Equine Symptomatic Lameness

poorly. Most important to recognize are the tree points width and angle and the saddle support area of the horse. Although most riders know there should be two to three fingers width of clearance at the withers there also must be sufficient clearance all around the withers. The tree point angle at the gullet plate needs to be parallel to the shoulder angle not the wither angle in order to allow the shoulder to properly move. The cartilage will protect the shoulder bone as it moves between the deeplying spinalis rhomboid and longissimus muscles and the superficial trapezius during motion of the horse. However if the tree points are forward facing or too narrow this cartilage can be severely and irreparably damaged over time. One behaviour that can result from poorly adjusted tree points is what is termed stubbornness. 
Remember this area over the withers is where the stallion bites the mare to immobilize her and prepare her for breeding. Its the same result with different inputs the rider is on top urging the horse forward reflexively however the horse doesnt want to move.
If the saddle is too long for the saddle support area the most commonly affected vertebrae are the last two thoracic and the first two lumbar T17T18L1L2. Additional issues arise when the panels twist or fall to the side and shove the vertebral spinous processes to the left or the right because of greater muscling at one shoulder. The resulting pain also causes the horse to protectively tighten the back muscle longissimus dorsi which further pulls the vertebrae out of alignment. Therefore either direct trauma or secondary muscle traction are to blame for a resulting occurrence of lameness.
If correctly fitted and centered the channel of the saddle protects the dorsal spinous processes. However when the rider sits too far back or the panels of the saddle are too long the horse may experience tremendous pain over the lumbar transverse processes which are not designed to carry the weight of a rider and saddle. The horse then hollows its back hyperextends and the result is sacroiliac hock and stifle problems. Although there could be health related issues not caused by saddle fit it is worth considering the saddle as instigator before investigating other potential causes.
Many veterinarians have not been trained in saddle fitting and may actually not recognize the consequences of poor saddle fit. Veterinarians are generally called