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.

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 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.

Lifting weights with fewer reps (3-11) builds more muscle than doing lots of reps (20-28) in young guys new to training, especially after 8 weeks.

Lifting lighter weights many times in a row might help grow certain slow-twitch muscles better because they stay working longer.