It’s great your dog loves to play Peggy, but the intensity of this repetitive activity could be doing him more harm than good.
Chasing after a ball if done to excess can be harmful for dogs, even fit working dogs. Very often they get too tired, but it takes a while for you to realise this, and by the time you do they will have become overtired, over-exercised and risked putting unnecessary pressure on their joints.
The sudden changes of direction that are involved in playing ball can also put too much stress on your dog’s legs so try to avoid this.