There are many fairly subtle symptoms of back soreness in horses, including:
Abnormal standing and/or shifting stance frequently while tied or in cross ties
Flinching or pinning his ears as the saddle is lowered to his back
Pinning his ears when you mount
Stiffness turning right or left
Reluctance to work, when this hasn’t been a problem in the past.
More obvious symptoms of back soreness include:
Dipping of horse’s back as you palpate either side or both sides of it with moderate pressure, on either side of his backbone. Use a knuckle to put pressure on the long muscle of his back (each side of the spine, about 2 inches away from the bones). Horses with backs that aren’t painful usually don’t mind a fair amount of pressure on these muscles.
Head tossing, ear pinning, moving around, etc., as you palpate the back muscles.
Why is my horse’s back sore?
Stifle soreness-tightening back muscles to avoid moving the stifles up and forward during locomotion.
Poorly fitting saddle (this is a fairly infrequent cause if the saddle is good quality and well-cared for, unless combined with a poorly-sitting rider).
Riding the horse with his head too high or too low, or in draw reins that the horse tries to oppose.
Rider sitting crooked or off to one side (Look at your saddle -both the seat and the underside, as soon as you dismount after an hour’s ride-to spot potential crookedness in your seat position. If you are sitting crooked, there will be a larger wear pattern on the side you favor).
A fall in the pasture.
Bruising of the back from horse play: Horses (especially geldings) play rough!
Getting cast in the stall (This can happen when no one is around, but evidence is there if you check: “trashed stall bedding”, knocked over water buckets, hoof marks on the stall walls, abrasions on the horse’s head)