Humans are bipedal which is why we load the weight using our lower body. Dogs are quadripeds and can utilize different gait patterns to offload weight how they seem fit. So yes, some breeds are push and pull however, breaking it down to it's most simple form, the dog's PULL the sled, as in they lead it. They do not PUSH it. Therefore most working dogs will have a larger musculature on the front 2 legs as they're pulling. Leg drive is derived from the front 2 legs.