I'm sorry I said "hard running." Repetative, very long distance running is hard on their bones and joints. But hard running in the way I am thinking, is bounding around, romping, running for short bursts, off and on, varying the gaits, trot, run, walk, stopping for a bit to sniff something....getting a little tired out, but not excessively. If you're out playing with the pup, this can be done. It's the repetative type exercise which is not so good...same pace, same gaits, long distances. A pup needs some exercise. Building a little muscle and blood flow to the ligaments and tendons is all good for supporting the bones. This is my opinion though, but it comes from discussing this with my vet when I got my Doberman. I really wanted to make sure I wasn't over doing anything, but this Dobe needed an energy outlet big time. And along with that some good mental stimulation from training. Just make sure your pup is supervised when not confined in a safe area or crate. With good training and raising your pup will grow out of this. Best wishes.