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Canadian researchers have come up with a new way to extract people power: a knee brace that can harvest enough energy during a relaxed stroll to keep half-a-dozen cellphones working.

Volunteers wearing one biomechanical energy harvester on each leg were able to generate five watts of electricity with little extra effort. If they walked quickly, they could generate 13 watts, said Max Donelan, a kinesiology professor at Simon Fraser University in Burnaby, B.C.

He and his colleagues report on their invention today in the prestigious journal Science.

The harvester looks like an orthopedic knee brace, but Dr. Donelan said it works much like regenerative braking in hybrid cars, in which energy normally dissipated during braking drives a generator instead.

He and his colleagues rigged the brace with a generator system that can turn physical motion into electrical energy as the hamstring muscle slows down the swinging knee.

"Think of the body as an amazing battery," Dr. Donelan said. "The average person stores as much energy in fat as a 1,000-kilogram battery."

Their harvester generates slightly less power than a backpack invented in 2005 by University of Pennsylvania biology professor Lawrence Rome, which enabled U.S. marines to power cellphones, night-vision goggles and other equipment without lugging extra batteries.

The backpack takes advantage of the hip's movement to generate electricity, but people have to carry a relatively heavy load in order to produce the power. The packs Dr. Rome used in his experiments weighed between 20 and 38 kilograms, to generate up to 7.4 watts of power. Yesterday, he said he hopes to produce a commercial product that weighs significantly less and produces more electricity.

Energy-harvesting shoes have also been invented, but are not yet on the market. They collect energy from the motion of walking, but generate only 0.8 watts of power.

Dr. Donelan has set up a company, Bionic Power Inc., to develop a commercial model of his knee-brace device that should be ready for field testing in one year. The experimental versions weigh 1.5 kilograms each, but Dr. Donelan said he should be able to make a much lighter one.

The company is hoping for military clients, said chief executive officer Yad Garcha. The idea is that soldiers would carry one battery to power all their electronic devices; the battery would recharge while they walk.

Mr. Garcha won't discuss how much the device would cost, but says right now it is too expensive for the average consumer in the developed world, let alone those in poor countries who don't have access to electricity.

About a quarter of the world's population lives without an electricity supply, and Dr. Donelan hopes that one day children in developing countries will be able to use the device to power laptop computers.

"They'll have to take a break from their homework to go outside and play to generate power to do their homework," he said.

Rescue workers could use it to recharge batteries, he said. So could geologists or anyone working in a remote location.

An expert in the biomechanics of walking, Dr. Donelan has been working on the project for more than 10 years.

When we walk, he said, the hamstring slows down the knee as we swing each leg through the air. The knee-brace device helps the muscle do this, and then harvests the extra energy.

Six volunteers wore the devices while walking slowly on a treadmill, and the researchers measured their oxygen use and carbon-dioxide output to see how much energy it took to walk with the brace.

"You have people walking, carrying the device, but it is not generating any power. Then without them knowing, you click a button on our control system and it starts to generate power. And they don't even know you've done that," Dr. Donelan said.

But they noticed when he turned it off.

"They can tell because they have learned to turn down the muscles at the right time, so now their legs are extending a little more than they like to, a little bit wildly," Dr. Donelan said.

It took them about 10 strides to adjust.

"It is something they learned to take advantage of very quickly, so then they miss it when it's gone."