The USEF Rule Book states that the “position in motion” at the “posting trot, inclined forward; galloping and jumping, same inclination as the posting trot” (EQ 108, #4). When the horse is travelling in a medium frame, the rider’s upper-body inclination is about 20 degrees in front of the vertical. This is achieved at the posting trot by the rider closing the hip angle and posting with more weight on the crotch than the buttocks in the sitting phase.
The angle of the upper body depends upon the degree of collection of the horse. For example, if a horse looks its best being shown in a long frame in an under-saddle class, then the rider’s upper body would be inclined slightly more than 20 degrees to match the flatter carriage of the horse. On the other end of the spectrum, if a rider is asked to perform a “working trot with a lengthening of stride” in a USEF Talent Search Class, then the upper body would become slightly more erect so that the rider’s weight could subtly help the legs as a driving aid.
In George Morris’ famous book, Hunter Seat Equitation, he says that “the correct use of the back is a very subtle thing both to learn and to teach, much more so than, for instance, the rider’s leg. The weight of the rider’s trunk coordinates and clarifies all of the aids. Therefore it is of major consequence that one has the best possible back position in order to function.”
The idea of “form to function” is relevant in many sports, with the emphasis being on how a particular position makes an activity easier to do and prevents performance errors and injuries. Whether it is how a tennis player holds the racket and follows through with the swing, or how a springboard diver gets enough height in the air and enters the water with no splash, the degree of accomplishment depends upon the quality of the athlete’s form. Our sport is just the same, with form being important for the success of the rider, not only in an equitation class in which rider form is judged, but also in hunter classes in which the rider’s good or bad form affects the horse’s performance.