A way to reverse ageing has been discovered which allows withered muscle to rebuild itself by turning back a “biological clock”.

The effect has already been demonstrated on human muscle tissue in the laboratory.

Scientists in the US believe the breakthrough could lead to new treatments that rejuvenate and strengthen ageing bodies or combat degenerative diseases.

Their findings also underline the importance of staying active for older people, since this reduced age-related muscle loss.

Professor Irina Conboy, from the University of California at Berkeley, said: “Our study shows that the ability of old human muscle to be maintained and repaired by muscle stem cells can be restored to youthful vigour given the right mix of biochemical signals.”

Previously the same team had shown that molecular “messages” from muscle cells alter with age to affect tissue repair. As people get older, their ability to restore and rebuild lost muscle is weakened.

The US researchers, working with colleagues from the Institute of Sports Medicine and Centre of Healthy Ageing at the University of Copenhagen in Denmark, compared muscle tissue samples from around 30 healthy men. Half the volunteers were young 21 to 24-year-olds and half aged between 68 and 74.

At the start of the study, samples of muscle tissue were surgically removed from the participants' thighs. The men then had the leg from which the biopsies were taken immobilised in a cast for two weeks so that their muscles atrophied.

After the casts were removed, the men exercised with weights to rebuild their wasted muscles. The scientists found that during the exercise period the muscles of younger volunteers had four times more regenerative stem cells engaged in tissue repair than those of older participants. Old muscle also showed signs of damaging inflammation and scarring.

Analysis of the samples revealed for the first time a biological pathway involved in muscle repair that relied on an enzyme called mitogen-activated protein kinase (MAPK). The enzyme, a type of active protein, stimulated a biological “switch” on muscle stem cells called Notch that triggered growth.