Once the intensity of the exercise becomes so high, that you body is producing more lactate than it can remove, your body will have moved from the aerobic system, to the anaerobic system, which means your body has gone from a system that is predominantly using fat and carbohydrates as energy, to one that uses muscle stored glycogen or other more local energy stores. It is during this training that you are more likely to increase the size of your muscles as an adaptation.