If you lift heavy weights (85% of your max) instead of light ones (30% of your max) for nine weeks, you’ll get stronger on exercises like squats and deadlifts—even if your muscles don’t grow any bigger than if you’d lifted light weights.

Lifting heavier weights for nine weeks made young men stronger in a max lift test than lifting lighter weights, even when both groups worked until they couldn't do another rep.

Lifting heavier weights might help you get stronger a bit more than lifting lighter weights, but the difference is so small that we can’t be sure it’s real — it could just be luck.

Lifting heavy weights might help you build slightly more muscle than lifting light weights if you're new to training, but the difference isn't big enough to say for sure it's real.

If you're a healthy young adult lifting weights, lifting heavier weights (fewer reps) helps you get stronger faster than lifting light weights (more reps), but lifting medium weights works almost just as well as heavy weights — the difference isn't big enough to say for sure.

For exercises that move just one joint—like bicep curls—lifting heavier weights leads to better strength gains than lifting lighter weights, even if both are done until exhaustion.