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 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.

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.

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 don’t work out start lifting weights, their muscles grow more because of changes inside the muscle itself—like more hormone sensors and activation signals—rather than because of hormones floating in their blood.

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.