Useful back exercises for your horse
There are various theories about why horses get 'kissing spines'. Heredity is likely involved, as well as improper loading of the back during riding.
Not all horses suffer from the condition, even if x-rays show that they have it. In attempt to prevent 'kissing spines' developing, it's a good idea to train their back, and aim to ride your horse with the right form.
Good exercises for back training
Back ups
You don't have to back up ten steps in a row, but just one, with the horse in shape, and then ride forward directly. In the moment when the horse takes the first step forward, the back muscles are engaged.
Hill work
Climbing is good for your horse's back. Climbing up is better exercise than climbing down for a horse with back problems. If you have a healthy horse and just want to train their back for preventive purposes, climbing down is fine. You can strengthen their back muscles on the downhill, by doing halts at regular intervals on the way down. When you climb, do it at a walk, and make sure your horse lowers its head so they can navigate the route.
Work in low form
Working your horse in a low form, but with the hindquarters well under them, is excellent for stretching and strengthening the back muscles. Thanks to the inclination of the vertebrae - where the thoracic vertebrae lean forward and the lumbar vertebrae lean back - the back muscles are properly arched if the horse settles down with the hind legs and walks in a low form with head down.
Gymnastics jumping
If you are confident in letting your horse go without a rider, try some gymnastics jumping. Jumping a series of bounces of three to four poles is good exercise for your horse's back, as they have to arch and bring their back up with each jump.