As mice get older, a key cellular switch called mTORC1 in their muscles turns down genes that keep muscle structure strong, turns up genes linked to inflammation, and flips on signals seen when nerves stop talking to muscles. Giving them a drug called rapamycin helps reverse some of these changes, suggesting this switch plays a big role in how muscles age.

As mice get older, the places where nerves connect to muscles show big changes in genes tied to inflammation and tissue breakdown — especially in a key area called the neuromuscular junction — which might weaken the connection and affect muscle function.

As we get older, a protein in our muscles called mTORC1 can become overactive, which slows down the body's cleanup process for damaged parts inside cells. Taking rapamycin might help turn that cleanup process back on.

In older mice with overactive muscle metabolism, giving them a drug called rapamycin changes the types of fats in their muscles — some go up, some go down — which might mean the drug helps reset how aging muscles handle fat.

Rapamycin helps 'rejuvenate' some muscles in old mice but not others—turns out, it depends on the muscle and how connected it is to nerves. In muscles that are more affected by aging, the drug might actually make nerve-related problems worse.

Older male mice have bigger changes in muscle chemicals after taking rapamycin than older female mice, suggesting their bodies respond differently to this drug as they age.