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 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.

For young active men, doing lighter weights with more reps three times a week for a month might cause less muscle damage than lifting heavy weights with fewer reps, based on a blood test that measures muscle stress.

Lifting heavy weights at least twice a week, doing full movements for a few sets each time, and doing it early in your workout can help healthy adults get stronger compared to not exercising or doing other types of training.

Lifting heavy weights fewer times, three days a week for a month, helps young active men build more muscle and get stronger than lifting lighter weights many times.

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.