The stifle joint is the equivalent of a knee in a human.  It's about where the hind leg meets the body, and bends back.  A stifled cow has injured it.

We've had this happen with hiplocked calves before, once or twice.  They generally got over it if I recall correctly, but we've never had a cow do it.

Let us know how it turns out, but I would think it's going to take some time.