Yeah, a stifle injury is like you straining your knee (to make an anology)...obviously, horses get all of their push from their rear legs so any muscle/ligament ailments would severely reflect in the horse's ability to perform...I've seen it happen before and it usually requires significant care and time....if it is reoccuring then it would seem to me as though there are imperfections in the horse's stride that put more stress on that particular leg and area of that leg...How is the horse's conformation?..good luck with it...