WHO would win an arm-wrestling contest between a robot and a human? That's easy, you say - after all, machines can move boulders and crush steel, so surely a robot would overpower a human?

On 7 March 2005, 300 people gathered in a ballroom in San Diego, California, to find out. Under the watchful eye of a professional arm-wrestling official, three robotic arms took turns squaring off against a human. One by one, the electrically powered challengers were soundly defeated by their unyielding opponent, a 17-year-old girl.

It was a poor showing by the bots, but the result was not so surprising to Yoseph Bar-Cohen, the organiser of the event. The catch was that the robot arms used artificial muscles, a still-nascent technology which mimics the action of the human equivalent. "They look like a muscle, but the mechanism inside is different," says Bar-Cohen, a physicist at NASA's Jet Propulsion ...