For women who haven't lifted weights before, doing traditional heavy strength training with slow, controlled movements for 6 weeks gives the best overall muscle growth in all types of muscle fibers.

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.

We just don’t know yet if light or heavy weights make muscles grow better — the results could swing either way in future studies, so we need more 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.

Even though muscles get bigger after 10 weeks of weightlifting, the individual muscle fibers don’t necessarily get thicker — the growth might come from how the fibers are arranged, not from each one swelling up.

Lifting heavier weights might make your fast-twitch muscle fibers grow more than lighter weights, but the results aren't totally certain.