11 year old Boxer

My 11 year old Boxer female looks emaciated. The winter is not kind to her. Her entire rear is nothing but bones. Her backbone sticks out as well as her ribs and when she eats her stomach will look huge. She eats and she's been wormed. She has lots of energy but she has so much trouble sitting down and she shakes if sitting up. I would like to put some weight and muscle on her quickly especially during this cold weather. The vet says all her tests check out and she's healthy other than bone issues and hip problems. I have her on a mature dog dry food. Please help!