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 young men lift weights to build muscle, their muscle-building process speeds up at first but slows down after 10 weeks, even if they keep increasing the difficulty.

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.

After 10 weeks of working out, men who are already somewhat active don't build muscle as efficiently as before, and their bodies don't respond as strongly to exercise—this happens whether they're doing cardio or weight training.

For men who exercise regularly, doing both weight training and intense cardio together doesn't slow down muscle building compared to just weight training alone, even after 10 weeks of working out.

When young men who haven't trained before do one-arm exercises for eight weeks, their resting muscle building speed goes up by almost half in that arm, but the part that builds muscle fibers stays the same, showing different parts of muscle react differently to exercise.