Unpowered Ankle Exoskeleton Takes the Load Off a Long Hike

Evolution has had four million years to tinker with the locomotion experiment called walking on two feet. With that much time for natural selection to keep what works and chuck what doesn’t, there wouldn’t seem to be much room for improvement.

But what about those tired feet you get after standing on them all day? Or the dull ache that starts creeping up your legs after walking for a few hours? It turns out that even evolutionary fine-tuning leaves room for improvement, and a number of public and private sector engineers have been working on exoskeletons to improve bipedal efficiency.

Researchers have now built an ankle exoskeleton that requires no external chemical or electrical power and decreases the energy a person uses for walking by more than 7 percent, about the equivalent of taking off a 10-pound backpack. The team, from Carnegie Mellon, North Carolina State and the University of North Carolina, Chapel Hill, say this walking efficiency improvement is on par with units that run on powered motors. It could also be a simple and cheap way to help people whose work keeps them on their feet all day. Learn more and see images below.

(Walking on a treadmill with a prototype of the unpowered ankle exoskeleton. Gifs above created from video courtesy Collins et al./Nature.)

“We asked ourselves years ago, ‘Is there a way to assist a human in the task of walking by reducing their own energy use without needing an additional energy source?’ The answer to this question, it turns out, is ‘yes,’” said Steven Collins, a Carnegie Mellon mechanical engineer who collaborated on the project.“Think of nurses, emergency response workers, soldiers, or the millions of other people who walk many hours a day—7% would make a difference to them.”

Their innovation is a wearable device that uses a mechanical clutch and spring to do some of the work normally performed by the calf muscle and Achilles tendon. This shared work reduces the metabolic energy the body needs to expend to operate the lower legs when walking. The results of their work were published today in the journal Nature.

(a, The exoskeleton comprises rigid sections attached to the human shank and foot and hinged at the ankle. A passive clutch mechanism and series spring act in parallel with the calf muscles and Achilles tendon. b, Participant walking with the device. Load cells measured spring force. c, The passive clutch mechanism has no electronics, but instead uses a ratchet and pawl that mechanically engage the spring when the foot is on the ground and disengage it when the foot is in the air. Image and caption courtesy Collins et al./Nature.)

(The unpowered exoskeleton clutch, which engages and disengages a spring in parallel with the user’s Achilles tendon. Photo courtesy of Steve Collins.)

Their effort takes a different tack than what many projects to build exoskeletons take. Instead of augmenting the power of the wearer by using motors, pumps and other machines, this work focuses on off-loading force the muscle normally must generate. This method lowers the energy the body must use to walk, so a wearer would be able to walk farther and longer while carrying heavier loads.

Study coauthor Collins told Nature that the idea for the unpowered ankle exoskeleton originally came from early work building walking robots, which are up to 100 times less efficient that humans. They used some of the natural abilities of human legs to improve the robots, and Collins realized some of their knowledge could be used to improve human movement. By looking at human gait and anatomy as an imperfect system, they were able to discover where structural improvements could be made to upgrade what nature has given us.

“It’s sort of like changing the structure of the body to make it more efficient,” Stevens told Nature. “Instead of replacing metabolic energy with some replacement source, we have to make the whole system more efficient. In theory, these types of structural improvements [that we made] could have been discovered by evolution.”



(Walking through the woods with an unpowered, effort-reducing ankle exoskeleton. The walking-assist clutch engages a spring in parallel with the Achilles tendon when the foot is on the ground, off-loading the calf muscles and making walking easier. Gif created from video courtesy of Lisa Lau and Steve Collins.)