By DAVID DERBYSHIRE

Last updated at 00:43 08 February 2008

It gives a whole new meaning to the phrase power walking.

A gadget that converts the movement of the knees while walking into electricity has been unveiled.

On a gentle stroll, the knee-brace generates enough electricity to run a mobile phone.

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Although the prototype is bulky - like something Robocop might have attached to his leg - smaller versions could one day be incorporated into clothing.

It could allow joggers to power up their iPods while running or commuters to charge their mobile phones while dashing for a train.

The inventors even believe it will help soldiers cut down on the number of batteries they need to carry into battle.

Inventor Dr Arthur Kuo, of the University of Michigan, said the device - called a "biomechanical energy harvester" - worked in the same way as the power-generating brakes found in hybrid cars.

These collect energy that would otherwise be lost as heat when a car slows down. The knee brace harvests energy lost when someone bends their knee after swinging the leg forward to take a step.

Dr Kuo said knee joints were ideally suited for generating electricity.

"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," he said.

"We believe that when you're slowing down the knee at the end of swinging the leg, most of that energy is just wasted."

Dr Kuo and colleagues from the Simon Fraser University in Vancouver and University of Pittsburgh tested their invention on six men walking on a treadmill at a leisurely two miles an hour.

With a device on each leg, the volunteers generated around five watts of electricity while using very little extra energy to walk.

That would be enough power to run ten mobile phones at the same time. By running, one volunteer was able to generate 54 watts of power - enough for a conventional light bulb.

Dr Kuo, who published his test results in the journal Science, said his device needed refining.

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

"We hope to improve the device so it is easier to carry."