During Sam's spine surgery recovery, we found that swim therapy helped him develop back his atrophied muscles without putting a strain on his back and limbs. Swim therapy is also great for low impact exercise for older dogs, for taking off some pounds on an overweight dog, and just for a good, fun cardio workout.
Here are a list of resources to help you find physical therapy and swim therapy for your gimpy dog, if you should ever need it.
NOTE: Please work with your vet to ensure that swim or any other type of therapy is suitable to your dog's recovery.
Land of Pure Gold Foundation list of physical therapy and rehab assistance in the US and Canada
International Association of Animal Massage & Bodywork/ Association of Canine Water Therapy
The Canine Rehabilitation Center (good links)