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...

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 scientists measure changes in muscle cells after strength training using two different methods, the results don't match up well - so they should be looked at separately rather than compared directly.

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.

When untrained young men lift lighter weights until they get tired for 10 weeks, their cells get better at cleaning up and renewing their energy factories, which helps them handle lots of exercise.