Obviously, with the children's sleighs like ours, it would be uncomfortable and even damaging for the dog, because there will be a very large pressure on the croup. All that pulling effort actually should go evenly and smoothly to the sleigh, without pressing too hard on any spot on the dog's body, so for our needs a weight pulling harness was the best, according to the professional advice. You can see that there is no pressure on the tail area if the dog pulls. The load distributes evenly to the dog's back.
This is a cleverly made and inexpensive harness that fits well. I bought it on Ebay from there very nice and helpful US sellers - 5kpetproducts. They make these harnesses themselves with a number of other products, and helped me choose the size properly.
We have had lots of fun with it last year; we even gave a ride to my younger brother...
...only he was not very good at driving this vehicle and got lost somewhere on the way.
I could not see any difference in the physical shape of the dog, frankly. He is fully grown and, while I think such workouts are beneficial for him, I do not believe they can make his chest wider or muscles bulkier. That was just time well spent.
If you have a large-size adult strong, healthy and energetic dog, you can try this, too - chances are you will both enjoy playing like this in the snow. And then, who knows, you might become a fan of weight pulling or sledding! Just make sure that you start with empty sleighs and increase the load gradually, and be patient while teaching the dog.