Using the lower leg as a sensing point ultimately became part of Mr. James's plan. He decided to use a combination of sensors, measuring the heel strike rate and the pressures being imposed on the leg with two piezoelectric strain gauges on the lower part of the leg while, at the same time, monitoring the angle of the knee with a magnetic sensor in the knee. Two microprocessor chips in the upper part of the leg analyzed the data from the sensors at a rate of 50 times a second and used motors -- initially ones designed for the oil industry -- to constantly adjust the hydraulic pressure to reflect a user's walking pattern.
With their ability to change the properties of the device almost instantly, the C-Leg's processors allow wearers to change their pace rapidly or to shift weight naturally to walk down stairs or slopes. The leg is powered by an internal lithium ion battery that can operate for 30 to 35 hours before it needs recharging. (The C-Leg, however, does not allow amputees to walk up stairs naturally. Dr. Hans Dietl, chief executive of Otto Bock Austria, said that was unlikely to change. To allow natural stair climbing, the leg would have to lift its owner's weight, using a battery and motors too heavy to be practical.)
The prosthetists' initial reaction to Mr. James's development, which started to come together late in 1986, was not encouraging. ''They mostly said, 'You're going to tell an amputee to carry around a battery, take off his leg and plug it into the wall at night?' '' Mr. James recalled. Still, a prosthetist in Edmonton, Stan Wlodarchzyk, agreed to work with Mr. James at no charge, as did one of his patients, a teacher named Tony Slemp who became the test subject during the long software refinement period. Mr. James eventually licensed the leg design to Otto Bock, which refined the technology at its research center in Vienna.
One of a small number of testers for Otto Bock was Adele Fifield, who had lost a leg to cancer as a teenager. For her, the most obvious difference after 22 years of wearing artificial legs was that she stopped falling. But in some ways that was the least of the improvements. Ms. Fifield, who grew up in a fishing village in the Canadian province of Newfoundland and Labrador felt she had been freed from a mental drain.
''There wasn't a lot to do in small-town Newfoundland,'' Ms. Fifield said, ''so you tended to walk around the community. I remember walking and trying to carry on a conversation with my friends and finding it very difficult to do both things at once. You had to be conscious of every step. And that was on level ground.'' But with her C-Leg, Ms. Fifield confidently strolled along a hilly path, crisscrossed by roots, marked by mud and filled at points with loose rock and uneven flagstones. ''I didn't enjoy going for a walk the way I do now.''
Despite the early concerns of some prosthetists, Ms. Fifield has not found it particularly troublesome that the battery's charge lasts only about 30 hours. Only once in the last four years has she forgotten to charge it. she said. In that case, she revived the leg by plugging it in under her desk for two hours the next morning. On another occasion, the leg ran out of power and went stiff when she was wilderness camping, forcing her to hobble around for the rest of the trip.
But as the director of the National Amputee Center of the War Amps, a Canadian charity that provides financial help for people in need of artificial limbs, Ms. Fifield has one concern about the C-Leg: its cost. ''That's the only thing I hate about the whole thing,'' she said.
The C-Leg, which was introduced to the United States market in 1999, costs $40,000 to $50,000, compared with $15,000 for a nonelectronic hydraulic model. The company declined to say how many units it had sold but acknowledged that the number was quite small.
For some people, like Mr. Grimsley, insurers pick up the entire cost, but Ms. Fifield believes that they are a minority in North America. ''Five years ago, I used to say that my artificial leg cost as much as my car. Now my artificial leg costs twice as much as my car.''
Blatchford-Endolite is waiting for regulatory approval to introduce a new combined hydraulic-pneumatic electronic leg that a company official said would be considerably less expensive (he declined to offer a price) than the C-Leg while offering similar features, including the ability to walk down stairs and over uneven terrain.
As for Mr. Grimsley, he was caught on Broadway with his C-Leg partly removed by medical personnel when the towers collapsed on Sept. 11. After taking shelter against a building and then in a store, Mr. Grimsley got his C-Leg back on and was able to walk away from the disaster scene until he could flag down an ambulance, which took him to a hospital for treatment of his irritated stump.
A few weeks after the disaster he had his C-Leg swapped for a factory loaner because dirt had impaired the mechanical performance of the leg, but it still worked. Mr. Grimsley's office is now in a Port Authority data processing center on Staten Island.
And while he is still coming to terms with that day's attacks, he has no complaints about his leg. ''It's really beautiful technology,'' he said.
Photos: CONFIDENCE -- Adele Fifield, who has worn other artificial legs, says that the C-Leg freed her from a mental struggle during walks in rural Newfoundland and Labrador. (Ian Austen for The New York Times)(pg. G4); LIFESAVER -- Curtis Grimsley used his Otto Bock C-Leg to walk down from the 70th floor of the World Trade Center on Sept. 11. He says the device saved his life. (Norman Y. Lono for The New York Times)(pg. G1) Chart: ''A Computer in Tennis Shoes'' In any walking, the C-Leg must perform two contradictory tasks. At times, it must swing freely; at other points, it must be stable enough to support its user's weight. Completing these tasks depends on several sensors: two strain gauges measure the forces being applied to the leg and the rate at which its heel hits the ground, while a magnetic sensor tracks the angle of the knee. Two microprocessors analyze the sensor's data and adjust the hydraulic valves that control the knee. 1. When a user places a natural leg on a step below, the strain gauges will notice a significant decrease in the load on the C-Leg. The microprocessors reduce the hydraulic pressure on the knee, allowing it to bend. 2. The magnetic sensors detect the increasing knee bend. As the hydraulic pressure is further reduced, the leg swings forward. 3. As the C-Leg lands on the next stair, the knee straightens and the load on the artificial leg increases. The microprocessors will instruct the motors to raise the hydraulic pressure of the leg to support the user's weight. (Source: Otto Bock Orthopedic Industry)(pg. G4)