primarily producing force isometrically, without doing any work, during the stance phase of walking, but still using substantial metabolic energy,” Collins explained. “This is the opposite of regenerative braking. It’s as if every time you push on the brake pedal in your car, you burn a little bit of gas.”
With this insight in mind, the team created an ankle exoskeleton that offloads some of the clutching muscle forces of the calf, reducing the overall metabolic rate. The device uses a spring that acts like the Achilles’ tendon and a clutch that mimics the calf muscles. The difference is that the spring and clutch do not expend any energy the way tendons and muscles do. The device reduces the load placed on the calf muscles and the spring stores and releases elastic energy. The clutch engages the spring while the foot is on the ground, disengaging it while the foot is in the air.
“The unpowered exoskeleton works in parallel with your muscles, thereby decreasing muscle force and the metabolic energy needed for contractions,” said Sawicki.
One of the long-term goals of Collins and Sawicki’s project is to use energy-efficient exoskeletons to assist individuals with mobility issues.
“You can imagine these lightweight efficient devices being worn on the affected limb to help people with the permanent aftereffects of stroke,” Collins said. “We’re hopeful that designs that use similar techniques can help people who have had a stroke walk more easily. We’re still a little ways away from doing that, but we certainly plan to try.”
The team intends to test the current device with individuals who have a variety of mobility issues to determine what designs might work best for different populations. They are also interested in developing exoskeleton components for the knee and the hip, where they believe they may be able to garner even larger benefits.
Editor’s note: This story was adapted from materials provided by Carnegie Mellon University and the National Science Foundation.
