People whose muscles grow a lot with training might get that boost because of special cells that help muscle fibers expand, while those who don’t gain as much might not see the same cell activity — but scientists aren’t fully agreed on this yet.

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 really big—more than 22% larger—they need extra help from special cells to add more nuclei so they can keep growing.

When muscles get bigger, tiny repair cells called satellite cells usually wake up to help—but new research shows muscles can still grow bigger even if those cells are turned off, using other tricks inside the muscle fibers themselves.

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