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.

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 eat way less food than usual, giving them a high dose of testosterone makes some muscle-building signals in their cells stronger, but it doesn’t stop their muscles from breaking down — just like in guys who get a fake pill.