When young guys who don't usually exercise lift weights until they're tired, it wakes up special muscle repair cells and helps build stronger muscles—no matter how heavy the weights are.

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 untrained young men lift lighter weights until they're tired for 10 weeks, it changes how their fast-twitch muscle cells organize themselves to better support muscle growth.

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.

Lifting weights until you're tired, whether with light or heavy weights, makes your muscles bigger and stronger in the same way for beginners.

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.