Lifting heavy weights three times a week for six weeks makes fast-twitch muscle fibers grow bigger by about 12% in people who don't usually exercise, showing it especially targets these powerful muscles.

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

When young guys who haven't lifted weights before do strength training for 10 weeks, their muscle fibers grow in a way that makes them stronger, not just bigger with extra fluid. It's like building real muscle instead of just puffing it up.

When untrained young men lift weights until they can't anymore three times a week for six weeks, and they do the same total amount of work, their muscles grow about the same amount whether they lift heavy or light weights.

If you lift weights the same total amount each week, it doesn’t really matter whether you do it in one big session or spread it out over several days—your muscles will grow about the same either way.

If young men lift light weights with very short breaks between sets for 8 weeks, their upper arms grow more than if they lift heavy weights with long breaks—even if they do the same total amount of lifting—because the short breaks create more muscle-burning stress that helps muscles grow bigger.