Kristy here is some info you may find interesting. Even though it is on English saddles the function of the tree is all the same and you can see how important a solid tree is for weight distrabution. I'll keep looking for the vet page on what damage can come from treeless saddles.
Principles of Correct Saddle Fit
Saddle Fit: Understanding the basics
Saddle Fit: A word about panel design and technology
Saddle Fit: Understanding the basics - Tree, Panels, and Motion
The basic principles of saddle fitting are relatively easy to understand. The primary principles of saddle fitting are analogous to fitting your foot correctly in a pair of supportive athletic shoes. A well-fitting saddle spreads the rider's weight-per-square-inch evenly over the panels, avoiding excessive or uneven pressure under any part of the tree or panels. The saddle must also be correctly balanced so that neither the back nor front is either perched too high or wedged too low.
Front and back gussets enable the panel
to be shaped for even weight distribution
and correct balance.
The purpose of the tree is to distribute the rider’s weight as evenly as possible over the broad, strong sweep of muscular surface of the horse's back. The tree is also designed to protect the crucially sensitive spine and central nervous system from impact shock or pressure. The panel that the saddle sits on cushions the tree, helps to balance the saddle, and provides a close, stable purchase between the saddle and the horse’s back. In order to achieve correct balance for the rider and even distribution of weight for the horse, the tree must be the correct width, and of vital importance, the tree must be the correct shape for the horse.
The tree points must be a reasonable match for the angle of the horse's shoulder. This aspect of tree fit is relatively easy to assess, and often the investigation delves no deeper than this. What is harder to discern is whether the other dimensions of the tree are equally well-matched to the shape of the horse’s body. Once the tree is covered by the seat on top and the panels below, the exact fit characteristics of a particular tree are often more challenging to work out. It is quite helpful if the fitter has some familiarity with the overall shape and dimensions of the particular tree in question to be confident that the structural contours of the tree are a close match for the contours of the horse’s body.
If, for example, the space between the rails of the tree is too narrow for a horse with a short, dome-shaped wither, the saddle will pinch and pivot over the stirrup bars. One clue to this might be that the back of the saddle tends to flip up and down in rising trot. In this instance, if the spacing of the rails is too narrow or angular for the width and spread of a particular horse’s back, the narrow area under the stirrup bars forms a sort of stricture where the contour of the tree deviates significantly from the contour of the horse’s back. As the rider weights the stirrup bars, this becomes like a fulcrum, levering the back of the saddle up and down when the horse is in motion. The visual effect may be somewhat disguised either by thick gusseted panels or by trying to level the back panels with a riser pad, but nothing can compensate for the underlying problem of a tree whose shape is a poor match for the horse under the stirrup bars, even if it the fit appears to be a fine through the tree points.
There are many factors that can affect whether a particular saddle remains stable on the horse through the horse’s full range of motion, but the first question to ask is: Does the shape of the tree in this saddle conform well in all its dimensions to the shape of the horse’s back?
What is crucial when evaluating a horse for saddle fitting purposes is not really whether the saddle looks like a good fit when the horse is standing in the cross ties, but what happens when the horse is in motion under a rider. The saddle must be stable and well balanced through the entire range of equine bio-dynamics in order to perform its two critical functions of protecting the spine and distributing the rider's weight comfortably and evenly.
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Saddle Fit: A word about panel design and technology
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 makes the prospect of rounding and engaging the back for correct athletic movement a great deal more inviting.
This gusseted back panel distributes
the rider's weight evenly over the
horse's back muscles.
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 girthing configuration and panels 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.
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.
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