When muscles grow from lifting weights, some types of muscle fibers might not get as strong or reliable as others, according to the numbers.

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.

Scientists aren't sure yet if a newer type of lighter weight training works better than traditional heavy lifting for building a specific kind of muscle fiber in adults, because there haven't been enough direct comparisons and the few studies done so far have mixed results.

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

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.

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.