Best Foot Forward

Hugh Herr was a teenage mountain-climbing phenom when a blizzard took his legs. Now the MIT professor has used science to get them back, building artificial limbs so advanced, they force us to rethink what "able-bodied" means.

Still, he thought his new legs could be better. In 1985, when he was 21, he enrolled at Millersville University, a small school near Lancaster. There he met a computer science professor named Don Eidam who pushed Herr into trigonometry, and then calculus, before Herr fell in love with physics. “My intellectual birth,” he says, “was in college.” As the semester wore on, Herr climbed less and studied more. He began working with a Pennsylvania prosthetist named Barry Gosthnian who wanted to use hydraulics to cushion the painful and repetitive grinding of an upper leg against a prosthetic limb. The goal was for the prosthetic socket to respond not just to different levels of stress, but also to different areas of stress—almost like the way a four-wheel-drive car distributes the task of holding the road. The two began working late into the night when Herr was an undergraduate, experimenting with urethane and polymers. By 1990 they had a patent for a socket.

Over the next several years—as he worked toward a master’s at MIT in mechanical engineering, a Ph.D. in biophysics at Harvard, and then a postdoc back at MIT in biomedical devices—Herr went on to devise 10 iterations of a knee joint, 10 iterations of an ankle joint, and several iterations of leg braces. Each version was a little lighter, a little more pliable, a little more durable. The prosthetic ankle he invented worked first in alignment with the foot structure, like a tendon, connecting the muscle to the bone. It then spanned the joint, like a ligament, and connected bone to bone. Herr’s prosthetic knee employs something called magnetorheological fluid, a suspension of micrometer-sized magnetic particles in a carrier fluid, like oil. MR fluid is used to reinforce Kevlar armor; Herr used it to help vary the resistance of the knee as it moves. The original prosthetic with MR fluid weighed 10 pounds. The latest version is down to 3 pounds, and has four times the torque. Both the knee and the ankle mimic the functionality, but not always the form, of the human leg. And that’s what Herr wanted. He wanted something that was both humanlike and better than human. He wanted to challenge the idea that an amputee is, by definition, disabled.

“We as humans sometimes believe the human form is the most beautiful,” Herr says. “But there’s clearly form and structure that is beautiful and non-human. You attach machines to the body and there’s an obsession to make it humanlike. That’s from people being ashamed of having a machine attached to the body. I want that robotic look to be absolutely gorgeous.”

From left, the PowerBoot exoskeleton, Herr’s latest and boldest invention yet; Herr with his computer-controlled PowerFoot (shown attached to a knee prosthetic). (photographs by dennis kleiman)


Herr earned his professorship at MIT in 2004. For the next few years, at the start of every semester, he told his class nothing about his background, about the legs he had designed for himself. Toward the end of each semester, Herr walked back and forth and asked his students whether his gait was “normal” or “pathological” (i.e., uneven). About half said normal, and the others voted pathological. Then he lifted up his pant leg and began taking apart his prosthetic, piece by piece, delighting in the stunned silence in the room. His gait, he explained, was pathological. Despite the seeming evenness of his stride, no one had devised a prosthetic foot that could produce a normal gait.

Until now. With a $7.2 million grant from the Department of Veterans Affairs, Herr helped devise (and, of course, test) a robotic foot run by a computer that can read the terrain and slope of a surface and react accordingly. The PowerFoot One is the first prosthesis to allow for a normal gait—and it actually outperforms a human foot. It provides a source of power that propels the leg forward, and does it more efficiently than the human leg. It has springs that boost acceleration when needed, and relaxes when at rest. It has a black rubber sole taken from a shoe, but the rest is a sci-fi jumble of plastic, metal, and micro-processors, along with a 25-volt rechargeable battery.

Time named the PowerFoot one of its Best Inventions of the Year in 2007. Herr helped found a company to produce the PowerFoot, which will be available on the market later this year at a price yet to be disclosed. “It’s a remarkable difference [from past models],” Herr says, with a sudden charge in his voice. “One can walk very fast. You kind of fly up a hill. The toe points down on descending steps. It relaxes and conforms to ground surface. It even droops when you’re sitting, like the human foot.” He’s finally done it—copied nature but done it one better, because this foot will never ache or itch or fall asleep on you.

And yet Herr has already moved on to the next challenge. Sitting now in an empty classroom, his students gone, Herr flips open his laptop and click on a file. A screenshot shows one of his Ph.D. students wearing an exoskeleton. It’s a “pelvic girdle,” Herr says, that’s wrapped around his waist and attached to metal supports on the outside of his legs that extend to the ground and bend where the body might bend. Herr hits play on his computer and the man starts jogging in place with so little effort that he looks as though he’s on the moon. Herr opens another file and a man in the same contraption is bouncing as if he’s on a cloud. This is a prosthetic device for either the disabled or the able-bodied. And instead of matching the energy output of a normal limb, the exoskeleton far surpasses it, literally counteracting gravity. Herr thinks this could work in war zones—imagine rucksacks that can transport the usual 50-pound load and also reduce the weight felt by the wearer by 100 pounds. He thinks it could have other uses, too, like delaying and even preventing hip-replacement surgery.

The leg exoskeleton, or Power-Boot, as Herr has named it, is backed by several medical venture-capital firms, chief among them WFD Ventures of New York. It could hit the marketplace “in just a few years,” he says. And Herr imagines a time soon after that day when there will be no stigma attached to prosthetics.

He likes to sit outside in shorts on sunny days and watch passers-by gawk at his PowerFoot. He wonders why, when he goes through security at Logan, the TSA staff asks whether he would like a private security screening. “Why the hell should we be ashamed of any wearable device on our body?” he asks. “When it’s highly functional and beautiful, most people celebrate.”

In the end, everything can be made better, even his own arms, which, now that he’s seen what he’s capable of creating, Herr won’t rule out amputating someday. “Not yet,” he says, smiling.

“But when the technology improves…perhaps.”


Eric Adelson is a former senior writer for ESPN the Magazine.