It's often heard: "whatever you work on, on one side of the horse, you should work on the other side of the horse."
This is training sentiment is put into practice to keep a "balanced" approach. This helps to manage the explicit training of one side over the other. It prevents horses from becoming extremely one sided strictly due to preferences of the rider.
However, this does not help to alleviate the sidenedness already present in a horse. If a scale is unbalanced to start with, adding equal weight continuously to both sides will never balance it. A person must first assess the difference in weight of the sides, and then proceed to add and/or subtract weight from one or both sides in minor adjustments to eventually find an even and balanced position for the scales. So to must the rider do for their horse.
1) WARM UP
In order to assertain an accurate reading of the sidedness of your horse, they must be warmed up fully on both sides, otherwise the rider will glean false assessments.
A horse should have 5 to 15 minutes of walk on a free or loose rein, being able to swing and bob their head on straight lines, shallow figures, and large circles. In the trot and canter, the rider should take either a two point seat or light seat and go rising in the trot, to alleviate pressure on the back muscles. Again, the horse should be allowed on shallow figures, large circles and serpentines, and directed to frequent changes of direction. The horse should be encouraged to stretch forward and downward into the contact while maintain rhythm and thrust from the hind. Transitions between gaits are also important to help a horse warm up and encourage them to go "on the aids". A warmup should be around 10 to 25 minutes, depending on the horse, their age, fitness level, type of work that is being trained, and the specific goals for that day.
2) ASSESSMENT OF THE SIDEDNESS OF YOUR HORSE
Once the horse has warmed up both ways equally the rider will be better able to assess the horse's sidedness. This is like adding weights to your scale, and letting the scale settle into place before analyzing the balance.
At this point, smaller circles and simple lateral movements are used to see what side is more hollow and what side is more dominant.
Flex your horse at the throatlatch, bringing the head so you can see the eye and nose right, and then left. Which way was easier? That might be your more hollow side. Which way was harder to flex? That might be your more dominant side.
Try working in a small circle or volte in both directions. Does your horse maintain the bend and step underneith themselves maintaining impulsion better to one side than the other? Does this seem to correlate to the result of the throatlatch flexion?
Try a simple leg yeild from the centerline or quarterline to the track. I find moving towards the track, especially in this "assessment" period easier as it gives the rider a guage for straigtness. Furthermore, the wall and corner support your outside aids in the recovery steps of your movement when back on the rail. In the leg yeild, was one way easier than the other? Did the horse trail their hind legs out more noticably in one direction over the other? Did the shoulder buldge out more in one direction over the other? Was it easier in one direction over the other to maintain a parallel line to the track, rather than moving on an angle to the track in more-or-less a straight diagonal?
Try spiralling in and out in leg yeild from a 20 meter circle to a small 10 meter circle. Again, is it easier to maintain flexion and impulsion one way over the other? Did the horse bulge out through the shoulder one way more than the other?
Try doing a series of serpentines down the arena. Is it easier changing the bend from one direction to the other?
You may find it helpful to have a friend be in the arena with you during this initial assessment period of the horse a) take notes and record your feelings of the sidedness of your horse and b) make observations on the apearance of sidedness from the ground.
Once you have recorded these findings, it is helpful to go through them and make correlations between all your assessed movements. Was there one way that was noticeably more "bulgey" in the shoulder? Was one way easier in general to flex than the other? In all your movements, was one way more hollow, and one way more dominant?
If yes, you have discovered the starting balance of your scale. The heavy, bulging, dominant side is the side of the scale with more weight, and the light, hollow side is the side of the scale with less weight.
3) BALANCING THE SCALE
Now that you have asessed the initial values of your scale, it is time to either add or subtract weight from either side to balance the scales. It is at this point that I must reaffirm that a balanced and straight horse does not result from neither overtraining one side over the other, nor from riding both sides exactly the same. The horse exibits different characteristcs on either side, therefore must be ridden slightly differently on either side to relate to these differences.
The dominant side is the side that is heavy, bulging through the shoulder, and stiff through the throatlatch. This side must be encouraged to flex through the neck and ribcage, and be encouraged to lift up the shoulder and step up with the inside hind leg. Try incorporating lots of volte's within the work to this side. Start and finish your long sides with volte's to this side, and put volte's in the middle of lateral work to this direction. Go deep into corners, and really focus on bending around the inside leg and lifting up the shoulder. In transitions, concentrate again, on keeping this dominant side soft and flexible, with that shoulder lifted, and that hind leg up and pushing forward.
The hollow side is often perceived to be the light side of the horse. A horse often has an easier time to bend their neck and body to this direction. In addition, the horse often trails out through the hind end towards this direction, and will find it harder to carry weight on that hind leg, as it is often not under the body, but off to the side. Work on lateral movements putting this hind leg under the middle of the body while maintaining impulsion would be a good idea. This could consist of travers or haunches in with the dominant side on the inside and the hollow rein on the outside. Leg yeilding into the hollow rein both towards the track or spiralling on a circle would also help encourage stretching and lengthening into contact on this side.
When riding to address the sidedness of your horse, you are helping to balance them out into a more even and straight mount. This will in turn help to create a happier horse with more stamina and better ease of movement.