Bending of a horse has been a topic of discussion for thousands of years. The discussion is mostly related to anatomy and biomechanics and since different people have different concepts about them.
The word, bending, in English implies that a horse can bend its body like a “C” where the neck of the horse is bent to the inside and the hindquarters are also to the inside of a circle. A horse can indeed do that by compressing the rib cage on the inside. However, to meet the requirements of correct bending as described by the FEI, the hindlegs of the horse must be in the same tracks as the front legs. This is also called straightness because the hind legs of the horse should travel in the same tracks as the front legs on both straight and curved lines. This does not happen when the neck is bent to the inside and the rib cage is compressed on the inside. Although the horse may feel to be bent like a “C,” the hindquarters are on the inside of the circle. The hind legs are not in the same tracks as the front legs.
Since there is no lateral flexibility in the thoracic and lumbar vertebrae of a horse, the only way the hind legs can travel in the same tracks as the front legs is through both lateral and longitudinal rotation of the pelvis so that the hind quarters are engaged, and the inside hind leg can travel more underneath the body and closer to the outside hind leg of the horse. This lateral and longitudinal rotation is created in the lumbo-sacral and sacra-iliac joint. The neck of the horse stays centered between the shoulder blades with inside flexion only at the poll. Turning the neck to the inside would push the outside shoulder to the outside. Therefore, rotation of the pelvis is necessary since the path of the inside hind leg is shorter than the outside hind leg. When the horse is straight on a curved line, the hindlegs are in the same track as the front legs. There will be a small amount of compression of the rib cage on the inside as needed, but a rider will not feel the rib cage being pushed to the outside of the circle. Instead, the rider will feel the backbone of the horse centered between his/her two “sit bones” (ilial tuberosities.)
Hence, the more bend that is desired, the more that engagement is required.
The anatomy of bending is further discussed in Harmonic Dressage, Part 2
Photos courtesy of Christian Simonson - view his website by Clicking/Tapping Here