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.

Whether you lift light weights many times or heavy weights fewer times—both can make you stronger in exercises like squats and deadlifts, as long as you push to exhaustion.

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

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.

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.

Whether you train with one joint (like leg extensions) or multiple joints (like squats), you get just as strong at the movement you train — but if you train a simple movement, you get a bit stronger at holding still too, compared to complex movements.