...The principles of the seat advocated herein remain the same for all types of riding. There are only minor variations in the length of the stirrup-straps, and in the resulting change in the forward inclination of the body. It will be found that the shorter the stirrup becomes, the more the body must be inclined to the front to remain in perfect balance, and thus minimize interference with the horse's efforts. ...
...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...
...it may be said that the seat to be described is an exceedingly simple and natural one. In fact, most children, as soon as they have gained a little confidence on a horse's back, assume the correct posture instinctively, Unfortunately, their natural tendencies are often ruined under the tutelage of the ignorant grooms and unqualified riding teachers abounding in this country...
Stirrups and Legs
...For hacking and normal training of the horse, when he is not to be schooled in jumping, the rule for the length of the stirrup straps is as follows: Being seated as described above, with the legs hanging down in a natural position by the horse's sides and the feet out of the stirrups, the treads of the stirrups should hang even with the center of the large bones on the inner sides of the ankle joints. This is a general rule...
...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...
- Harry Chamberlin, Riding & Schooling Horses