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.

When some guys get much bigger from weight training and others don’t, it’s not because the big responders have more of these male hormones inside their muscles — both groups have about the same levels, so something else must be causing the difference.

When young guys who’ve never lifted before train with weights for 4 months, those whose muscles produce more of a specific protein (called androgen receptor) tend to grow bigger muscles—even if they all do the same workouts and eat the same food.