When untrained young men lift lighter weights many times until they can't lift anymore, it builds just as much muscle in their calf muscles as lifting heavier weights fewer times. Both ways made their muscles about 10% bigger after 8 weeks.

If you do push-ups or bench presses with light weights until you can't do another rep, twice a week for two months, your arms and chest will grow just as much — so push-ups can be just as good as gym machines for building muscle, if you push yourself just as hard.

Even lifting light weights really slowly until you can't do another rep can make your muscles bigger, just like lifting heavy weights.

If you lift weights until you can’t do another rep—whether you use heavy weights or light ones—you’ll end up with about the same muscle growth after nine weeks, as long as you’re putting in the same effort and number of reps.

When untrained young men lift weights until they can't lift anymore, their calf muscles grow bigger by 7-14%, no matter how heavy the weights are. This shows that pushing hard is what makes muscles grow, not how heavy the weight is.

Even if you lift the same weight every time without making it heavier, your arms can still get noticeably bigger in just two months — at least when you're just starting out and you're a young woman.