"sufficiently severe to impair the required performance" There's the key phrase. Required performance in big eq classes is jumping 3'6" fences, spreads that are wider than that, perfect changes, rollbacks, etc etc. You have to have a pretty sound horse for that. If your horse is in pain you'll get penalized. But if your horse had a hitch/catch, or maybe he was a little stiff, you aren't going to penalized. Hunter classes are the other extreme. You jog into the ring, you get a few steps in front of the judge, and if your horse jogged stiff your out. Maybe it's 0 degrees and there wasn't any room for you to walk your horse. Better hope he jogs sound.