When you lift heavy weights, tiny cells in your muscles wake up and stick to muscle fibers, and this seems to help your muscles grow bigger faster.

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.

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.

Lifting heavy weights makes your muscles stronger, and that happens because your brain gets better at telling your muscles how to work together, and your muscle fibers themselves grow bigger with help from special repair cells.

Doing light weightlifting while restricting blood flow to the muscles can cause a big jump in special muscle repair cells, and this effect gets even bigger if you do it twice in a row.

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.