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...

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.

When young guys who already work out do a lot of high-rep weight training for a short time, their muscles get bigger but their energy-producing parts actually become less dense, meaning bigger muscles don’t always mean more energy factories inside them.

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.

Lifting weights for three months makes the muscle fibers in young men's biceps get bigger — both the endurance-type and power-type fibers.