The withers is formed by the 9 first thoracic vertebrae spinous, corresponding to the 9 pairs of ribs attached to the sternum. The ribs form the non-deformable part of the rib cage (fig.11). Its form is visually assessed by his side more or less salient and more or less prolonged. It starts at the top of the scapula and ends at the apex of the 9th dorsal vertebra at an angle more or less opened with the line of the back.
We propose a method of assessment, resulting in an appreciation ranging from "short" to "extended", by measuring the distance from the top (4th backbone) at its base (9th backbone) (fig.11).
This measure has to be modulated by the General visual assessment, depending on the shape and size of the shoulder and the place of the top of the withers compared to the level of the elbow.
One of the withers' features is to determine the saddle positioning. It is desirable that the position of the rider in the saddle does not move too forward or backwards the center of gravity of the horse which would disrupt its natural balance.
This is to better meet this balance that racing jockeys ride with so short Stirrups! Furthermore, in stretching forward when neck goes down, the spinous forming withers contribute to lift the back and relieve the back muscles.
More prominent is the withers, more its leverage effect will be effective. A long and prominent withers will be preferable to a short and embedded withers, both to promote the natural balance of the rider horse couple and the work of the rider in the mobilization of the horse's back.
The top of the withers is in principle placed directly above the tip of the elbow. Too "forward" or too "backward" to this axis will compromise the ideal location of the saddle.
The musculature of the withers will play a role in his elevation over the jump and the mechanism of constraints absorption at landing. All things being equal, a naturally muscular Withers is preferable.
On this subject, this is again a matter of interactions: characteristics that can compensate for a withers too short or embedded are the shape and the dimensions of the back and the natural balance of the gaits.
It will be easier to compensate for the disadvantages of a short withers with a long, flexible, rather right back, and with a natural balance linked among other things to a good range of motion of the shoulder.
At the other extreme, a horse with a very long withers and a rather short back do "saddle poorly" and load the hindquarters with the weight of the rider; This can affect the quality of the propulsion and the articulation of the back in movement. If the two extremes should be avoided, it will be preferable to search a withers well erupted, muscular and sufficiently long to ensure proper positioning of the saddle over the center of gravity of the horse.