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

Lifting lighter weights might make your slow-twitch muscles grow more than your fast-twitch muscles, according to some new ideas.

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

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

When adults lift heavy weights, their fast-twitch muscle fibers grow much more than their slow-twitch ones, based on several studies.

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