3 muscles are responsible for horses having the potential of experiencing back pain. They are the longus/colli (muscle under neck), iliopsoas (lumbar spine to hip) and the rectus abdominus (belly muscle). When these 3 muscles are working correctly, they will enable your horse to collect and round up. The topline muscle needs to be relaxed too.
All of these muscles are powered by the nerves from the spine. When a subluxation exists (bone of the spine not moving correctly), then these muscles weaken and the topline gets tight, thus creating a hollow back and an inability to collect up and move forward. This will even affect the front limbs from extending forward. All the exercises in the world will not improve the weakened muscles until you have the horse’s spine adjusted. Then exercises will improve the movement.