
Hi
I would feed high protein diet to improve the physique along with a measured excercise program. the amount of food and length of excercise depends how old your dog is, puppies and young dogs should not be over exercised to avoid problems with skeletol development.
Muscle mass will be increased with more exercise, but i would recommend increased on lead road walking to improve overall muscle tone. That should show improvement in feet, pasterns and muscle condition.
The type of food you choose is a personal choice, yours or your dog, whether you supplement the existing food, or go for an all-in food, the alternatives are endless.