The management is really key for these horses.
Did you also x-ray the neck?
I have a gelding with KS. Like vxf111, he never bucked, or seemed cold-backed, or even really misbehaved. But he had some quirks that I thought, once considering all of them, might be related to pain. He was funny about major shifts in footing, for the most part. Tense in his neck, which I always thought was weird (and later proved to be related to cervical arthitis). He had shifting “lamenesses”, that seemed to present more as general stifle weaknesses, but estrone didn’t make much a difference and he was already conditioned on foothills/full turnout. And he never flexed positive on any limbs… I kept calling my sport vet and my chiro-vet, and both thought these were just “young horse weaknesses”. They thought I was possibly wasting money to pursue further diagnostics.
He was a headscratcher for a while because I genuinely felt like this was the highest quality horse I have ever owned, but we weren’t progressing well and some things, such as cantering on the bit on grass, and lunging, seemed genuinely difficult to impossible for him. He would not misbehave, but he would get very tense and tight - not the “Track tight” but, “I cannot do what you are asking me to do” tight.
The biggest clue it was something physical was that, I could get a fantastic ride out of him W/T/C in ring-side/sand footing… but could barely scratch a respectable training-level (dressage) ride out of him on the grass… which was problematic because my home-base is a grass ring and for the longest time I felt like a horrible rider/“retrainer”, because he had been off the track for almost two years and was MILES behind the progress I made with far more difficult OTTBs… At first I thought possible laminitis? Low grade sole pain? Had his fetlock to hooves x-rayed, and his rads were pristine.
…but then I’d bring him to my trainers’ for her to watch him while I rode to help me break through this difficulty, and he would give me a fantastic ride that would make her say “and what were you complaining about before?”
It took me a few rides off property to realize it wasn’t a training issue, which I really believed for the two years I had him.
Rads of neck & spine showed enough that I said wow, he’s really been dealing with a lot… and hiding it. Major bone spur, multiple remodeling sites in the neck, several processes remodeled in the spine… injected & mesotherapy and really making sure the saddle fit. That was a fiasco, but finally found him the right saddle. But he was a different horse, completely – all the things that seemed difficult before, like the footing, and the tensity in the neck… kapoof.
Injections 1x year, mesotherapy, hind shoes (I found that for him, he had great feet, but putting the hind shoes on gave him that extra protection so he did not feel he had to compensate for some low-grade sole pain from being barefoot on MA rocky topsoil 24/7), REALLY staying on top of saddle fit, an anti-inflam supplement (I do devil’s claw/yucca, we are not showing ATM), full 24/7 turnout, ample ample ample warm up before any collected or “on the bit” work (I actually handwalk 2m, and then jog-in-hand for 2m before getting on, and then usually go for a 5-10m walk hack around property before ring work), have all made a big difference.
Also, I try to limit the lunging. I do not know if it is the neck, or the spine, or both, but he genuinely seems to have a hard time with it. Keeping the hard work (dressage) to 1-2x a week and hacking out the rest of the week has made him progress further in his training than schooling dressage 3-5x a week and hacking the other days. YMMV.
Every horse, KS or otherwise, is individual in that regard… but every KS horse I know is substantially better and healthier for being out 24/7. Stalling does cause stiffness, aches, and residual back pain.
Keeping them in shape and moving is paramount too. It does not have to be in dressage work, either. I would avoid Art-2-Ride, by the way – you may have someone try to rope you into that, but they also dismiss that KS is a legitimate disease and that horses cannot correctly work over their topline if they are in pain.