When men lift weights, their testosterone helps their muscles grow by turning on certain signals in the muscle cells. This hormone response can last up to two days after a workout, making muscles more responsive to testosterone during that time.

Young men need normal testosterone levels to get the full muscle-building benefits from weight training — low testosterone blunts the body’s muscle-growth signals.

Taking much more testosterone than your body naturally makes can make your muscles bigger and stronger—even if you don’t lift weights or do any exercise.

When young men lift weights for 12 weeks, their short-term spikes in testosterone and IGF-1 after workouts don’t seem to predict whether they’ll gain more muscle or get stronger.

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.

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.