...A jumper , when actually sitting in the saddle, will have his thigh bone at the same angle to the ground as someone who you might call 'chair seated'. But, since his lower leg goes backward, under him, with an acute angle behind the knee, it brings the lower leg and the stirrup (his base of support) back UNDER the rider's weight, instead of in front of it (a true chair seat)...
A jumper does that, not because it gives him a good seat, but because that is a compromise he accepts in exchange for being able to get off the horse's back when jumping. Jockey:
The problems with a chair seat, for typical riding, is that your weight is concentrated above the back, so a horse who spins or scoots sideways can scoot out from under you. It also tends to shift your weight too far back since your knees normally need to be clear of the horse's shoulders - and if your thigh is parallel to the back, that will shove your rump a thigh's distance back. That is about 20 inches for me, and I've got a 16" seat in my saddle!
And in a chair seat, you are resting on your rump - like one does in a chair. So your legs cannot act as springs to to absorb shock. Those problems still exist with a horizontal thigh even if one brings the heel back under the hip. But a jockey (pictured) or a jumper is focused on a seat that will help him in his sport. It isn't meant to be an all-around seat. And it differs from a chair seat because the intent is not to stay on the horse's back at all, but to rise above it.
...In hacking, since the distances ridden are usually not great, and the time on the horse's back is short, the stirrups may be adjusted primarily to suit the rider's comfort. They should be fairly long. In breaking and training a young horse, there is a marked advantage in having the stirrups quite long, since the rider's legs are then well down around his mount, where they may be employed strongly as aids in teaching the lessons at hand, and also wrapped about him to provide security of seat in case the youngster bucks or violently plays up. No matter what the length of the stirrup is, the body is always inclined to the front
; slightly with long stirrups, and progressively farther as the stirrups become shorter...
...For the beginner, it is well to have the stirrups a little on the long side, rather than too short, as this permits, and almost forces the rider to work the thighs and knees well down around the horse, and thus overcome the usual instinctive tendency to raise the knees, which makes the seat unstable and weakens the grip of the knee and calves. It is the ability to grip with the calves of the legs, and to a much lesser extent with the knees and thighs, that provides the strength of seat through which a good rider stays with his horse when difficulties, such as shying, plunging, stumbling, bucking or jumping arise.
No matter how much the stirrups are shortened, it must be understood that the stirrup-straps, when the seat is correct, always remain vertical, and that as a result of shorter stirrups, the knees, though raised, go very little farther to the front....as the stirrups are shortened, the seat and buttocks are necessarily pushed farther back on the cantle. This demands more forward inclination of the body from the hips
What Harry Chamberlin considered ideal is about what I call the "Baby Bear" setting for my stirrups. The cantle of a western saddle doesn't allow one the freedom of an English saddle. At the Baby Bear setting, my rump can be smacked by the top of the cantle when trotting in two point.
The next hole lower is my Momma Bear setting. For most riding, in my saddle
, it is probably the best. Secure enough, forward enough while giving the horse some freedom. But on a trail, I usually use the Poppa Bear setting - one hole lower still. It sucks my hips a little more forward (since I usually want my stirrup straps vertical) and keeps me well wrapped around my horse. I choose to ride Bandit in a way where an occasional balk, jump sideways, or small buck is part of the price of riding. If my legs were long enough to hook my toes together under his belly, I'd do that!
But the Momma Bear setting is best. I just get nervous sometimes.