The above picture illustrates the similarities between human and equine anatomies. Although the horse does not have a clavicle (collarbone) connecting the shoulders, and a hoof has evolved where the middle finger should be, the muscular/skeletal structures ate still very similar. Of course, the major difference between horse and human is that the horse is built on the horizontal plane; the human on the vertical. However, both of these beings share the same aging and conditioning pitfall: gravity. The horse becomes swaybacked and the human looses height because of compacting vertebrae.
By using the above picture, we are able to use analogies and comparative descriptions when referring to a horse’s static position or biomechanical movement. When comparing it to movement in a human it van be more easily understood.
You will rarely find a horse with both shoulders muscled the same. Most horses are left handed, and have more muscle mass on their left shoulder. Therefore with a symmetrical saddle, the right shoulder may be moving correctly, but since the left tree point offers more resistance to the motion of the left shoulder, that shoulder may move more vertically instead of back.
The resistive motion of that shoulder has another downfall. Because this shoulder does not exercise the thoracic trapezius (wither muscle) evenly, the wither muscle behind the good shoulder will be better developed. The non-fitted saddle may therefore fall to the left, causing an even greater impediment on the movement of the left shoulder by the left tree point.
A mentioned earlier, the horse does not have a clavicle, or collarbone. A horse often may have not only one shoulder more muscled; it can also have one shoulder more forward than the other. As the symmetrical saddle moves forward due to the energy of the moving horse, the saddle will move further in the direction of the forward shoulder. The rider’s reactive response to this saddle falling in one direction will cause that rider to place more weight on the opposite seat bone. To complicate matters, the rider will often compensate for straightness by leaning with the shoulders in the opposite direction to make up for the dropped hip.
The supraspinous ligaments are a tough fibrous tissue that run on each side of the spinal processes of the back. Connecting the hip to the poll of the horse, these ligaments are responsible for holding the horse’s head in the neutral position. Because these ligaments have a set length, the head and neck can remain relaxed when the back is up. However, when there is too much weight on the horse’s back, the back will sag. This lengthens the path of these ligaments, which will cause the head and neck to rise. In this position, the horse will experience stress in the poll area. To complicate matters in the dressage discipline, where a horse is asked to be on the vertical, this pressure becomes even worse. This underlines another reason why the horse’s back needs to be up at all times.
As the horse is moving forward, his hips will follow the motion of the movement of the diagonal legs. This in turn will cause these ligaments to move back and forth along the spine with the horse’s motion. This path along the spine must therefore remain clear of any pressure. If not, this will cause the ligaments to shorten their path, again placing stress at the poll, or even causing resistance in the movement of the hip.
In the saddle, the horizontal panels (panels that sit under the saddle on top of the horse’s back) have to far enough apart (gullet width) to allow these ligaments the freedom to move with the diagonal motion of the horse. If not, it will be the pressure of these panels against the ligaments that will shorten the movement of these ligaments, thereby creating poll stress and shortness of stride.
Another complication here is when a horse has one shoulder that is more forward than the other.