People Power: A Generator on Your Knee

Feb. 7, 2008 — -- In a lab at Simon Fraser University in Canada, volunteers walked on a treadmill with tiny generators mounted on their knees. Every time they took a step, they created a small amount of electricity by flexing a joint on the generator.

The result: about five watts of power — which may not sound like much, but it's 10 times as much as you need to power a cell phone.

Or maybe, say makers of the generator, the device could be used by a diabetic to run an insulin pump. Perhaps a person who's lost an arm could use it to power a prosthetic one. In poor countries, this knee power might be a cheap source of electricity for laptops.

All without breaking a sweat.

"People are an excellent source of portable power," said Dr. Max Donelan, who led the experiment, in an e-mail to ABC News. "An average-sized person stores as much energy in fat as a 1,000 kilogram (2,200 pound) battery."

But how to harness that energy? There have been experiments before, with heavy backpacks, or shoes that were outfitted to create a current when pressed down on the ground. But the generators were clumsy, and produced little power.

Donelan's team took some off-the-shelf parts — a small generator and an orthopedist's leg brace — and fitted them to six volunteers. Donelan and his colleagues said it was the kind of idea that comes up over a beer. The test subjects, all male, walked on a treadmill, while the researchers monitored their breathing to see how hard they were working.

The concept was simple, and it worked. The test subjects were asked to walk at less than three miles an hour. The extra burden of the brace and generator were minimal to them.

If they worked harder, they could easily triple their electrical output. One volunteer tried running with the brace on, and got up to 54 watts — enough to power an average light bulb.

Donelan and his colleagues published their work in this week's edition of the journal Science.

"Our findings are important because portable electricity represents much more than just a convenience to some people," said Dr. Donelan in his e-mail. "It allows a soldier to communicate, navigate and get home safely. It dramatically improves the quality of life for stroke victims, amputees, and others who rely on power-assisted medical devices to get around.