Very simply to start is that back is very much like a suspension
bridge with support on all four corners but nothing under the barrel
of the body (the "free span" from the point of wither to the LS joint)
to hold it up but the spine and the muscles of the back.
The back works to support the rib cage and connect the hindquarters
and the forequarters. Without us astride it is carrying a  large
amount of weight just in the contents of the rib cage. Nature has
provided the horse with all the needed muscle structure though to add
strength and more support to all this.
A good thing for one to do would be to look at a picture of the horses
skeletal system with a horse standing with it's legs correctly under
them self's. Take notice how all the bones align and are in place. If
one can visualize moving the legs out in park position and watch the
back drop. Think what is going to happen to all the vertebrae.

To give one a good idea of this would be to take  a spring  from a
inside of a pen and bend both ends up while holding and pushing your
elbows up and out ( your arms being the horses legs),  and see how the
top of the spring bends into each other and the bottom spreads out.The
support has been taken away, the spring drops into a ventro position.
It  also has caused all the top loops of the spring to pinch into each
other.  Now bring your elbows back down where your forearm is
perpendicular to the ground like a horses leg, while holding the
spring . The spring goes back to being level and everything back and
aligned up. The spring level could hold some amount of weight if added
but the ventro held spring could not hold the same amount of weight as
the level position with out buckling in more because it has no support
under the four corners.

When a rider mounts a horse with a level back the horse has the
ability to pull it's muscles up to hold a certain amount weight and
/or pull without stress by the legs being in the correct support
position but move that support out on one end or both ends the ability
for the horse to lift and engage those same muscles has been taken
away. The horse is now unable to keep the vertebrae from buckling down
and into each other and the muscle are unable to pulls them self's up
to support as well. This is not only a stress to the hard tissue but
also to all the soft tissue of the horse back. If one watches closely
you can see the horse now braces the legs, hips and shoulders and neck
for the added pull and weight about to be applied. This is where all
the strain in other areas mentioned in the article comes in.

This can be made even worse by that rider that does not use there own
impulsion from there own feet and legs to mount but pulls on the front
and back of the saddle and pulls and drags all their body weight up on
the horse.
If one spends a bit of time thinking about it is rather simple to
understand .
 Another good way to test it on ones self would be to kneel on the
floor on hands and knee's keeping your limbs under each corner of your
body. Now just move your knees out back behind you and feel what
happens to your own back, it drops from level and feel the strain on
your hips as well as lower back and shoulders. The longer you hold the
more you feel the strain now move your hands out in front of you and
feel that. Having a 2 or 3 year old child would be a good test to ask
them to climb on your back when level and then do it again with just
your knees out behind and then do it with your arms out as well. How
much do you have to brace in both positions?
 Next would be in this human park position  with the child aboard to
pull your limbs back under you lift your back and move forward. Then
do it starting with a level back position. Last would be in the parked
out position to move backwards with the child aboard. Be ready and
have the liniment ready because chances are you with feel all this in
the morning.

All this can  weaken muscles and they can become atrophied along with
causing misalignment of the spine and hips, pushing out the joint
lubricant and wearing away joint cartilage creating bone on bone
eventually possibly causing bone wear or even chipping or fracturing.

Ok, here is one ... when a horse is standing normally, there are small
spaces between each of his vertebrae which cushion the edges of the 
and prevent them from impinging (touching) one another.  When he is 
out, his back goes into a concave curve (ventroflexes) and these spaces 
diminished, bringing the edges of the vertebrae closer together.  When 
climb on him, adding weight to the already concave back, it becomes 
more concave, and those little spaces become ever smaller.  This can 
real problems in the back, especially neurological damage. which may 
manifest itself in odd lameness and lack of soundness in the hind legs.

So, I don't ask a horse to park out to mount, I use a block!

Lee Z