The modified mice: They were also found to be aggressive

'Supermouse' goes head-to-head with normal mouse

It also lives longer, and breeds later in life compared with its standard laboratory cousin.

The research has been conducted at Case Western Reserve University in Cleveland, Ohio.

Details of the scientists' new transgenic animals are published in the Journal of Biological Chemistry.

The mice were produced to study the biochemistry at play in metabolism and could aid the understanding of human health and disease.

The GM rodents can run five to six kilometres at a speed of 20 meters per minute on a treadmill, for up to six hours before stopping.

The performance was tested on rodent treadmills

He told BBC News: "The muscles of these mice have many more mitochondria. These are the little 'engines' in the cell that produce energy. For some reason, the number of mitochondria are around 10 times more than we see in the muscle of their littermates."

The mice over-express a gene responsible for the enzyme phosphoenolypyruvate carboxykinases (PEPCK-C). Normal expression is in the liver, in the production of glucose.

The scientists found their new mice would eat twice as much as normal mice - but weigh half as much. They could also give birth at three years old - which in human terms is akin to an 80-year-old woman giving birth.

The research is featured in the Journal of Biological Chemistry

But Professor Hanson played this down. "Right now, this is impossible to do - putting a gene into muscle. It's unethical. And I don't think you'd want to do this. These animals are rather aggressive, we've noticed."

Scientists say such work is more likely to help them understand human conditions, such as those which lead to wasting of the muscles.