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.

Young guys need their natural testosterone to get the full muscle-building benefits from weightlifting — if their testosterone is blocked with drugs, their muscles don’t grow as much, even if they train just as hard.

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.

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.

Even if your body releases a lot of muscle-building hormones after a workout, that doesn’t mean you’ll grow bigger muscles any faster than someone whose body releases way less—because studies show both people end up with the same muscle gains.