Young blood could help revive tired ageing muscles, researchers suggest.
Muscles weaken as people age
Old people's muscles are known not to heal in the same way young people's do, but a Stanford University team suggests it is old blood that is to blame.
The study found special stem cells come to the rescue of damaged young muscles, but are not triggered in older ones.
Writing in Nature, the team say tests on mice suggest something in young blood spurs the stem cells into action to repair the muscle damage.
It had been recognised that old muscles had the capacity to repair themselves, but that - for some reason - they failed to do so.
The Stanford researchers focussed on muscle stem cells, called satellite cells, that are spread throughout muscle tissue.
In young mice and humans, the cells come to life if they are needed to repair damaged muscle.
But the team found that they fail to come to the rescue of older muscle - even though they are still present.
In their tests, the team surgically connected the circulatory systems of an old mouse with that of a young one, or to another old mouse.
They then damaged muscle in the older mice.
If old mice were connected to young ones, and therefore had 'young blood' flowing through their bodies, they healed normally.
However, when old mice were connected to other old mice, and were sharing old blood, they healed slowly.
Dr Thomas Rando, who led the research, said it could be the chemical mix surrounding the cells, rather than the cells themselves, which triggered the repair mechanism.
"We need to consider the possibility that the niche in which stem cells sit is as important in terms of stem cell aging as the cells themselves."
Dr Rando said finding the youth-promoting factors in the blood was "as big a fishing expedition as you can possibly imagine".
Dr Anne McArdle, from the School of Clinical Sciences at the University of Liverpool, said: "We know that muscles age, they get significantly smaller and weaker.
"But if they are damaged, they don't recover."
Dr McArdle said: "We know from this study that there is something in the blood of younger animals that's not in the old.
"This is an exciting leap in the research. It proves the fact that you can reverse this problem.
"But it's no small task to identify the factor in the blood that's involved."