As a horse builds musculature, the width of the gullet plate and width of the tree will change. An example here is a young thoroughbred built heavy on the forehand. The humerus is collapsed, shoulder straight, camped under (legs lean to the rear of the horse, instead of squarely under). This horse would now require a narrow gullet plate with a narrow tree. As the horse is allowed to move without resistance, the back muscles will develop, pulling the humerus up, which will in turn move the shoulder back and position the front legs under the horse. This shifting of weight from the front to the rear of the horse is what is referred to “coming up in the wither”. The saddle will remain a narrow gullet plate, but he may now need a medium tree. As the horse develops through lateral and collection work, his trapezius will develop mass, and more of his weight is shifted to the rear. Now the saddle’s gullet plate may need to be widened, and the saddle tree may need to be narrowed – something that is not readily doable with most English saddles. We call these disposable saddles, since they are not able to change with the needs of the horse.
The human analogy here is Arnold Schwarzenegger (or any major body builder, for that matter). Most men’s necks come off their shoulders perpendicularly, or at 90 degrees. The trapezium muscle lays flat to their collar bone. However, in Arnold’s frame, the trapezius comes directly off the tips of his shoulders, and attaches on a narrower angle, somewhat below his ears. This is where much of the confusion arises concerning seeming muscle atrophy along the wither muscle. Instead, correct muscular movement causes correct muscle mass. For the trapezius, this equates to the angles becoming narrower (not atrophying), requiring a saddle to fit that narrower frame.
As you can see, it is extremely important that saddles have the ability to change both gullet plate width and tree size, as the horse correctly builds muscles. If not, the saddle becomes the training block for the developing horse.
The abdominal, or stomach muscle, is the largest and least understood muscle of the horse. However, it is also the most essential. In studying the biomechanics of any animal, you will find that all motion is made possible by interaction between two forces. In this case, the stronger the stomach muscles are, the stronger the back muscle can be. The best example is the swaybacked horse. This condition is due to weak abdominal muscles.
The human analogy here is illustrated with people who have back pain. A prescribed remedy includes exercising the abdominal to compensate for the overworked back muscles, which are doing all of the work holding the body erect.
The bottom line is: if a horse can move freely, taking into consideration the different angles of different breeds and various degrees of conditioning, the saddle should not interfere with the movement if it has been correctly fitted.
Horse Anatomy – Biomechanics of the Humerus and Shoulder
The muscle inserted in the femur is the Latissimus dorsi (back muscle). The position of the humerus tells a lot about the horse. It tells how developed the back muscles are, and how much weight of the horse is being carried on the forehand. The panels of the saddles should fit entirely on this muscle.
When a horse is moving correctly forward, the axis of motion of the fore leg is at the top of the humerus. As the humerus straightens, it pulls the latissimus muscle. Each time this limb moves correctly, it builds strength. As the muscle builds strength it builds muscle mass, which in turn pulls the humerus back. This development of the back muscle is what is responsible for bringing a horse off his forehand. This is how the development of the frame of a horse is accomplished. The result of the biomechanical movement of the humerus forward will cause the shoulder to move back.