Scientists looked at only 8 small studies about how different weight training affects muscle growth. They found there isn't enough research yet to say for sure which exercises work best for building specific types of muscle fibers.

Scientists aren't sure if lifting more weights always makes muscles grow bigger, because their research shows the results are often unclear and sometimes show no effect at all.

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.

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.

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.

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.