Sherri, I can tell you they can't pull without the rear. I participated in a weight pulling demo once years ago. My berner was cart trained but not weight conditioned and she was 7 yrs old. We started with about 10 dogs including Sibs, Mals, a Great Dane, some GSs, Sammys, and other pure-bred dogs. Each dog pulled on level to slightly sloping up hill ground and we started with a 100 lb gig (3 wheeled thingy) and the dog had to pull this from a dead stop a distance of about 16'. All the owner could do is call the dogs name. After each dog had a try those that didn't/couldn't pull dropped out and they added weight at about 50 lb increments after each round of pulls. At the 550 lb level the four dogs left were my Jeni, a Mal and a couple other sled dogs (either both Sibs or a Sib and a Sammy) The Dane and a GS didn't even make the first pull because they didn't have the proper rear for digging in and "driving" forward. Too much angulation in the GS so energy was wasted. This is of particular concern in our Berners today as many many have too much bend of stifle and couldn't pull a load on their best day.
Sandie Hawkins