“It’s a real exciting milestone for the field of assistive devices. They’ve taken an assistive device and lowered the cost of human walking. That’s kind of a big deal because walking is already really cheap, and they did it with a very simple, but clever device,” said Prof Thomas Roberts of Brown University, who was not involved in the project.

The device is the result of 8 years of patient and incremental work. Dr Collins and his colleagues succeeded where many researchers in the past had failed by performing careful analyses of the biomechanics of human walking and then designing a simple, ultra-light-weight device that relieved the calf muscle of its efforts when it wasn’t doing any productive work.

Ultrasound imaging studies had revealed that the calf muscle exerts energy not only when propelling the body forward, but also when it performs a clutch-like action, holding the Achilles tendon taut.

“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,” said Dr Collins, who is the lead author of the paper published in the journal Nature.

“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 researchers created an ankle exoskeleton that offloads some of the clutching muscle forces of the calf, reducing the overall metabolic rate.

A mechanical clutch engages when the foot is on the ground and disengages when the foot is in the air, to avoid interfering with toe clearance.

This clutch takes over the effort of the calf, producing force without using consuming any energy and thereby reducing the overall metabolic rate.

“This unexpected and unprecedented result, with the potential to improve such a familiar human activity as walking, was discovered during a fundamental scientific study of mechanically augmented ankle function. It is a great example of how basic research can lead to new beneficial devices,” said Dr Jordan Berg of National Science Foundation, who was not involved in the project.

One of the long-term goals of the project is to use lightweight, 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. 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,” Dr Collins said.

In the future, he and his colleagues plan 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.