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

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.

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.

If you're already used to lifting weights, it doesn't really matter if you change up how heavy you lift, how many reps you do, or how long you rest—your muscles will grow about the same either way.

When young men who don't normally exercise do eight weeks of weight training, their fast-twitch muscle fibers get bigger, but their slow-twitch fibers stay the same size and the overall mix of muscle types doesn't change.