When men drank protein shakes, their slow-twitch muscle fibers had more repair cells packed into each square millimeter—but this didn’t happen in their fast-twitch fibers, even though those fibers grew bigger with protein and concentric training.

The study found that doing certain types of exercise increased muscle repair cells in both slow and fast muscle fibers, but adding whey protein didn’t boost those cells in slow fibers—it only helped fast fibers grow bigger when combined with the right exercise.