You might have guessed the muscles of college athletes rebuild faster than their middle aged counterparts.
But now, researchers at the University of California have traced the causes to the behavior of stem cells.
"When we asked muscle stem cells to rebuild the muscle young muscle stem cells did it very well and old muscle stem cells failed," said Irina Conboy from the University of California.
Conboy teamed with colleagues in Denmark, and used casts to immobilize the legs of volunteers so their muscles would atrophy. After the casts were removed, both groups were asked to exercise.
While the muscle tissue of the young volunteers began to rejuvenate, the tissue of the older group began to scar and become inflamed -- suggesting the muscle cells were not replenishing themselves.
"So we were actually fortunate enough to identify. Why that happens at the level of molecules," said Conboy.
Conby's team identified three molecules that help instruct those stem cells about when to begin creating new muscle tissue. They also found evidence that the levels of those messenger molecules change as we age.
So in separate experiments they re-adjusted the levels to match those of a younger person. They say the result was a potentially age reversing effect in older tissue samples.
"And what is clinically relevant is that by changing molecules back to young stages, we changed cells back from being old to being young," said Conboy.
Conboy says the technique has rejuvenated muscle cells in both human tissue samples and in mouse models and while human trials are still years away, she believes the compounds, delivered through the blood, could potentially stimulate stem cell activity in a wide range of tissue and perhaps slowing the effects of aging on the body.
"So we don't think that they will immediately become 20 years old. In lab conditions muscles taken from older people were greatly rejuvenated. But we don't expect the same rejuvenation will happen to the whole person. But we hope this unfortunately decline in strength and agility will be forestalled or delayed," said Conboy.
In the near term, Conboy's study produced some discouraging results for older athletes. It found that vigorous exercise after periods of inactivity did little to rebuild healthy tissue in older muscles, suggesting that older people might benefit more from a lighter, but more consistent exercise program.