When you lift light weights until you can't anymore, it fully uses your fast-twitch muscle fibers even though the weight isn't heavy.

If you're a young man who already lifts weights, doing workouts with either light weights (lots of reps) or heavy weights (fewer reps)—as long as you push yourself until you can't do another rep—will make your muscles grow just as much either way.

Lifting weights until you can't lift anymore makes muscles grow bigger no matter how heavy the weights are, but using heavier weights builds more strength than lighter 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.

When people who are already fit push themselves as hard as they can during weight training, it doesn't matter if they lift heavy weights for a few reps or lighter weights for many reps—both ways build muscle about the same.

When you lift weights until you can't anymore, you build similar muscle size no matter how heavy the weights are, but lifting heavier weights makes you stronger and faster at moving them than lighter weights.