Professor Lee Sweeney explains the research methods and findings

The scientists told a meeting of the American Society for Cell Biology in San Francisco that the technique might eventually ensure that humans will be able to retain muscle strength into old age.

It could also help treat the effects on muscles of certain degenerative diseases.

Presenting the findings, Dr Lee Sweeney of the University of Pennsylvania Medical Centre said: "Our results show that it may be possible to preserve muscle size and strength in old age using this approach."

Ethical dilemma

But he also warned of possible ethical and safety related objections to developing the technique for use in humans. The injections could be a "perfect perfomance enhancer", he said.

The treatment would be undetectable in the blood of athletes and, Dr Sweeney says: "You build muscle mass and strength without exercise."

The technique involves injecting a virus carrying a growth-producing gene into muscle cells. The virus passes the gene - responsible for producing insulin growth factor (IGF-1) - into the muscle cells. Once in place the gene causes the muscle to produce IGF-1, which leads to more muscle growth.

The muscles increased in strength by up to 27% in elderly mice - normally mammals lose up to a third of their muscle mass and strength as they age.

But the effect is only seen in muscles that directly receive the injections, so effective treatment may only be possible by giving scores of injections to patients.