Sorry to read about your horses back. You don't say exactly where in the back
the problem is. But I'll take a shot at it anyway.
It's interesting to me that you can lounge the horse without issue and it only
comes about when bearing the weight of the rider. I would consider the rider's
influence in the problem as well as the saddle.
When palpating the back you say you get a "pain" reaction after riding and
then no reaction after some time off. I would feel for heat in the area, when
the pain reaction is gone. This would indicate a HyperTonic muscle to me, that
is "bracing" or "splinting" the area. This can be in reaction to previous
pain, or to a subluxation somewhere in the axial skeleton. It could be a
vertebrae or an Illia. The heat comes fromt he amount of energy the muscle is
expending more than from inflammation. Because this muscle is working so hard
and is partially contracted it does not have it's normal range or motion and
can be easily over worked. I assume you are stretching the horse and yourself
before serious work.
I would also check for hypertonicity of the hamstrings and gluteals. Some of
this added tension is going to be a reaction and some may be the cause.
The investment in the DVM/Chiro is realitively cheap when yu prorate it over
the long haul. The longer the issue persists the greater the possibility of
some movement aberation setting in, and the possibility of some chronic non-
specific lameness. Also the Vet can tell you if soft tissue work is in order
and should be able to recommend someone who is qualified in your area.
I read in your later posts that you have gotten some massage advice so I won't
bore you with more of the same.
It is important to role out other medical issues, the rider, the tack as well
as making sure that all of the soft tissue compensation is addressed.
Certified Advanced Rolfer