Both light and heavy lifting can boost the number of muscle repair cells in slow-twitch fibers, which might help muscles recover or adapt over time.

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.

Even though muscles got bigger, the actual muscle fibers didn’t get thicker or change type—so the growth might be due to other factors like fluid or connective tissue.

The slow-twitch muscle fibers got bigger no matter what kind of workout or drink the men used—whether they lifted slowly up or down, or drank protein or sugar water.

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.