I don't think it has anything to do with the dogs being kept on a set up. I've had dogs that would never run a mill run a jenny. I've had dogs that weren't even bulldogs run a jenny but look at a mill like WTF is this. The jenny is simply more comfortable for the dog. Feet are on the ground. The dog itself isn't off the ground on something that could possibly be loud with a little rock to it. Having the dogs do different things at a young age will acclimate them to everything. The trick for me and the jenny was to find something the dogs wanted to chase, and for my dogs, that's a Kong toy.