My experience has showed me that breeding during a keep can and weaken a male dog. I boxed competitively and competed in other contact sports.... And yes I believe breeding/sex weaken the rear end of a male, canine or otherwise. Some dogs can over come this and some can not, but the dog is weaker. I would say if you breed less than 7 days away from show time, your not looking at the best interest of the dog. ( male dogs specifically)