Gadgets are great until they run out of juice and become a temporary door stop - thankfully a few boffins have invented a way of recharging batteries literally while you walk. 

Using the body's movement to power device is nothing new - many watches do this, but scientists working at Simon Fraser University Locomotion Laboratory in Burnaby, Canada have come up with a device that attaches to your knees and uses it bending motion to create electricity. This knee brace has a drive train that converts mechanical energy into electricity.

"A similar principle is used in hybrid cars to make electricity when you press the brakes; it's called generative braking," said Max Donelan, director at the laboratory.

The recharging knee braces produces five watts of power, which is enough to charge up ten mobile phones. At the moment the braces weigh 1.6kg and walking with it involves a bit more effort to walk about But when the recharger is turned on it actually slows down the leg decreasing the energy required to walk. "Our generator actually helps your muscles out," Donelan told Science magazine, "by decelerating your limbs for you."

Running produces more power at 13 watts. One minute of this pace could give you 30 minutes of talk time on the blower - although you'd be so out of breath you may well need 30 minutes just to have a "quick" chat.

Dr Arthur Kuo, of the University of Michigan in Ann Arbor, said that people were a rich source of energy - much like Keanu Reeves in the Matrix.

"An average-sized person stores as much energy in fat as a 1,000kg battery. People use muscle to convert this stored chemical energy into positive mechanical work," said Kuo. "This work can be performed at a high rate, with 100W easily sustainable."

So if you don't mind partially resembling the Borg for the sake of charging up you mobile, this could be just the ticket.