Returning to the issue of bending in the ribs.
Most certainly the rider can use his legs to bend the horse. The question is, which of the two legs plays the greater part and if in fact the rider's legs indeed are meant for bending. Using them, the inner leg at the girth balances the actions of the outside leg, which, positioned a bit further back, initiates a sideways shift of the croup. Question. What happens to the center of action? Where, while neck and ribs are bent, is it to be found?
Just as well if not better the rider can use hand and rein to bend the horse. Turning the inside wrist nails up shortens the rein mechanically. It effects a so to speak mechanical shortening and/or bending not only of the neck but of the flexible middle section as well. The reason: neck and ribs are united by one large muscle system, which centers around the shoulders, shown here.
The rider's inner leg at the girth fulfills two functions. It not only balances the action of the outside leg. It as well may activate the inner hind leg. The rider's neck, shoulders, hands and reins, however, have additional and inherently more logical means for bending and directing the horse. A rotation of the rider's neck into the direction of movement indicates a sideways bending of the horse's neck. A forward movement of his outside shoulder effects a shift of the forehand into the new direction of movement. The indirect outside or the direct inside rein speak to and position the hind legs. The rider via the horse's jaw speaks to the horse's center of locomotion.
And - if needed the rider's inside wrist nails up shortens the horse's inner side. Both methods (hand or leg) have the self same obvious problem. They shorten one side. Accepting the fact that bending in the ribs or for that matter a strong sideways distribution of the neck are effected by non-locomotive muscle systems one comes to understand that actively shortening one of the horse's side works against impulsion.
The impact of the sideways setting of the neck on the length of the horse's vertebra (i.e. neck, backbone sacrum and tail) called Längsbiegung in German equitation is another story. It effects deeper muscle systems. A greater understanding of tonic muscle systems (responsible for posture and touched by the neck's setting) and phasic muscle systems (responsible for locomotion and utilitarian motions, such as the horse's scratching of the ear) are next on study-horsemanship's research agenda.
