Muscle fiber hypertrophy in response to 6 weeks of high-volume resistance training in trained young men is largely attributed to sarcoplasmic hypertrophy

Even the muscles that are built for endurance can get bigger—like, over 10% bigger—if you do the right kind of strength training for a few months. This goes against what people used to think.

When young guys who already work out do a lot of high-volume weight training for 6 weeks, their muscles grow mostly because of fluid and non-contracting parts inside the muscle cells, not because they’re building more of the actual muscle-building proteins — and this change sticks around for over a week after they stop training.

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 heavy weights for six weeks changes specific proteins in the muscles of fit young men. This suggests their muscles are switching to a way of making energy that doesn't require oxygen.

After young guys stop intense weight training, their muscle proteins stay low for over a week, suggesting it's not just temporary swelling or soreness.

Even though muscle fibers get 23% bigger with high-rep training, the amount of actin protein inside doesn’t go up, and the usual tight link between fiber size and actin breaks down.