When beginners lift heavier weights, they do more total work overall, but it doesn't actually make them stronger or build more muscle than using medium weights.

Lifting heavier weights for fewer reps makes your legs much stronger than lifting lighter weights for more reps, but both types of training help your upper body a little bit too, and this might work differently for different muscles.

When lifting weights with your arms, whether you use heavy or light weights, you get about the same improvement in how many reps you can do with both heavy and light weights.

For young women new to weight training, lifting weights until you can't do more reps (either 5-7 reps per set or 10-14 reps per set) three times a week for two months gives about the same muscle growth and strength gains, no matter which rep range you use.

Lifting heavier weights for fewer reps makes well-trained young men stronger faster than lifting lighter weights for more reps, especially in exercises like squats and bench presses.

Lifting heavy weights makes your legs much stronger than lifting light weights, but for your arms, both heavy and light weights make you about equally stronger.