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Unpowered exoboot takes some of the hard work out of hiking

This lightweight, wearable exoskeleton fits around your foot and ankle and can reduce the amount of energy you expend on walking.

Carnegie Mellon University College of Engineering

Humans are well-tuned for walking, which is a good, low-impact form of exercise. But there are some instances where a little extra help might be needed -- for someone who needs to move around but is mobility impaired, for instance, such as a stroke patient; or for people who are on their feet a lot, such as an infantryman.

A newly designed, unpowered boot by researchers at Carnegie Mellon and North Carolina University uses spring power to reduce the energy expended in walking by around 7 percent. That seems like a small amount, but it's on a par with powered devices of a similar nature, and will help optimise the human gait.

The carbon fibre device, which weighs as much as a normal shoe -- about 500 grams -- uses no power. Instead, with each step, an unpowered clutch engages a spring parallel with the Achilles tendon, taking load from the calf muscles.

"The unpowered exoskeleton is like a catapult. It has a spring that mimics the action of your Achilles tendon, and works in parallel with your calf muscles to reduce the load placed upon them," said the study's co-author Dr. Gregory Sawicki, a biomedical engineer and locomotion physiologist in the joint NC State/University of North Carolina-Chapel Hill Department of Biomedical Engineering.

"The clutch is essential to engage the spring only while the foot is on the ground, allowing it to store and then release elastic energy," said Dr. Sawicki. "Later it automatically disengages to allow free motion while the foot is in the air."

Stephen Thrift

The device was tested on nine able-bodied adults with different springs to find the "Goldilocks" spring -- too stiff or too loose, they found, and the spring added to the work the walker had to do, rather than easing it. The final spring, naturally, was somewhere in the middle.

Dr. Sawicki said that a 7 percent reduction in metabolic energy was roughly equivalent to removing a 4.5 kilogram (10 pound) backpack -- a not insignificant improvement. "Though it's surprising that we were able to achieve this advantage over a system strongly shaped by evolution, this study shows that there's still a lot to learn about human biomechanics and a seemingly simple behaviour like walking," he said.

The full study, "Reducing the energy cost of human walking using an unpowered exoskeleton", is published in the journal Nature (subscription required).