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.

Lifting weights until you're tired, whether with light or heavy weights, makes your muscles bigger and stronger in the same way for beginners.

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.

Whether you lift heavy weights for fewer reps or light weights for more reps—as long as you push until you can’t do another rep—your muscles grow about the same amount.

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.