Some men gained more nuclei inside their fast-twitch muscle fibers when they drank protein and lifted up, while others gained them when they did the slow lowering motion—even without protein—showing that adding nuclei doesn’t always go hand-in-hand with muscle growth.

The study found that muscle cells add new nuclei in two weird ways: either with protein and lifting up (concentric), or with no protein and lowering down (eccentric)—but not in other combinations, showing it’s not just about getting bigger, but how you train and what you eat.