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 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.

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

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.

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.

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.