Athletes with artificial legs now run nearly as fast as the world’s best runners, and scientists are working on artificial limbs that will respond to a person’s thoughts. In the future, will losing a limb no longer mean that a person is disabled?

How good are today’s prostheses?
The advances in the past few years have been dazzling, and for those who have experienced the trauma of losing a body part, the new prostheses provide a startling degree of power and maneuverability. Wearing an artificial left arm to replace the one he lost in an accident, Willie Stewart of California two years ago became the first amputee to kayak the Grand Canyon rapids. Maj. David Rozelle, who lost his right foot to a land mine in Iraq in 2003, went on to run in the New York City Marathon on his high-tech replacement. With his two carbon-fiber composite “Cheetah” legs, South African sprinter Oscar Pistorius can propel himself 400 meters in a little over 46 seconds—nearly as fast as the fastest able-bodied runners in the world (see below).

How do these limbs work?
Constructed of titanium, silicone, and other cutting-edge materials, most employ computer chips and other miniaturized electronic components. The battery-operated “C-Leg,” developed by a German company, is among the more sophisticated offerings. As it encounters variations in terrain, the C-Leg’s sensors send data to a microprocessor, which automatically adjusts the flow of hydraulic fluid within the leg. The “Utah Arm,” made by Motion Control of Salt Lake City, relies on electric signals generated when the user twitches a muscle. These “myoelectric” impulses are transmitted to electrodes on the surface of the skin, which are connected to computer chips in the limb. The chips then guide the limb in the desired direction. The “i-Limb,” an artificial hand introduced last year, is equipped with tiny motors that allow each finger to move and even wrap around an object.

What’s driving these advances?
As is often the case with technology, war. Thanks to improved body armor and battlefield medicine, only 10 percent of all casualties in Iraq are fatalities. The vast majority of the wounded are felled not by bullets but by explosive devices, which tend to blow off limbs. So thousands of soldiers are coming home after amputations. As a result, the Pentagon and the Department of Veterans Affairs in the past few years have pumped $70 million into limb technology. But these products are available not just to veterans; devices developed under the auspices of the government make their way to the marketplace and become available to the general public. With nearly 2 million Americans missing an appendage, annual sales of prostheses have nearly doubled, to almost $1 billion, over the past few years.

Are users happy with their new limbs?
While nobody would say that their prosthetic device has truly replaced their lost limb, most say their lives have been greatly enhanced. “If you walk fast, it walks fast,” said Mike McNaughton, a National Guardsman who lost his right leg in Afghanistan in 2002 and now wears a battery-powered computerized prosthesis. “I played with my son during his soccer practice the other day.” Karl Chapin, whose right hand was blown off in Vietnam, uses his new i-Limb to tie his shoes, drive a car, and turn a doorknob. “For the first time, I was at a banquet and I actually held a plate,” he says. “It was a tremendous feeling.”

Are there drawbacks?
Yes. For many, the biggest negative is the expense. The C-Leg costs at least $40,000; the i-Limb goes for $65,000. Most insurers won’t cover these high-tech limbs, placing them out of reach for many. And for all their sophistication, the new artificial limbs still have a hard time replicating the human body’s complex movements. To get an artificial leg to do its job, for example, the wearer has to deliberately fling forward what is essentially dead weight. “Every step. I think about every step,” says former soldier Melissa Stockwell, who lost her left leg in Iraq in 2004. By one estimate, the best artificial foot-ankle systems can supply only 50 percent to 60 percent of normal functioning; knee systems, 30 percent; and arm-hand systems, just 5 percent.

Can these problems be solved?
There is some very promising research. In the burgeoning field of “neuroprosthetics,” scientists are experimenting with implanting tiny chips in any remaining arm or leg muscles. These chips would pick up nerve impulses from the brain and send instructions to the prosthesis, and feedback from sensors in the limb would help guide the movement. “You think about it and the mechanical appliance responds,” explains University of Miami physical therapist Robert Gailey Jr. “That is the ultimate goal.”

Is that a realistic goal?
It appears so. Just last week, the journal Nature reported that laboratory monkeys were able to control mechanical arms using only their thoughts. Scientists had implanted tiny sensors in the monkeys’ brains that carried signals from the brain to a mechanical arm. With practice, the monkeys were able to use the arm to reach for food, grab it, and bring it to their mouths. A truly bionic appendage—one that operates purely by thought and can do everything the original can—is still just science fiction. But scientists say it’s coming. “This is a quest,” says Frank Moss, director of MIT’s Media Lab, “to blur the distinction between human ability and disability.”

Pistorius on the run
Oscar Pistorius of South Africa was born without the fibula in his lower legs, and he had both legs amputated below the knee when he was 11 months old. Learning to walk with prostheses, he played rugby as a teenager and later took up running. Pistorius was soon sprinting on his J-shaped carbon-fiber blades, and now holds several world Paralympic records. But he didn’t attract much notice until last year’s South African National Championships, when he placed second, setting off a debate over whether he should be allowed to keep competing against able-bodied athletes. In January, the International Association of Athletics Federations barred him from entering the Beijing Olympics, ruling that his springy prostheses give him an advantage because they’re more efficient than the human foot. But in May, the Court of Arbitration for Sport reversed the ban. Pistorius still must meet the Olympic qualifying time of 45.55 seconds in the 400 meters to earn an automatic berth, and he’s not sure he can make it. If he doesn’t, he says, he’ll be back in four years, at the 2012 Olympics in London. “It’s not just about me,” he says. “It’s about the extra opportunity for amputee athletes.”