If young guys are given a drug that lowers their testosterone, they don’t build much muscle from weight training—while others with normal testosterone gain about 1.5 kg of muscle in 6 weeks. This shows testosterone is key for getting the most out of lifting weights.

When young men have low testosterone on purpose for the study, their muscles don’t respond as well to weight training at the cellular level — testosterone seems to help muscles fully 'feel' and react to the workout.

For men who lift weights regularly, the body's testosterone reaction after workouts was about the same whether they did short, intense sessions or longer, higher-volume ones, and this didn't really change over 8 weeks of training.

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.

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.

Even after a super hard workout, the body’s baseline testosterone level doesn’t stay high the next day — it goes back to normal quickly.