Back pain in horses is one of the most difficult problems to diagnose because the symptoms can be vague and the underlying cause may be in another area. Dr. Andrew Fiske-Jackson indicates at the Horse.com that horses with back pain may show poor performance, rearing or bucking under the saddle, sensitivity when grooming or being cold backed. The symptoms are really vague and could be caused by many problems other than back pain. Lots of horses are referred to as being cold backed and again this is a subjective test but some horses with back pain do not want to move their backs and you can determine this by lightly pressing a pointed - but not sharp - object along the back in a linear fashion and the horse will generally extend or flex the back depending on the area. Too much extension or flexion could indicate hypersensitivity and is abnormal but not moving the back at all could indicate back pain.
Dr Fiske-Jackson indicates that in many horses, back pain is secondary to lameness in the forelimbs or hindlimbs and the pain is secondary to abnormal movement of the lower legs, causing a strain on the back. A lot of back pain diagnoses begin by ruling out other problems as 74 percent of horses with back pain are also lame. Your veterinarian can attempt to numb the back with local anesthetic and see how the horse performs, but riding a horse with a numbed back is not without risk so if you cannot see the problem without a rider, it can difficult to determine improvement. However, veterinarians can also use inertial measure units to measure the back movement and this is done by placing small sensors on the back to check for reduced back movement when exercising. Also, ultrasound can be used to detect loss of muscle mass in horses with back pain.