Hollow, Sway, or Dropped Back Horses

 and although its difficult to understand and can be hard to attain it can ensure that
your horse is comfortable and its back will stay strong throughout its life.
Your Riding Skills
Even if your horse knows how to use its hindquarters and core muscles to hold up a rider your riding can counteract any attempt by your horse to carry you well. If you are too heavy for the horse or ride like a sack of potatoesbouncing and pounding on your horses back with every stepyour horse will get a sore back its muscles can weaken and its back may become hollow. 

AgeRelated Hollow or Sway Backs
As a horse ages its muscles weaken. This can lead to the swayback we often associate with old horses. This is just one of the reasons why a horse needs to get exercise throughout its life to keep muscles in tone. Lack of muscle at any age can cause a horses back to hollow out.
Correcting a Hollow Back
Like most horse problems good training and riding will help prevent and may even slightly correct a hollow back. A saddle that fits your horse properly may also help a hollow backed horse. This can take time and may require you to work with a coach for the best results. If the problem is congenital however there is little to do but make sure the horse is taught how to carry a rider so its muscles stay strong. There are also ground exercises that can help a horse with a hollow back.