A study by the University of Veterinary Medicine, Vienna showed that young hunting dogs who are trained using the same weight as adult dogs, can cause damage to the joints and tendons, especially of the front legs.
This is because the extra weight that young dogs carry causes them to tilt forward in much the same manner that a person would who was carrying a heavy load. The researchers felt that adult dogs are suited for carrying loads but young dogs that are growing should use adjusted weight instead of the same weight as adult dogs.
They suggested that young dogs in training be checked regularly by a specialist to be sure that there is no damage to joints, tendons and muscles.
It stands to reason that if a dog associates pain with an activity, he will not enjoy the job or game and may not perform to his best ability, especially if he is being trained for competition.
Although this study was conducted primarily on hunting retrievers, many breeds of dogs enjoy the game of fetch. This means that all dog owners who play fetch with their dogs should be aware of how much weight the object is and not allow young dogs to carry heavy objects.
I personally had a Border Collie who as an adult, loved to play with a bowling ball and would successfully put his canines in the holes and pick it up and carry it for a short distance. You never know what a dog will fancy and play with!