People new to weight training who have more of a certain protein in their muscles (called ERα) tend to gain more muscle size after 10 weeks of leg exercises — so this protein might help explain why some people build muscle faster than others.

Doing 10 weeks of leg exercises that strengthen your knee muscles boosts one specific hormone receptor in your muscles (ERα), but doesn’t change two others (AR and ERβ).

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.

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.

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.

If you're a young, fit guy who lifts weights, having more androgen receptors in your muscles might mean you gain more muscle size and strength after 12 weeks of training compared to others.