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.

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.

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

When young men who don't exercise start lifting weights, any number of reps between 3 and 28 changes their fast-twitch muscle fibers in the same way, making them more efficient, and this happens after 8 weeks of training.

Even though muscles got bigger, the actual muscle fibers didn’t get thicker or change type—so the growth might be due to other factors like fluid or connective tissue.

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