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.

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 young guys who’ve never lifted weights before train with weights for 16 weeks, those whose muscles develop more androgen receptors tend to grow bigger muscles—and this receptor increase might explain about a quarter of why some people grow stronger or bigger than others.

If you're a young, fit guy who already lifts weights, doing more weight training for 12 weeks won't change how many androgen receptors are in your muscles—but the number you started with can tell you how much bigger your muscles will get.

Some people who lift weights and end up with more of a certain protein in their muscles tend to grow bigger muscles, but not everyone sees this link—some people with lots of the protein don’t grow much, and others with less do.

People who build more muscle from lifting weights might be more sensitive to testosterone because their muscles have more 'testosterone receivers' after training — but scientists aren’t completely sure yet.