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 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.

This is about how often you do special blood flow restriction workouts. It says that training more than 5 days a week might make your slow-twitch muscle fibers grow bigger (by about 19%) compared to training less often, but scientists aren't completely sure yet because the research is mixed.

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.