Joe has wonky back legs, but nothing shows on X-ray. Two years ago, we were told to keep him on strict rest for something like 12 weeks. No sofa, no running, leash walks only. Up and down the stairs only to go to bed at night and come down in the morning. That got him "right" again, but we're still careful about lots of stairs, big hills, strenuous walks, serious running, and the like. We let him do some of everything, but not all at once, and we make sure he rests after. If he does something crazy, I give him 1/2 a Rimadyl before he starts to hurt.
And every time he limps, I panic.