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.

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

Lifting heavier weights makes your fast-twitch muscle fibers grow bigger than lifting lighter weights does.

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 heavy weights might make your fast-twitch muscle fibers grow bigger than your slow-twitch ones, according to some new research.