If you're already fit and lift weights, your muscle cells don't seem to add more helper cells or nuclei, even when you change up your routine—meaning those extra cells might not be needed to make your muscles bigger.

If you're already fit and lift weights regularly, adding variety to your workouts or sticking to the same routine won't noticeably change the number of special muscle repair cells or their nuclei — your muscles stay pretty much the same on that front.

When you lift weights and your muscles get bigger, the number of special repair cells and nuclei inside them tends to go up a bit — but that’s not the whole story; other things are also helping your muscles grow.

Some people get much stronger and build more muscle from weight training than others—this might be because their muscle cells grow more new nuclei, but not every study agrees on this.

When your muscles get bigger from lifting weights, tiny repair cells called satellite cells stick themselves onto muscle fibers to give them extra nuclei—like adding more workers to a factory—so the muscle can keep growing big and strong.

When you lift weights or put extra stress on your muscles, tiny repair cells called satellite cells usually wake up and get busy—this helps your muscles grow bigger, but sometimes muscles can still grow even if these cells don’t activate.