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.

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.

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.

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 heavier weights might make your fast-twitch muscle fibers grow more than lighter weights, but the results aren't totally certain.

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.