The following information on back soreness was provided by North Carolina State University College of Veterinary Medicine.
There are two major areas of back soreness that need to be examined and ruled out before any major medical or surgical treatment of hind limb joints like the stifles or hocks are instituted.
The Saddle Area
The saddle area includes 4 to 8 inches below the point of the withers (the pommel area) and under the back portion of the saddle (the cantle area). Soreness in the saddle area relates to the horse's back conformation, saddle fit, and how the rider "rides" or sits in the saddle.
Soreness in the pommel area corresponds to the most concentrated area of pressures generated by the saddle tree and the pull of the girth and stirrups in both English and Western saddles. One example of many would be if the girth is not tight enough and allows the tree to rock back and forth, this will cause soreness in the pommel area even with the best of riders. The pommel area is the site where the rider's weight naturally focuses via body position and weight in the stirrups. A rider that carries his/her weight back too far in the saddle can cause soreness in the cantle area. There can be additional non-saddle related stresses to this withers area in certain situations, such as a blanket that pulls or a horse that rolls in its stall or field in a particular way as to bruise the pommel area. Withers conformation, either higher or lower than usual, may also be a contributing factor to specific soreness.
After a thorough examination, which may include the rider riding for the veterinarian, some treatment plans can be developed. They may include ponying and/or longeing to rest the back area, saddle fit evaluation,and pad changes. A thick pad that covers the entire saddle area could act as a "back bandage." In spite of the fact that this type of pad might elevate the rider too far away from the horse's back, it helps in the healing process. Once the back is healed the training or competition pad can be put back into use. Any pad that offers folds, ridges or pockets that can potentially develop pressure points between the saddle and back should be avoided.
The Rump Area
Soreness in the rump area can include pain in the sacroiliac joint, hip joints, gluteal or semi-membranous muscles, etc. This particular pain can come from a direct pull or trauma to the particular area. Also, this pain can be referred from pain originating from any joint below the hip where the horse has to compensate for over-contraction of a muscle to try to relieve pain on the joint, especially while carrying the rider in a particular exercise.
Horses that have normal hind foot conformation, which includes reasonable heel height, have an imaginary line extending from the coronet band that reaches the forearm. So, if the horse has low heels and long toes, the imaginary line will land along the belly line. These horses could have a higher risk of gluteal soreness and possibly increased risk of suspensory ligament inflammation. Correction would be to raise the heels or decrease the breakover via trimming or shoeing.