Bell

I'm currently visiting my grandparents and they have three dogs. This is Bell, she is 12 and quite a character. Unfortunately she has a bad back leg at the moment and is limping pretty badly. The vets don't know why as they have not been able to find anything wrong. This has been very restrictive for Bell, who is normally very active. She's not allowed to go for walks and isn't meant to run around. How do you tell a normally very happy and active dog she can't do any of those things? She literally doesn't understand! She seems to find it very difficult when the other dogs are being taken for a walk in the mornings.