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.

When men and women do strength training, their big, powerful muscle fibers grow about the same amount—neither sex consistently gains more muscle than the other.

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

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

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

When women who haven't trained before do weightlifting for 6 weeks, their fast-twitch muscle fibers change: some decrease while others increase, and this happens no matter how fast or hard they train.