If young guys lift weights for eight weeks—whether they go heavy or do more reps—they’ll likely build muscle in their upper body and back, and that can be seen with ultrasound scans.

If young guys lift heavy weights with fewer reps, they’ll likely build more chest and back muscle than if they lift lighter weights with more reps — at least over 8 weeks.

If you're already fit and lift weights, doing anywhere from 5 to 20 sets per muscle group each week will make you stronger and build more muscle — and it doesn't matter much which number you pick within that range.

If young men lift weights with the same total effort—whether they use heavy weights with long breaks or light weights with short breaks—they’ll still grow about the same amount of muscle. So, different ways of lifting can both work.

If you're a young man who already trains regularly, doing more sets with lighter weights for 6 weeks can make your thigh muscle grow a little bigger — but doing fewer sets with heavier weights won't make it grow at all.

If you lift light weights with short breaks between sets for eight weeks, your upper arms might grow bigger than if you lift heavy weights with long breaks—even if you do the same total amount of work. Light weights + quick rest = more muscle growth in this study.