You don't want to push your dog too much at this age. Pitbulls grow in stages. Usually they get all their height first. Then they start to thicken up and the head will spread and thicken. When your dog hits about a year old is when hes gotten most of his height. And then the dog will begin to thicken up. Around this time you can start puttin weight on him or her if you like. But running your dog does wonders. If you dog likes to swim...swimmin is best for the dog. Gets their wind up and gets them ripped with veins. But natuarally a pitbull will become muscular, just got to give him time.