We have all thrown around the T word since we became cognizant of saddles. Mostly we talk about whether we like a narrow or wide twist. Hmmmmm... have we ever considered the horse we are riding in this conversation? Not very often.
Just what IS a twist? The twist is actually supposed to accommodate the horse's shape, not the rider's. It is the area in the saddle tree that actually twists the wood or plastic tree angle from the withers to the angle of the back. That the twist is in an area that determines our comfort is just a coincidence.
Why is this important? Most of us have been purchasing saddles with twists we like and we have not been taking the horse's shape into consideration. If the twist is too narrow for your horse, whether in an English, Western, or Australian saddle, it will pinch the primary muscle in the horse, that needs to develop. Not only will the trapezius muscle not develop, it will atrophy in many cases because the pinched area also becomes a pivot point where all the rider's weight focuses when the horse tries to utilize this muscle (ie. it flexes this muscle and lifts the narrowed area instead of having the room to flex into).
If the twist is too wide for a horse it can cut into this same muscle right at the top and dig into the spine. In this case the tree will essentially hang on either sides of your horses's long wither spinous processes.
Hold your bicep tightly and flex it. Try to imagine building up your bicep with the constant pain of either of these two kinds of constriction. You would compensate somehow. Horses do too and that is one reason why so many hocks and stifles are being injected.
When you purchase a saddle, use a saddle fitter who can talk you through whether your horse requires a wide, medium, or narrow twist. So what about your comfort? Let's just say I hope you are comfortable with the build of the horse you purchased, because his or her shape should be the first consideration when you purchase a saddle... after all, your weight is on their back, not vice versa.