In my last litter I had one. I had previously had a small litter and there were 2 in it. I had read about doing physical therapy using massage on the puppy. You hold it on its stomach on your lap and gently massage starting at the withers slowly rub in a downward motion down the front legs so that the legs start pointing straight down instead of out to the side. It doesn't take long before the puppy is much looser in the joints and relaxed. You also hold the front legs gently together while massaging the shoulders. Spend some time rubbing from the back down the ribs to help shape the rib cage to a normal round shape instead of flat. It really took only a week of gentle massage to get this little one more relaxed, sleeping on her side, sitting up. We used bathmats upside down, but they have to be a loose weave type not slick. We also helped her by placing her back legs under her body to take steps. Once she figured out she could get around she really took off. She is a gorgeous puppy now, great keel and angles. You would never know she ever had a problem. I have found that this method was easier then what I went through with the pups in the older litter. None of the puppies ended up with any sign of an issue due to being flat in the ribs.
I've had a couple of smaller litters with fat puppies that looked like they wanted to trend toward swimmers. I'll put egg crate foam mattress pads under the whelping fleece to make for a soft but irregular turf for them to manuever on. I had one little fella that I would lay on my lap, on his side, and just massage his rib cage from his back toward his belly for a while in the evenings much like one of other posters has described. He was slower to walk at first but it didn't take long before he was up and about with the rest of the litter.
