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.

In young men who don't exercise, fast-twitch muscle fibers are more tightly packed with muscle-building parts than slow-twitch fibers. When they start lifting weights, this difference stays the same, but all muscle fibers get bigger.

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.

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 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.

If you're already used to lifting weights, it doesn't really matter if you change up how heavy you lift, how many reps you do, or how long you rest—your muscles will grow about the same either way.