Lifting light weights without pushing to exhaustion doesn't make untrained young men stronger or grow their slow-twitch muscle fibers, showing that just doing more reps isn't enough if you don't get tired.

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're tired, whether with light or heavy weights, makes your muscles bigger and stronger in the same way for beginners.

Lifting lighter weights with more reps might make your slow-twitch muscles grow bigger than lifting heavier weights with fewer reps.

Lifting lighter weights might make slow-twitch muscle fibers grow more than lifting heavier weights, but scientists aren't totally sure yet because the results aren't clear.

Lifting heavier weights makes you stronger and improves your brain-muscle connection more than lifting lighter weights, even if you do the same total amount of work and build the same muscle size.