Even if you lift light weights, if you go until you can’t do another rep, your muscles end up using almost all the same fibers as if you lifted heavy — which might be why both ways make muscles grow similarly.

Even if you lift light or heavy weights, if you push until you can’t do another rep, your muscles end up using nearly all the same fibers — so they grow similarly.

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 light or heavy weights until failure doesn’t clearly make your fast-twitch muscle fibers bigger in your thighs — the data is too mixed to say one is better.

When your muscles get really tired during a workout, your body has to turn on more muscle fibers to keep going — and that might help them grow bigger.

Whether you lift light weights or heavy weights until you can't do another rep, your slow-twitch muscle fibers in your thighs grow about the same amount — but we can't be sure because the results are too uncertain.