When you lift weights, lighter weights might build more of your slow-twitch muscle fibers, while heavier weights might build more of your fast-twitch muscle fibers, but scientists aren't totally sure yet.

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

When adults lift heavy weights, their fast-twitch muscle fibers grow much bigger than their slow-twitch fibers - sometimes 10-15% more. But when they do lighter weights with restricted blood flow, both fiber types grow more equally or the slow-twitch ones grow more.

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.

Slow-twitch muscle fibers (type I) can work longer without getting tired compared to fast-twitch fibers (type II), which might help you lift weights for more time before exhaustion.

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