PHOTO: OTTO BOCK

STREAMLINED: Like many of the latest prostheses for sprinting, this leaf spring, made by the German company Otto Bock, has no heel.

As streamlined in form as they are sophisticated in engineering, 21st-century sprinting prosthetics are a radical departure from the past. Unlike an earlier generation of artificial devices, which sought with well-intentioned literal-mindedness to recreate the highly complex articulation of the human foot, the heel-less carbon-fiber leaf spring used by nearly all Paralympics sprinters today was designed with a different purpose: quite simply, to allow the athlete to move as quickly as possible. Of what use, after all, is a heel to a sprinter who runs entirely on the balls of his feet? At the Athens Paralympics last year this journalist saw only one lone sprinter—in the 400—wearing an old-fashioned prosthesis with an artificial foot inside a track shoe, and watching him limp seventy meters behind the field was a painful reminder of just how revolutionary was the changeover.

But were it not for a freak water-skiing accident in 1976, current technology might not be all that different from what it used to be. When 21-year-old Van Philips, a college student in Arizona, sought to resume an active sports life shortly after losing his foot, he was appalled to find the best running prostheses on the market clumsy and painful. "Some of the feet available in those days were made of balsa wood," Philips recalled. "They were light but they had no flexibility. Some had ankle movement, but no method of storing energy. Many were a sloppy fit." There must be a better way, he thought.

And so necessity once again gave birth to an invention, this one designed, patented and built by Philips, who changed to an engineering major in school and later founded a company, called Flex-Foot, that supplied several generations of elite disabled runners with performance-enhancing feet. One measure of the significance of Philips innovation is the addition last year of his first "C-Sprint" prosthetic—so named due to its curved shape—to the permanent collection of the National Museum of American History, along with drawings, sketches and prototypes tracing its conception and development. Philips sold his company in 1999 to Ossur, which has continued to improve and refine the basic design.

The kind of device worn by Pistorius, Frasure, and Shirley actually consists of three parts. The first is a hard socket made of polypropylene or woven carbon fiber composite materials that fits over the stump of the lower leg. A custom-fitted silicon rubber liner provides an interface between the two, protecting sensitive tissue while creating a snug fit. Finally, to replace the two bones that provide the support structure of the lower leg (the tibia and fibula) along with the ligaments and muscles that capture and generate energy, an ultra-stiff and curved length of carbon fiber about the width and thickness of a ski—a mix of woven and unidirectional fibers, on the one hand, and filament wound fibers, on the other—is bolted onto the outside of the socket. Compared to any other prosthesis, this inverted J-shaped spring is "the most effective at storing and releasing energy during walking and, in particular, recreational and competitive sports activities," concluded Linda J. Marks and John W. Michael in a clinical review published in the British Medical Journal.

"The key is that the polymer matrix has a combination of elastic and viscous behavior when subjected to cyclical load" as the prosthetic springs are compressed and "fired" with each step taken by the athlete, explains Mike Jenkins, a mechanical engineer and materials expert at Birmingham University in England who specializes in prosthetics. But even if advances in material design ensure that energy storage is maximized (up to 95 percent, in some cases) a carbon fiber foot "can never release more energy than it stores, and there will always be some dissipation or damping" caused by the imperfect interface between stump and socket, he adds.

Conclusion? "If you considered the case of identical twins, subjected to the same conditioning but one twin having two Cheetah limbs [made by Ossur], then I think the 'able-bodied' competitor would win," Jenkins said in an e-mail exchange. Ajit Chaudhari, a biomechanical engineer at Stanford University's Biomotion Laboratory, points out that a below-the-knee amputee is missing quite a few important muscles. Their absence, he explains, would outweigh any gains from increased efficiency or storing energy in the carbon springs.

Even new developments in osseointegration—the insertion of a titanium implant directly into the remaining femur bone, thus eliminating energy loss stemming from the need for a socket and silicon liner—would never yield more than a 100 percent return on the load, which is two to three times the weight of a sprinter at full throttle. Given that the muscles in an anatomical limb