From this Science News article Muscles remember past glory:
"Muscles hold memories of their former fitness in nuclei (green, shown on muscle fiber) that help the muscle bounce back to fitness when training begins after a period of inactivity.
Pumping up is easier for people who have been buff before, and now scientists think they know why — muscles retain a memory of their former fitness even as they wither from lack of use.
That memory is stored as DNA-containing nuclei, which proliferate when a muscle is exercised. Contrary to previous thinking, those nuclei aren’t lost when muscles atrophy, researchers report online August 16 in the Proceedings of the National Academy of Sciences. The extra nuclei form a type of muscle memory that allows the muscle to bounce back quickly when retrained.
The new study suggests that pumping muscles full of nuclei early in life could help stave off muscle loss with age."