Dogs carry around 60% of their weight through their forelimbs, and 40% of their weight through their hind limbs, and the front limbs actually are not attached by bone to the skeleton, which is what enables dogs to take longer strides with their front legs and thereby move more efficiently. The front limbs are attached to the skeleton by a group of muscles, and these muscles also support that 60% of the dog’s body weight. With our dogs, if we throw a ball, especially with those far to widely available ball chuckers, where we ask the dog to repeatedly run from a standstill to a fast run, sharply stop and often skid, jump or twist in order to get the ball, and then run fast back again, there are huge forces placed through that dog’s skeleton and muscles. Dogs’ muscles and skeletons are simply not designed for these kind of movements…it is not natural for a dog to brake, twist, jump and skid to a halt. This constant micro trauma that is placed on the muscles and joints can cause longer term damage and can also cause the dog to compensate for these injuries in the way that they move or stand, to take pressure off the painful area, thereby causing additional problems. On top of the damage that can easily be caused by these repetitive movements involved in chasing a ball, generally when a dog is engaging in ball chasing, they have not had the opportunity to fully “warm up” their muscles. No warm up and no cooldown = increased likelihood of damage. Can increase pain If you have a dog who already has arthritis, or has elbow or hip dysplasia for example, having them chasing a ball can mean that the joint disease will progress faster than it would without those activities. Dogs will still chase a ball even when they are in pain – dogs tolerate a huge amount of pain, and if a ball is thrown more often than not, adrenaline will kick in which will, temporarily, override the pain they feel. But once that adrenaline has gone, the pain will be back and most likely will be worse.