Hybrid-electric cars such as the Toyota Prius take advantage of stop-and-go driving using so-called "regenerative braking," where the energy normally dissipated as heat is used to drive a generator.

Now scientists can apply the same principle to walking and a stroll from Hall at CeBIT to Hall 6 could be enough to re-charge your mobile phone or Blackberry.

US and Canadian scientists adapted a knee brace to generate enough energy to power a mobile phone for 30 minutes from one minute of walking.

Walking is a lot like stop-and-go driving.Within each stride muscles are continuously accelerating and decelerating the body. Using a series of gears, the knee brace assists the hamstring to slow the body just before the foot hits the ground, and to generate electricity.

Sensors on the device switch the generator off for the remainder of each step to put less strain than if it was constantly producing energy. The 1.6kg device produced an average of 5 watts from a slow walk. "We also explored ways of generating more electricity and found that we can get as much as 13 watts from walking," explains Dr Max Donelan of Simon Fraser University in Canada, lead author of the paper. "

The knee brace is the latest development in a field known as "energy harvesting". "We're pretty effective batteries," Dr Donelan told BBC News. "In our fat we store the equivalent of about a 1,000kg battery."

Portable power will be used first by amputees to charge their powered prosthetic limbs.

Dr. Donelan has set up a spin-out company to develop a light weight, slim-line version of the knee brace. "That's about 18 months away, so it's not science fiction far in the future stuff." he notes.

Brace yourself for Knee-Power