When young men lift weights, if a specific protein in their muscles gets activated strongly exactly 5 hours after working out, they tend to build more muscle over the next 4 months—so this protein spike might tell us who’s going to grow the most from training.

In guys who are new to weightlifting, how strongly their muscles signal growth right after a workout (at the 5-hour mark) seems to predict how much muscle they’ll gain over the next 4 months.

When you lift weights, your muscles temporarily stop building protein, but right after you finish, they kick into high gear to repair and grow—this happens because key signaling molecules in your muscles get activated during recovery.

When young guys who’ve never lifted weights before start training, how much their muscles grow depends a lot on what’s happening inside the muscle itself—like changes in certain proteins—not so much on hormones floating in their blood.

In guys who are new to weightlifting, a spike in a certain body chemical (IL-6) right after a workout is linked to bigger muscles after 16 weeks of training—but that spike alone doesn’t fully explain why some guys get bigger than others, because other internal changes in the muscle also matter.

Doing short bursts of intense exercise can help your muscles make more proteins, especially the ones in energy factories inside cells, but it doesn’t always make your muscles bigger if you only do it for less than six weeks.