• Suspension of the thoracic cage:
The thoracic cage is linked to the forelegs by two main muscle groups:
- The jagged muscles that are attached to the upper part of the scapula (just in front of the withers) and support the base of the neck and the first eight ribs.
- The pectoral muscles that connect the sternum to the upper end of the humerus and to the edge of the scapula.
Through their contraction and stretching, these muscles will ensure a more or less forehand lightweight in the gaits and a more or less amplitude of the thoracic cage movement between the shoulders at the time of the beat of the obstacle and elevation of the forehand, and then at the time of reception.
• Raising and lowering of the back:
The vertebral apophyses that form the withers are linked to the forward nuchal ligament and the backward supra ligament. When the nuchal ligament extends during a neck lowering movement, it pulls these apophyses forward, stretching the supra-thorny ligament, which in turn stretches the apophyses of the dorsal vertebrae (and a bit the lumbar), causing the back rising and the work of the back-muscle groups.
On the other hand, if the back wires and contracts the thorny supra ligament, it pulls the apophyses of the withers back, soliciting the nuchal ligament and causing the neck rising.
This mechanism, is essential to the musculature of the topline, which is obtained by the longitudinal and lateral work of this part of the body.
This bodybuilding of the trunk allows to effectively withstand the weight of the rider. The longer, prominent and prolonged the withers is, the greater its effect on the stretching of the nuchal ligament and the supra-thorny ligament will be important and thus effective in terms of muscle solicitation.
Placement of the saddle:
The saddle shall be designed and positioned in such a way as to allow the withers to move and deform without adding any constraints. The saddle is placed behind the withers to let him loose.
It must also be placed so that the center of gravity of the "horse + rider" at the same distance from the shoulder and the basin as the center of gravity of the horse alone. Positioning greatly, it facilitates the balance work of the "horse + rider" together.
A prominent and long withers helps. However, be careful of the extremes: an overly prominent withers is more likely to injure itself, being too long causes the saddle to position too backward and affects the center of gravity of the pair.
For this reason, the PAX program recommends, on a scale of 1 for very short withers to 4 for the most prominent and prolonged, a score between 2.5 and 3.5 to be in an ideal comfort zone.
The back is the anatomical region that goes from the withers to the sacro iliac junction. It corresponds to the vertebral column segment composed by the last 9 thoracic vertebrae (dorsal vertebrae) and 6 lumbar vertebrae.
It stops at the sacrum, which is made up of welded vertebrae, unlike the dorsal and lumbar vertebrae which are articulated between them. Each vertebra is composed of a vertebral body from which 3 apophyses start: two cross-coastal apophyses and a thorny upward apophyse.
The shorter the back, with compact vertebral bodies, the closer the peaks of the thorny apophyses are, the higher the risk of "conflict" between these apophyses during back movements.
Conflicts cause inflammation that can bother movements and jumping.
The right and left vertebral column equerries formed by the thorny apophysis and the two transversely shaped apophyses of each vertebra serve as insertion points for back musculature, the most important of which are the long dorsal muscle and the erector spinae muscle.
The longer they are, the wider and deeper the back, and therefore able to accommodate a large and potentially powerful musculature.
It is the combined action of the back muscles that causes or facilitates its lowering and elevation.
The longer these muscles, with equal tone, the more effective their contraction and stretching will be.
They determine the extension of the thoraco-lumbar part of the spinal column and that of the sacred lombo joint and therefore play an important role in the shape of the trajectory.
The abdominal muscles, located under the spine, attach themselves to the ribs and lower face of the lumbar vertebrae. They cause the opposite effect of the previous ones. The simultaneous action of these two antagonistic muscle groups determines the constraint on the spine during locomotion and jump. Again, with the same muscle tone, the length of the back is more a guarantee.
This approach, which leads to a preference for long backs, must be modulated by another consideration: the correctness of the work.
The back, through the flexion and extension of the spine, allows to coordinate the efforts made by all the muscular groups of the horse to move and jump.
It is important that it would be able to work symmetrically and straight at the request of the rider.
This will require, in particular with the work of incursions, symmetrical muscling on both sides of the back and putting "shoulders in front of the hips" to ensure that the back is able to drive the energy exactly in the direction desired by the rider.
A "good back" is therefore as flexible and long as possible, as long as he can work straight!
The right, long, wide and flexible back of a horse are the ones that the rider can control...!
Beyond these considerations, the PAX graphic focuses on the shape of the segment that links the base of the withers to the sacroiliac joint. If the spine is very straight (if the spikes of the thorny apophyses are all at the same height), the amplitude of the spine extension motion will be greater than the potential amplitude of its contraction: With equal back length, the horse can open it more easily than the curve.
For the very hollow back, already naturally open, we observe the opposite and therefore less predisposition to the extension (less ability to "open the back" to orient the course of the jump).
For this reason, the PAX program favors straight backs but does not go to the extreme. On a scale of 1 for the lowest to 4 for the straightest, the "comfort zone" was fixed between notes 2.5 and 3.5.
It should be noted that the form of the back take into account the age and working conditions of the horse. The back of a mare tends to widen as gestations and muscular melting take place. Therefore, the context-sensitive nature of the notation must be considered.