The usual theory put forward as to why horses are one-sided like this is that it is the way the foetus lay in the womb. However, it is now considered more likely due to the fact that the horse has - as do all mammals - a split brain, with the two hemispheres responsible for controlling different functions.
Lets say our hypothetical horse is Stiff Left / Hollow Right. The obvious signs of this to the rider will be a heaviness in the left rein, difficulties in turning/bending - the horse will fall in on the left rein and out on the right rein and problems in canter depart. The horse is crooked.
What are the Biomechanics of the Crookedness?
The hind legs are not working with equal phases of thrust and carry. The left hind steps forward, but it doesn't do so far enough because he is carrying his pelvis at an angle. This means that the left hind doesn't reach as far forward as it should. It stays behind the vertical for too long and it thrusts more than it carries. It isn't supporting the left shoulder so the horse places to much weight on the left front leg and with it leans into the left rein.
The angle in the pelvis that causes the left hind not to step under enough has repercussions in the right hind leg in that it pushes it out to the right, from under the mass of the horse. This brings the horse's right hip in advance of his left one and puts him in a incorrect haunches in position to the right. The right hind is neither thrusting nor carrying, but idling off to the side. This can result in the muscles on the left side of the horses back becoming more developed that those on the right and cause the rider's seat to slide off the right side of the saddle. Very often a rider then tries to compensate for this sliding by leaning over to the left, twisting in the waist and collapsing over the left hip. In turn this causes the left hand to be carried higher and further back than the right. This over use of the left rein blocks the left hind leg even more. It steps even shorter, making the imbalance even worse. The horse becomes heavier and heavier in the left rein as a result of the excessive thrust and lack of support from the left hind leg.
Where do we begin when we start to straighten the horse?
With the rider - a crooked rider cannot straighten a crooked horse!
You have to become very aware of your own body and its position in space - proprioception* (for explanation see foot of page) and to make any adjustments that are necessary. Oh, if it were so easy!
Your instructor stands there and says: "You’re collapsing to the left / leaning forwards / pulling your heels up / riding with piano-playing hands."
It is the rider's responsibility to sit straight no matter what the horse is doing. When we think we are sitting straight and balanced, we are almost always leaning slightly forward, with one hip and shoulder more advanced and more weight on one seat bone. Even if a rider has been more or less straightened out by constant supervision from a teacher, the crookedness will sneak up on her again as soon as her attention wanders. I believe that this straightness is one of the qualities that distinguish the master from the average rider. The importance of straightness - both the horse's and the rider's cannot be overestimated. We can observe it in every rider we watch and in every horse we ride. If the horse's shoulders or haunches can deviate from their line by as little as a quarter of an inch, the horse immediately loses impulsion, suppleness, lightness, and longitudinal flexion. As soon as he is straightened out again, the purity of the gait will be restored as well. Of course, only a very straight seat is able to detect even the most subtle deviations from the straight position.
The thing that prevents you from making a once and for all adjustment is 'Muscle Memory'.
There is a key role between the mind and the muscles. Muscles memorise particular movements, especially when carried out over a prolonged period of time. These movements become implanted in the muscle and the mind and it is very difficult to change them. You have your habitual way of sitting -be it in a chair at your PC or in the saddle . It is unique to you. When you try to alter this position - for the good - after only a short time it will cause you discomfort because your muscles want to return to the position they know as normal. Unfortunately, the moment you take your mind off the new posture to give your attention to another positional fault or to give an aid, your body will automatically resume its habitual position. When you lose focus on what you are doing, your body does what it feels like and not what you want it to do!
It's as well to bear this in mind from your horse's perspective too. He has his own idea of what 'normal' is too and will try to revert to it in the same way. A strong horse can easily get a weak rider to sit where he wants them to. In other words he can get you to work around him. The knack is to get the horse - by maintaining you own correct posture - to work around you.
Straightness is the perfect ideal we ride towards every ride. It is a continuum that begins when we first get on a young horse's back. At this stage and throughout the Campaign School (UK up to Elem / US 3rd level approx.) we are concerned with ordinary straightness; the hind legs following in the same tracks as the front legs. The horse becomes longitudinally supple - stepping elastically from hind hoof to mouth into an even, living rein contact.
After this comes acquired or functional straightness. Now, the inside shoulder is placed in front of the inside hind, with a slight flexion of the poll to the inside (riding in position). Lateral work commences and the three joints in the hind leg are flexing and the weight is being taken more on them and the forehand is lightening.
Ultimate or absolute straightness is a for very advanced horse. He can be properly ridden from inside leg to outside rein and he is truly through and capable of all the advanced movements; piaffe, passage, pirouette, tempi changes, etc. Throughout all this increasingly more demanding work, you must never lose sight of The Basics - the longitudinal suppleness must remain and the horse must remain straight in relationship to the exercise being ridden.