Artificial muscles that can lift loads 200 times heavier than a human muscle could power the limbs of superstrong robots in the future, scientists say.

In tests the muscles, made from twisted strands of carbon yarn, were able to pull more than 100,000 times their own weight.

"The artificial muscles that we've developed can provide large, ultrafast contractions to lift weights that are 200 times heavier than possible for a natural muscle of the same size," said professor Ray Baughman from the University of Texas at Dallas.

"Because of their simplicity and high performance, these yarn muscles could be used for such diverse applications as robots, catheters for minimally invasive surgery, micromotors, mixers for microfluidic circuits, tunable optical systems, microvalves, positioners and even toys."

However, for technical reasons, they are unsuitable for replacing lost or damaged muscle in the human body.

The muscles are made from carbon nanotubes -- hollow strands of carbon 10,000 times thinner than human hair yet 100 times stronger than steel.

Yarn made from the nanotubes is soaked in wax and shaped into a coiled structure. When heated by electricity or a flash of light, the wax expands, causing the yarn to contract and twist.

The process goes into reverse when the heating is stopped and the yarn cools.

Results of tests of the artificial muscle have been published in the journal Science.

In terms of power-to-weight ratio, the muscles were four times more efficient than an internal combustion engine.

They were also capable of operating in temperatures above 1,000C, higher than the melting point of steel.

Prof Baughman said one possible application could be "smart suits" for firefighters, which would be designed to react to dangerous temperatures by providing greater protection.

"This intelligent materials function aspect is very important," he said.