There is a whole shape language to how horses with back pain move and it is often not one specific symptom, but multiple symptoms together that might suggest pain. As a stand alone these symptoms might be general weakness, greenness, any number of things. But when there’s two or more together, it’s likely back pain.
KS horses tend to have flat pelvis and hip movement, with a flat tail that does not have much fluidity between paces. Either held away from the body without much movement, or clamped down with tail hairs being slapped back and forth between hocks (side note: this is also sometimes a clinical sign of hock pain). When they jump or go through a corner you may see the tail turn to one side and may see more space between tail and buttock as they hold it out straight behind them.
The range of motion in their hind leg will appear as one forward motion versus a series of independent opening and closings (articulation) of joints: opening of the hip and stifle, then opening and reaching of the hock with forward extension of the limb, followed by flexion in the pastern and fetlock in loading phase. In a horse with back pain the whole hind limb seems to move all at once, think of placing your arms at your side and keeping the whole arm straight as you raise it forward to your shoulder, then keep it straight as you bring it back to your hips - very robotic. That is the best analogue I can give in a written description for how these horses move; there does not tend to be much bend or flexion of the limb in adduction phase. The try to keep the whole hind end straight versus articulate and push.
The (reduced) range of motion in the pastern and fetlock is another characteristic symptom of pain behind. They may demonstrate a stubbing toe-first landing behind where their pastern and fetlock are stiff before loading phase, very similar to how horses with (hind) caudal heel pain move. It can look like the fetlock knuckling over before ever landing. It’s most obvious as a stabby landing or toe dragging. Rather than reach behind and overtrack, they tend to bring that hind leg under them and wait for the force of landing to flex the pastern. The more experience with KS I have the more I see this symptom as a solid standalone for back pain; some other symptoms of KS (like stiff hocks) overlap with other clinical diseases and do not always equal back pain by themselves.
To me, KS is most obvious in the canter. There may not be much degree in separation of the hinds, very little reach or step under themselves, and often you will see them swap leads or prefer one leg for push off over the other right before fences. This can sometimes read as stifle pain. They might trail with their hocks, and tend to move the leg stiffly versus fully articulate it. When watching them canter away on a straight line the hock does not have much of a ‘pendulum’ motion but rather a snappy up & down one. Often paired with a snapping of the hoof and/or one hock that is held higher or away from the body, and some twisting of the hock in all three gaits but most obvious at the walk. Overall the picture would be of a weak[er] hind end and will look as if the horse is quick to offload weight behind.
The way they hold their neck is also sometimes a good tell, because they will stabilize offloading their hind limbs with their neck. KS is not always “stiff, fast, and inverted” - some personality types (especially the ‘economy mode’ horses) will jump in a more flat and long manner, tend to be more on the forehand and use their neck to balance that. These horses tend to be rein lame with a lot of contact because the contact/weight in the reins is restricting their ability to compensate behind.
In general as the demands of ridden work increase, the weakness behind becomes more obvious and will come with a subset of ‘behavioral problems’ that is the horse’s way of communicating they cannot physically perform what is asked in a pain-free way.
KS has so many symptoms and many horses are too generous until they reach their limit. KS also doesn’t always produce a limp – especially in the beginning before compensational injuries happen – it’s easy to miss until it isn’t. The best thing is to know your horse’s baseline on any given day - know what is a normal stride length for their conformation, know what they look like on their best days and compare it ruthlessly every day. Not every KS horse has the same symptoms and management and riding style all play into their success. Because KS is a chronic (pain) disease, they will have good and bad days just like us. Best we can do is tweak their management and their therapies in a way that works best for them. It takes a lot of trial and error to figure out what works best for your specific horse.