Understanding the structure of your Horse will allow you to appreciate the correct anatomical properties of suitable Girths and Saddle Pads.
From the perspective of the Society of Master Saddlers, please check the billets lay straight and even, running parallel to the ground; poor construction of the girth straps will influence where the saddle will sit on the horse. It is preferred to have two-sided elastic girth thus allowing the rib cage to move evenly. Ergonomically designed will allow for the elbow to retract unencumbered with each stride. Some horses have a narrower girth groove, <4" from elbow; this type of structure is well suited to the Narrow gage girth, as such from the Pro-Lite range. Wide barreled horses have tendency to push the girth forward and towards the elbow as well- this type may be a strong candidate for the Narrow gage.
Located behind the elbow is a network of nerves. Often impeded by ill fitting girths, Horses will present "girthiness", display tail wringing, ears pinned back- physical displays of tension. Current research has produced several different configurations of girths: some claim to be the fix all, some have fancy contraptions, and some- actually are anatomically correct and beneficial.
What we do know : ill-fitting saddles will run the girth into the elbow area and cause constriction to this sensitive area. Horses need stability from the equipment. You must use the same billets at the same holes. In consideration that the girth groove is average or well behind the girth groove, wider girths are more comfortable as it distributes pressures over a larger surface area. Narrow strips will bear all the pressure when tightened, acting as restricting straps.
One-sided elastic girths creates too much play, thus causes the instability and may allow saddle to twist unevenly on the back . If fasten too tightly it will restrict breathing; will cause muscle soreness, especially along the bottom of the girth line on the sternum.
Non-elastic girths may be uncomfortable and loosen as the horse exhales. This gives Rider opportunity to over tighten.
What is it, that you'd like to accomplish by the use of the Saddle Pad?
From the Perspective of the Society of Master Saddlers: Saddle pads protect the Horse from the Sadde, and the Saddle from the Horse. At times, shim and corrective padding is required to offset asymmetries, yet here we discuss general saddle pads. A change in depth of saddle pad from the original fitting objective, will alter the fit of the Saddle. Please advise your QSF, or fitter if you have altered the equipment as saddle pads may increase the pressure.
Preference should be made to a wither contour. This feature will allow the area under the tree point ( Trapezius) to be free of constriction. Some saddle pads have narrow bindings, and will interface directly with the horses back causing soreness. Without the wither contour, the saddle pad will strap down against the Trapezius , again causing constriction. Some new designs are exceptional examples of anatomically correct padding. Remember, simple is best.
Three of my favorites are the Lettia Cool Max Pro , features a plate Wither contour, thin quilt and wicking properties; Concept Spine-free Pad, features a 4 panel spine, straps set higher to attach to D-rings and shearling inset under panel area; and newest on the market, Equifit Essential Square- thin, anatomically built with a very generous plate Wither contour, made of cotton twill, slip-resistent flannel backed, with wicking properties.