I'm glad i found this forum. I have been having my 50lb female pull me in a wagon for about 2 months now. No doubt about it her hindlegs, lower back and thighs are complete rocks. I have compared them to three other larger boxers and one pit bull that does not pull, and she is by far the most cut, physical looking dog, and she loves it. An excellent website to give you the 101 on the art of weight pulling is pulldoggies.com. I am ordering a harness for my dog from there this week. That site lets you in on all the basics and knowledge needed in order for your dog to get started and continue to practice the sport. With my dog i also do bite work and tons of tennis ball throwing. I run her up and down a large grassy hill in my neighborhood and you cant help but notice her front shoulders and thighs ripping out her clossy coat. I have a homemade spring pole in the backyard that i use with her too. I always hold these homemade shutzhund trials just for fun in my yard. Im only 17 and have had my dog a year in a half. She has so much athletic talent but she's not real mean. also you might check out the american pulling alliance, and the previously listed iwpa. like someone said, their not built as broad as the american bulldogs or the pits, but they can do their fare share. Check out some master weight pullers, the Presa Canarios, at red-star-kennel.com, or some bulldogs at blueriverbulldogs.com, the bulldog site is only 35 min away from my town, i live in Topeka Ks.