She doesn't actually appear to be that croup high. I school quite a few stock horses/appendix horses, and they have no problems going down hill if you let them! But it is a gymnastic process for them to sit and lift and push from behind rather than just catch themselves in the front. Downhill is a reactive way of moving. We want to show the horse a pro-active way...where she uses her core and her hind limb assembly and lift the shoulder sling to choose the movement, choose the placement of her feet and do so with some controlled impulsion. These horses have a LOT of power in the rear, but won't comfortably control it if running into the ground out front.
To show them the way uphill, I find that all of the suppling gymnastics such as leg yields in and out on circles, riding squares, shoulder in, haunches in, out etc, intersperced with just moving out (not running, but a nice forward, big trot and up and open canter, even if I have to make it a hand gallop to get the horse excited to push and reach underneath itself, just short bouts of that, then come back to working gaits and the gymnastic figures (serpentines, circles, squares, mixing up whether it is straight or done in shoulder in or out or yielding, the horse has more ability to lift. The forward gaits wake up the belly muscles and the butt...our other best friend is our halt. Asking for REALLY forward halts...the horse steps up INTO the halt, lightly, but gathered (lifted and thinking uphill.) Then go from halt to walk or halt to trot etc and you should feel the horse lift and push forward smartly.
How do you know a halt is really forward? Because you will feel like at any second you could give an aid for trot or canter and the horse could just float up into it. That's the sensation of a good halt...and usually indicative that the horse has lifted its back, used its hindlimbs and is truly on the aids, ready to go. Even if the horse isn't yet ready to pick up a canter from a halt, we still get that feeling on the young or reschooled horse when the halt is right.
BTW, I do a lot of my shoulder-in, haunches in exercises on a straight line at walk across arena...even using poles on ground to help the horse understand "straight" lines, and for me to check i am doing correct figures. I may do a shoulder right (poles on ground on left side) in across to the end of the poles, then ask the horse to turn on haunches left /yield (still bent away from the direction of the turn), then SI back across, maybe with a 10 or 15 m walk circle in the center of that line etc.
Making accurate school figures is one of your tickets to a horse that can learn to use itself correctly, carry herself and the rider, flexible laterally and longitudinally. Keep in mind the contact you need to adjust through the exercises, always testing to see when she will stretch up and out to her bits as much as possible.
I have had some ex reiners and ex Western Pleasure horses that need me to really keep the bridle lifted for them, because they have been moving with the neck down. But they find that sweet spot to take the bridle and they love it...because it must feel REALLY good to use their neck and poll comfortably. They are nearly always light in the bridle and the challenge is getting them to really take contact and stretch to it...they may have been punished for touching the bit in their past lives.
Another friend to help a horse lift, is to do many or most transitions from a shoulder-fore attitude or position. With this little bend, the horse has to use its hind legs a little bit at least. And don't forget to use counterbending...there are places where the horse will get a little drop in a shoulder or stuck on a rein, a little counterbend for a few strides or even a half circle can do wonders. We humans have a persistent habit of allowing a horse to make the same 'error' at the same place in the menage over and over each time around. Don't do it! Catch yourself and thus yourself BEFORE that place where the horse ducks in or a haunch falls out...use a bit of counterbend to get the horse straight underneath itself and correct that issue. (those things are often locational...identify your locations of repeated error and PLAN ahead. It is best in animal training to not let the animal practise a behavior we DON'T want. After several repetitions, it begins to become part of the mental program, the default.
Even though it may sound like simply preventing a default error is about addressing the error and nothing to do with helping a downhill horse move up, it is a "place" where the horse may be 'falling' downhill or uneven loading....to GET to UPHILL and lifting and pushing, the horse has to learn to load all 4 feet, straight (even on a bend as you likely know) and swing through from behind....every stride, all the time. (As best we can manage.) So every corner we let the horse fall in or on a straight line letting him bow out or shove a hip in or whatnot, is a couple more strides he can't be in a straight position and thus not fully able to be uphill. I maybe think of it as any step or energy that is going out one side or falling behind or locked up in the neck is that degree of energy that is NOT able to contribute to uphill, swinging, forward.
