Both. The human body only evolved to take so much force, and all the training in the world can't overcome basic biology. Players today are running faster, jumping higher, and pushing off at crazier angles than ever before, and they're doing it from a younger age and more often than ever before. As a result you are going to see more and more injuries. Just look at baseball pitchers - they keep forcing these guys to go up against the absolute limits of the human arm and as a result their elbows and shoulders stay fukked up. For NBA and soccer brehs it's the knees and ankles that can't take it. In the NFL it's pretty much the entire fukking body.