Do you think a dog must be health tested properly before breeding no matter how drivey or powerful the dog is ? I think these bull and mastiff type dogs have ridiculous pain tolerance. A lot of working enthusiast folks state that working alone is natural selection. However, i think this high pain tolerance can mask undergoing issues in the dog. For example my pup just turned two and he was on his way to start training with heavier weights in pulling. I did the right thing and decided to get his hips xrayed and the results came out unsatisfactory. He never limped pulling or quit pulling yet he had this undergoing issue. Just look at this video, which is very sad this dog won't let go of the other even after it gets smashed on the head or kicked. If a dog has bad hips or elbows its own drive or aggression can mask the pain of joint problem and nobody would ever know. I removed the link sorry