A guy asked me this question and I was a little stumped. Back in the day they did a biopsy on a bodybuider and found the muscle fibers were no bigger than a guy who did no exercise. Does this mean he gain more muscle fibers from training? That sounds impossible or is it? Could it be the study was flawed? I can't find the study that he was talking about on the internet. He said it could be 40 years or so ago. I would assume that weight training makes a muscle fiber thicker but maybe both are happening but I can't imagine new muscle fibers growing.