Hugh Herr and I are navigating the snow-covered sidewalks of Cambridge, Massachusetts, in two distinctive styles. I test my steps and vary my stride as I make my way among clumps of slick wet snow. He plows straight ahead. We cross a shiny black street, watching for Boston's notorious cabs. On the far side, I balance on the ball of my left foot, stretch my right leg, then hop across a slushy puddle to the curb. Herr strides right through. The cold doesn't bother his feet, although he complains that they squeak when the weather is wet: They're made out of carbon fiber.
When he was 17, Herr got trapped in a blizzard during a mountain-climbing trip and lost both legs below the knees to frostbite. Shaken, the former C student focused on his studies, eventually earning a master's degree in mechanical engineering from MIT and a Ph.D. in biophysics from Harvard. Now, as a codirector of MIT's Leg Lab, he's one of a number of researchers developing radically improved prosthetics. The field is still closer to Captain Ahab's peg leg than Captain Picard's Borg implants, but researchers are making remarkable progress toward creating artificial limbs as good as human ones.
One reason Herr walks so well is that he still has his own knees. People whose legs have been amputated higher have trouble stopping suddenly or recovering from a stumble, let alone navigating icy sidewalks. Human knees adapt to changes in speed and terrain by bending at varying rates and stopping at different angles. Traditional artificial knees swing free while the leg is in motion, then lock as weight is put on that leg. The result is a stiff, awkward gait.
Otto Bock Orthopedics Industry of Duderstadt, Germany, is using computer chips to help prosthetic knees behave more like real ones. The company's new C-Leg contains a lithium-ion battery and a microprocessor that measures the angle of the knee and the rate at which it's bending 50 times per second. The computer uses that information to project what the amputee is trying to do and adjusts valves to change the flow of fluid within hydraulic chambers inside the knee, increasing or decreasing resistance as necessary. "Every step an above-knee amputee takes, as he comes down he has to be concerned that the leg is stable," says Todd Anderson, a prosthetist at the Minneapolis branch of Otto Bock. "But if we could develop a prosthesis that does exactly what an anatomical leg does, theoretically the brain is already programmed to respond to that."
To match each individual's natu-ral gait, the C-Leg must be custom-programmed by a prosthetist when fitted. The Leg Lab is working on an auto-adaptive knee incorporating sensors that determine the correct operating parameters without any help. Researchers in the Smart Integrated Lower Limb program at Sandia National Laboratories in Albuquerque, New Mexico, aim to go further. They are working on prosthetics that measure not only knee position but also the forces exerted on the foot as it strikes the ground. Those inputs would allow a more accurate simulation of normal limb motion.
"This isn't going to be the bionic man or anything like that; it's going to be something much simpler," says Diane Hurtado, the former manager of the Sandia project. Even a small improvement would mean a lot to amputees, because plenty of routine activities are beyond the scope of artificial legs. "Going from standing to walking up a set of stairs requires your leg to completely change configuration," says Hurtado.