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.

Whether you lift lighter weights or heavier weights until you can't do any more reps, both ways give you the same muscle growth and strength gains if you're already used to training.

Lifting heavy weights until you can't lift anymore makes young male athletes much stronger than lifting light weights over 8 weeks, even if both make muscles grow about the same size.

Lifting heavy weights three times a week for six weeks makes untrained young men much stronger than lifting light weights, even though both groups gain about the same amount of muscle size.

Lifting heavy weights until you can't lift anymore makes young male athletes much stronger than lifting lighter weights over 8 weeks, showing that heavier loads train muscles and nerves better.

When young male athletes push themselves as hard as they can in weight training, whether using heavy or light weights, their muscles grow about the same amount after 8 weeks.