Which is quite an accomplishment seeing as how he outweighs her by 50 lbs. (Tar 112, Abby 62).
Tar started limping, then he started going about on three legs. Off to the vet!
We did xrays, even did his hips (looking for hip dysplasia). No broken bones, hips look great. His tarsals (that kind of backward 'knee' on the hind legs of dogs) is swollen and they found a flat spot on one of the bones (congenital) that makes it easy to sprain. And that is what he has - a bad sprain.
It is Abby's habit to lay in wait for Tar and spring at him, even on the stairs, to gain the advantage her diminutive size does not afford. We are going to have to watch that. Tar, in the meantime, has an open prescription for anti-inflammatory and painkillers.