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 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 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 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.

There might be other types of repair cells in muscles, besides the well-known ones, that help muscles grow bigger — but we’re not sure if this happens in people yet.

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.