His first prosthetic legs were made of plaster of paris. The prosthetist who fitted them suggested he might one day be able to walk without canes. But nothing could keep Herr from the climbing wall. Seven weeks after doctors amputated his lower legs, he hopped in a car with one of his older brothers, Tony, and headed to a series of cliffs along the Susquehanna River. For years he had been doing things on rock faces that people said he couldn’t, but even he was amazed by his performance that day. Weak and recovering from the surgeries, Herr was wobbly on his new feet. On the rock wall it was a different story. “I felt more natural scrambling on all fours than walking,” he says.
By spring Herr was in a local machine shop experimenting with his artificial limbs. Every few weeks he headed to Philadelphia to meet with prosthetist Frank Malone for refittings and adjustments. Herr began tinkering with the design of his new legs, adjusting the length and playing with different materials to make them lighter. “I realized that my prostheses need not look human,” he says. “They are a blank slate: I could create any prosthetic device I wanted for form, function, and enhancement.”
Herr threw out his climbing shoes and glued climbing rubber directly to the bottom of his mechanical feet. Then he went to work on their shape. For expert pitches where he planned to stand on small rock edges the width of a dime, normal feet were a disadvantage. So he designed a prosthetic about the size of a baby’s foot. He created a pair of feet with toes made of laminated blades that he could jam into tiny rock fissures far too slim to hold a normal human foot. He made the height of the legs adjustable; at 7 feet 5 inches, he could reach handholds and footholds far beyond the range of any able-bodied climber. And he made his legs easier to move. Herr drilled holes throughout legs fashioned from aluminum tubes, making them so light that they just barely supported his weight, while increasing the number of pull-ups he could do and the distance and speed with which he could climb.
“From that personal experience, I realized that technology has an extraordinary capacity to heal, to rehabilitate, to augment, and that really set the tone for my entire professional life,” Herr says.
He spent a couple of years traveling and living in the climbing meccas of North America, burnishing a reputation for his skill that eventually won him a spot in the national Sports Hall of Fame. But even as Herr excelled on rock walls, he was increasingly frustrated with what he could do on the ground. His rigid prosthetics lacked the natural cushioning provided by the tendons of the ankles and feet. The sockets where his legs met the artificial limbs chafed when he walked, leaving him raw and bloody and putting powerful strains on his knees.
By 1985 Herr had had enough. When the doctors said no solution existed for the problem, his experiments moved beyond climbing feet and into the realm of medical devices. “The medical community was giving me these devices and saying, ‘This is the best, live with it,’” Herr says. “I just couldn’t accept that what they gave me was really the best that we could produce.”
First he tried stuffing the sockets with leather and rubber to cushion them. Then Jeff Batzer, his fellow Mount Washington survivor, introduced him to a prosthetist and orthotist named Barry Gosthnian. Gosthnian had been an Air Force mechanic in Vietnam and recalled the shock-absorbing hydraulic supports used in aircraft landing gear. Perhaps, he suggested, a hydraulic cushion of some sort could soften the impact in the socket.
Herr and Batzer toiled in a workshop to develop a better hydraulic socket. That fall, Herr enrolled at nearby Millersville University, a state school in central Pennsylvania. With a new reason to study, he developed a passion for math and physics, earning almost all A’s.
By the time Herr graduated from college, he had his first patent—shared with Gosthnian—and a prototype for a cushioned socket with inflatable bladders. The bladders, made out of soft, flexible polyurethane membranes, were located wherever weight-bearing portions of the leg stump pressed against the socket, cushioning the force and softening the pressure on the stump as needed. (His primary research subject? Himself, of course.) He also had an acceptance letter to a graduate program at MIT in mechanical engineering.
Seeing Herr stride casually across MIT’s campus in a rainstorm, wearing blue jeans and a pair of Italian leather loafers, it’s virtually impossible to tell that he is missing both lower legs. He moves with a seamless, flowing gait—hands in the pockets of his puffy, green jacket, his gaze roaming the grounds. But in his lab, Herr often goes shoeless, his pant legs hiked up to expose aluminum legs an inch in diameter atop sleek masses of silver gears and wires, which power flat black feet resembling the bottoms of flip-flops. “I think they are more attractive than human legs,” he says.
Herr began work on the PowerFoot about six years ago with a simple realization: No available prosthetic came close to replicating the beauty and simplicity of human locomotion. Even with the best available models, most amputees walked more slowly and had less balance. Their gaits were eccentric, and their devices often caused back problems. When a person with intact lower limbs walks, the amount of power the calf muscles expend increases with walking speed. Yet virtually all commercially available ankle and foot prosthetics are passive devices, containing spring mechanisms to absorb shock as a person walks but making no effort to replace the power-generating capabilities of the muscles in a person’s lower limbs.
Herr and his collaborators believed that the lack of ankle power was one of the main reasons amputees burn 30 percent more energy walking than do humans with intact lower limbs. Addressing that problem would be no easy task, however. “At the time Hugh started this, if you had asked anyone in prosthetics, they would have told you that the ankle requires so much power that you could not build a lightweight, compact, quiet one,” says Bruce Deffenbaugh, a longtime researcher at the MIT Media Lab who worked on the project. “But Hugh is unstoppable.”
Herr and his partners began collecting all that was known about the dynamics of the human leg and the interaction of its component structures. Where the literature was sparse, they tried to fill in the blanks by taking precise measurements of a healthy human leg and creating a mathematical model that spelled out how the different components of the leg interact. They had to ask fundamental questions about everyday behavior. How much power, for instance, does a normal calf muscle in a 5-foot 9-inch male generate right before the foot pushes off the ground? When that muscle flexes, how will it affect the stiffness of the tendons attached to it? How stiff is the ankle when a person attempts to slow down?