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, 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 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.

Lifting weights twice a week for six weeks—whether doing fewer heavy reps or more lighter ones—can help young guys' muscles use oxygen better, which means their muscles may get more efficient at producing energy.

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.

How long you lift weights—up to 6 months—doesn’t seem to change the number of muscle fibers you have, according to this claim.