The saddle will have a tendency to shift toward the shoulder that is more forward. This twisting action of the saddle will then cause the rear of the saddle to twist at the cantle (back of the saddle) across these supraspinous ligaments. This places more pressure on the ligament on the opposite side of the forward shoulder. This in turn will cause the horse to either tilt his head or hand on the rein.
The thoracic trapezius, or wither muscle, is probably the most complex muscle of the horse, also known as the ‘submission’ muscle. Many mammals use this muscle to carry their young. Gently carried in the mouth of the adult, the youngster is put into the fetal position which renders it motionless. This is where the stallion bites the mare to immobilize her during mounting.
Important: In the following paragraphs, we make reference to the width of the gullet plate of a saddle. This should not be confused with the width of the gullet, or the width of the tree. The width of the gullet (or saddle “channel”) is the distance between the horizontal panels. The width of the tree on the other hand, is basically the distance between the bottom of the tree points. Most saddles have a medium gullet plate, but can have a tree size anywhere between very narrow and very wide. It is generally about 29 cm (approx. 11.5”).
The trapezuis also delineates the path of nerves passing to the forelimbs and internal organs. When a massage therapist talks about meridians, a large percent of those nerves pass through this muscle.
It is extremely important that the saddle does not put any pressure on this muscle. Pressure on this muscle places the horse instinctual submission. This equates to resistance and sometimes a lack of forward movement. The part of the saddle that can present a problem here is called the gullet plate, and often veterinarians refer to this as the “vise grip” of the saddle.
The width of the trapezius muscle is usually indicative of a breed of horse, or a stage of development, and is referred to with respect to the position of the humerus/scapular bones.
As mentioned before, most saddles have a medium gullet plate. For some warmbloods the symmetrical saddle can distribute equal pressure from the top of the tree point to the bottom, causing the least amount of pressure on the wither muscle.
However, in the young warmbloods, thoroughbreds, saddlebreds and other high withered horses, the medium gullet plate will drop down on the skeletal process of the wither causing a lot of discomfort to the horse. If you want to position the saddle off the skeletal proportion of the withers, you will have to narrow the tree points inward (a narrower tree). This pinching action will lock the shoulders and prevent them from moving naturally. For this type of horse, a saddle will need to have a narrow gullet plate, with the tree adjusted to the correct tree width.
This is the paradox of a narrow wide tree, or wide narrow tree, and can be illustrated as follows: take your hands and put them in front of you, palm to palm as in a prayer position (fingers up). Spread them apart in this position, still essentially vertical, but 6 inches between them. This is a wide narrow tree. Place them again together, this time spreading only the bottom of your hands about 4 inches apart, and leaving the finger tips together. This is in effect a narrow wider tree.
The last groups of horses are the wide-withered horses; including Arabs, Morgans, Lipizzaners, Andalusians, Friesians, some Draft crosses, and some young Warmbloods. The ramifications of a medium gullet plate on this type of wither, is that the entire weight of the saddle and rider is on the wither muscle. With the medium gullet plate, there is no tree size that will adequately fit this type of wither. The horse will require a wide gullet plate, with a tree adjusted for the correct width of his shoulders.