During every footstep, leg muscles waste energy, as do most cars when brakes are applied.

So a group of researchers has created a novel system that converts wasted leg energy into electricity.

Perhaps it won't reduce demand for oil, coal or gas power, but the act of walking does show potential to recharge one's iPod, cell phone, GPS device, laptop computer, prosthetic limbs and other medical devices.

Adding to the bonuses, the device has the potential to make walking easier.

Doug Weber, a biomedical engineer with the University of Pittsburgh Department of Physical Medicine and Rehabilitation, is one of the authors of the paper, "Biomechanical Energy Harvesting: Generating electricity during walking with minimal user effort," which appears today in the journal Science.

It describes how the team's knee brace with an attached power pack generates an average of 5 watts of electricity with each step. That means one minute of walking can power a cell phone for a half-hour, and a half-mile stroll can power a laptop for almost an hour.

Dr. Weber, along with researchers from Simon Fraser University and the University of Michigan, says the first application of the technology could be as a source of power for soldiers and astronauts. But there are many other potential uses.

"Producing substantial energy with little extra effort makes this method well-suited for charging powered prosthetic limbs and other medical devices," the study says.

Any medical uses would require Food and Drug Administration approval.

The device also could provide a half-billion children in underdeveloped countries a reliable power source for laptop computers and other portable electronics.

The device captures muscle motion wasted when the leg quits swinging. It harvests energy from "antagonistic" muscles in the leg, including the hamstring, which helps to put the brakes on leg motion at the end of each step.

By helping to stop leg motion, the device has the potential to save a person some energy, while producing a strong supply of electricity. It's the same principle that hybrid vehicles use to recapture power otherwise wasted when brakes are applied.

The study's lead author, J.M Donelan of Simon Fraser University, came up with the idea of harvesting energy from limb motion. Dr. Weber took the challenge and helped design the prototype that transforms walking motion into electricity.

Dr. Weber said the project required a new design principle. He bought plastic gears at a hobby store and assembled them with "chewing gum and duct tape" to produce the first prototype.

An early version of the energy-producing leg brace is scheduled for display in the Science Museum of London, he said.

Other human energy-producing systems exist, including one that generates small amounts of energy in the heel of the shoe, and another that uses the undulating motion of walking to produce electricity in a heavy backpack.

But Dr. Weber said the method he helped develop produces a sizable amount of power without requiring heavy hardware and with little or no extra energy. Future systems actually could save leg energy.

"This approach produces several more magnitudes of power" than other available methods, Dr. Weber said. "Anything that needs electricity could benefit from it."