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.

Lowering weights slowly wakes up muscle stem cells right after exercise, but lifting weights regularly over time builds up more of these cells permanently.

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 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.

Doing exercises where you push the weight up (concentric) makes more muscle repair cells grow than doing exercises where you lower the weight slowly (eccentric), at least in young men training for 12 weeks.