I have a very skinny Pittbull named Scrappy. I was trying to figure out how long i should walk him to tire him out and make him put on weight. What i want most is to start toning his muscle as i plan on making him a weight puller. ALL my friends say put weights on his neck like a thick chain with a padlock, but i read somewhere that messes with the joint growth. they also say attach weights to his leash so that he can pull some weight and get real mucular but he is only 4 months old and that WOULD DEFINETLY mess with his joints. So what can i do, safe for puppies to get their muscles in good shape.