We just don’t know yet if light or heavy weights make muscles grow better — the results could swing either way in future studies, so we need more research.

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.

There are only a few small, short studies on this topic, and many didn’t even track whether people stuck to their workout plans — so we can’t trust the results very much.

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.

Whether you lift light weights or heavy weights until you can’t do another rep, your muscle fibers don’t seem to grow significantly more with one than the other — but we can’t be sure because the results are too uncertain.

All the studies looked only at leg muscles in young people — we don’t know if the same results apply to arms, women, or older adults.