In young men who don't exercise, slow-twitch muscle fibers have more energy-producing parts than fast-twitch fibers to start with, and even after 10 weeks of weight training—which boosts these parts overall—the slow-twitch ones still keep their lead.

When young guys who don't exercise start lifting weights for 10 weeks, the energy-producing parts inside their muscles grow faster than the muscles themselves.

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.

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 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.