Earlier this week, a team of scientists revealed the first unpowered wearable exoskeleton that decreases the energy required to walk. The ankle exo, as they call it, is stunningly simple: It acts as an auxiliary calf muscle, using little more than a straightforward spring and a mechanical ratchet that either tightly or loosely grips that spring, depending on the motion of your walking foot. Wearing the ankle exo decreases the energy a person expends when walking by 7 percent.

Maybe 7 percent doesn't sound like a lot. But consider this: If you were to walk the 2,200 miles of the Appalachian Trail wearing the exo, the 150 miles stretching Vermont would essentially come energy-free. Philippe Malcolm, a biomechanist at Harvard University who was not involved with developing the device, calls the number "very, very impressive… it's on par with the best powered devices."

"The real challenge in designing efficient exoskeletons is in understanding the human element."

How can such a simple change make such a big difference on the human body, and what could it mean for the future of human health and mobility? We asked Steven Collins and Greg Sawicki, two of the three inventors behind the ankle exo. The short answer: What makes the ankle exo so effective is its very simplicity.

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The power of doing less

Engineers, inventors, and scientists have spent more than a hundred years trying to hack the simple human act of walking using a number of strange and marvelous devices. But take a look back at the various forms of wearable technology and exoskeletons they've proposed and it becomes clear why they all failed: Instead of trying to augment the human body's natural motion, many inventions tried to outsmart and out-tinker biomechanics with clever contraptions.

Collins, of Carnegie Mellon University, points to GE's 1965 monster exoskeleton 'Hardiman' as the ultimate example of a failed approach to improving human movement. "It's massive, bulky, hard to interact with… and it totally encapsulates this age-old idea that for an exoskeleton to be successful, you have to carefully control every bit of movement at all times," he says.

"We really already are experts at walking."

Heavy-duty, complicated machines are doubly self-defeating when it comes to walking, says Sawicki, who works at the University of North Carolina at Chapel Hill. Sawicki says that our natural walking gait has been finely honed over 7 million years of evolution (and is continually improved throughout our lives). Any device that shifts that gait or adds even a subtle change to our natural biomechanics—even if the device is powered—invariably ends up making walking more energy expensive.

"We really already are experts at walking," he says. "Even if you only ask someone to walk with shorter or longer strides, or to relax when walking, you'll find that you've increased the energy they're exerting," which scientists measure through respiration.

The double whammy is that wearable devices make your legs heavier. "If you're trying to reduce the energy cost of walking, you have to offset the energy you're now expending because of the extra weight," Sawicki says.

A basic boost

The key insight behind the ankle exo—to change our natural gait as little as necessary—came after the team reviewed ultrasound imaging studies that revealed exactly how the ankle, knee, and hip joints work together to share the stress of walking. Collins and Sawicki used that newfound knowledge to create an exoskeleton that added just a dash of machine efficiency to a specific part of the walking gait.

What the ankle exo does, in a nutshell, is give your calves a bit of a break. As you walk, the springy Achilles' tendon in your heel helps your ankle bounce in an out of position for almost no energy. But your calf muscles (even though the muscles are staying still) have to work tirelessly to hold that Achilles heel taught so it can bounce. The ankle exo transfers some of that tendon springiness to a mechanical spring, so the calf doesn't have to hold as tightly.

You can do a lot with a 7 percent gain

The design is so basic, Collins says, that it's not too farfetched to imagine someone in the age of the Wright brothers, 100 years ago, inventing something similar—if they'd had the biomechanical knowledge and insight. Granted, the 1-pound ankle exo uses carbon fiber, which wasn't around in 1903. "But I suppose one could get a similar performance if you had a craftsman or really skilled artisan working with lightweight wood," Collins says. As for the ratchet in the ankle exo, "we use these complicated shapes that are really best suited for CNC machines, but if you knew what you were doing, I suppose you could manufacture that 100 years ago as well."

To its creators, the simple design behind ankle exo leads in many directions—from a future prosthetic device to help the disabled walk to a tool that may one day help soldiers carry heavier loads or help hikers tackle even longer journeys. You can do a lot with a 7 percent gain.

The ankle exo is also a message to future exoskeleton designers. "Most of the people that design exoskeletons are mechanical engineers and roboticists, and their focus is naturally on the machine itself," Collins says. "But I think the real challenge in designing efficient exoskeletons is in understanding the human element. Sometimes you need these devices to let things happen on their own."

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