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.

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.

Whether you use light or heavy weights, your muscles can grow about the same amount if you train hard enough—both methods work for building muscle size.

If you're a young guy who already lifts weights, doing workouts until you can't do another rep for 12 weeks will make your legs and shoulders just as strong whether you use light or heavy weights—but for the bench press, heavy weights make you stronger than light ones.

If you only work out one leg with resistance training, that leg gets stronger, but the other leg doesn’t get stronger at all—even if you don’t train it.

To get stronger, it matters more how heavy the weight is than whether you push to your absolute limit—lifting heavier weights makes you stronger, no matter if you stop short of failure.