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.

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

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.

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

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

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