Researchers have developed a device that generates electrical power from the swing of a walking person's knee.

With each stride the leg accelerates and then decelerates, using energy both for moving and braking.

Max Donelan and his colleagues reasoned that a device that helps the leg decelerate could generate power without requiring much more energy from the person.

The principle is similar to that by which some hybrid-electric cars produce electricity from braking.

With the device, a minute of walking can power a cell phone for 10 minutes, Donelan, of Simon Fraser University in Burnaby, British Columbia, said. Other potential uses include powering a portable GPS unit, motorized prosthesis, or implanted drug pump.

Donelan and colleagues at the University of Pittsburgh and the University of Michigan report development of the device in today's issue of the journal Science.

The first practical use for the generator is likely to be in producing power for artificial limbs, said Donelan, who, with his coauthors, has founded a company to develop the device commercially.

The generator weighs about 31/2 pounds, so users burn energy carrying it on their knee but do not notice whether it is switched on or off when walking on a treadmill, he said.

They miss it when it is removed because they get used to its extra braking action, he said.

With one generator on each knee, people walking on a treadmill generated about five watts of power.

Lawrence C. Rome of the University of Pennsylvania called the development "extremely clever."

Other people have thought of rotational devices around joints, Rome said, "but what's really clever is these guys only turn on resistance when the person is trying to brake, so it helps you."

Rome, who was not part of the research team, previously developed a backpack that generates electricity from the movement of the person carrying it.

The backpack was comfortable, so it was an easy sell, he said. Whether the new knee generator is practical will depend on whether it is comfortable enough for people to want to wear it, Rome said.

Arthur Kuo, an associate professor of mechanical engineering at the University of Michigan and a coauthor of the report, called the device "a cocktail-napkin idea."

"There is power to be harvested from various places in the body, and you can use that to generate electricity. The knee is probably the best place," Kuo said.

"During walking, you dissipate energy in various places, when your foot hits the ground, for example. You have to make up for this by performing work with your muscles. . . . We believe that when you're slowing down the knee at the end of swinging the leg, most of that energy normally is just wasted."

The prototype is bulky, he said, but the energy generation part has very little effect on the wearer.

"We hope to improve the device so that it is easier to carry, and to retain the energy-harvesting capabilities," he said.

The research was funded by the National Sciences and Engineering Research Council of Canada, the Michael Smith Foundation for Health Research, and the Canadian Institute of Health Research.