Swaybacks or low-backed horses are those with visibly bent-backed postures along the middle of their backs. The condition is clinically known as lordosis and normally occur in horses due to old age and years of hard training, as well as those who have had numerous pregnancies. However, there are also instances when it can occur earlier than normal if it is passed on genetically.
Fitting a regular saddle on a swaybacked horses can cause saddle bridging wherein the saddle and rider's weight abnormally gets centered on the withers and cantle area while the middle "bridges" because of the lack of muscles there. This can be uncomfortable to both horse and rider and can cause abnormal spots on the wither and cantle area; an indication of an ill-fitting saddle.
To correct this, use a gel saddle pad like the Middle Riser Anti-Slip Gel Pad. This gel pad comes with extra padding in the middle that can fill up the lack of muscles in the area and can you and your horse a stable, more comfortable ride.