Body muscles tend to retain a memory of past fitness as DNA-containing nuclei, which proliferate when a muscle is exercised, even as they become weak due to lack of use.

The finding was reported online by researchers on August 16 in the Proceedings of the National Academy of Sciences.

From Sciencenews.org:

“It does fly in the face of a lot of peer-reviewed, published data,” he says. But the selective death of just some of the nuclei in a muscle cell would require a special kind of apoptosis. “The conventional wisdom doesn’t make much sense from a cell and molecular perspective,” Schwartz says. Gunderson’s group has come up with an explanation that seems more plausible. “Their data just feels right.”

If the results hold up in people, sports agencies may want to reconsider how long they ban athletes suspended for taking steroids. Previous research has shown that testosterone boosts the number of nuclei in muscle cells beyond the amount produced by working out. “If you have nuclei that last forever, then you would also have an advantage that could last forever,” Gundersen says.

Well, maybe not exactly forever. As people age, their ability to build muscle mass declines. The new study suggests that pumping muscles full of nuclei early in life could help stave off muscle loss with age. “This could be an argument for mandatory physical training in schools,” Saltin says.

It was suggested by study leader Kristian Gundersen, a physiologist at the University of Oslo in Norway that these findings demonstrate that exercising early in life could help in fending off frailness in the elderly and raised questions about how long doping athletes should be barred from competitions.