A correctly fitting tree is vital, but so are correctly fitting panels. In general, the larger, broader and more level the weight-bearing surface (as opposed to curvy and dome-shaped), the more comfortable the horse will be in his saddle and the more stable the panel will be when the horse is in motion. From the horse’s standpoint, a stable, yielding panel probably makes the prospect of rounding and engaging the back for correct athletic movement a great deal more inviting.
Panels are cut on many different patterns with different fit characteristics. What is important to maintaining good support, correct balance, and even weight distribution is that the entire surface of the panel must conform in shape and angulation to the contours of the horse’s back when the horse is engaged and on the aides.
The saddle fitter’s challenge is to choose a panel pattern that will complement the shape of the back while accommodating the changing contours of the horse as he moves through the full range of motion under saddle. This is why it is impossible to make a full and definitive evaluation of correct saddle fit with the horse standing in the aisle. An expressive, big-moving horse may put considerable dynamic stress on the stability of the saddle. As he engages the large muscles of the back and rounds up under the saddle, he transmits the enormous power of the hindquarters through the spine into forward momentum. Regardless of how well a saddle may seem to fit when the horse is unmounted and at rest, the picture may be quite different when an athletic horse is engaged and on the aids. Even a saddle that appears to fit quite well may end up pivoting and surfing all over the back when put to the acid stability test. In some instances it is obvious in the course of a ride that the saddle has inched forward over the shoulders and therefore is not stable. This is a situation that frustrates many riders on wide, barrel-shaped horses with expressive gaits. Admittedly this type of horse provides the saddle fitter a challenge in stabilizing the saddle, though starting with the best possible shape in the tree is a crucial first step.
There are auxiliary design features in the panel type and material, and the girthing configuration that can also be helpful. The situation may actually be grimmer, however, for a horse with a more hollow contour around the withers and behind the scapula who is laboring under an unstable saddle. Rather than sliding over the shoulder, the saddle may continuously jam against the back of the shoulder, or bounce around behind on hard, round panels. If at the end of a ride the saddle is still where the rider put it, the unstable panels or ill-fitting tree may not be visually apparent to the rider. Thus a less-than-stellar performance caused by a poorly-fitting saddle is often attributed to training issues or some other cause.
Bear in mind the close analogy between saddles for horses and shoes for human athletes. Imagine a top athlete running a marathon in a pair of penny loafers that are a size too small. With sufficient grit it could be done, but not particularly well, and probably not without consequences over the long run. A skilled saddle fitter must have in-depth understanding of equine biodynamics and a capacity for seeing and evaluating what a horse does with his back and body with a rider aboard. In the same way that human athletes are unique and idiosyncratic in how they perform athletically, horses are as well.
Many of the classic problems in fitting performance horses require understanding not only how a particular horse is put together conformationally, but how the horse in question engages his back, how straight or crooked he moves in various gaits, how well balanced he is longitudinally and laterally. However well a saddle may appear to fit when the horse is standing in the cross ties, what matters is the dynamic effect the combination of horse and rider have on the stability and balance of the saddle is when the horse is on the aids.