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.

If you lift heavy weights, you get better at doing more reps with heavy weights; if you lift light weights, you get better at doing more reps with light weights — your muscles adapt specifically to the weight you use.

If you lift heavy weights (80–85% of your max) instead of doing lots of lighter reps (60% of your max), your legs get stronger faster — after 6 weeks, you’ll likely gain 5–6 kg more in leg strength.

Using light weights for arm exercises can make your arms slightly more muscular, but using light or heavy weights for leg exercises doesn’t change leg muscle size in young women who haven’t trained before.

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.

Lifting heavier weights (fewer reps) leads to bigger strength gains than lifting lighter weights (more reps), even if both are done until exhaustion.