An important part of maintaining quality of movement is exercise and stretching between treatments. Every horse is different so discuss with your veterinarian if this stretch is appropriate for your horse, how often your horse should be doing theses stretches and how many reps are appropriate.
This is a stretch for the whole back. Have your horse stand somewhat square. Using a treat try to get them to reach strait out in front of themselves. You may need a barrier such as a gate, stall guard or fence, to prevent them from just walking forward. This extends the entire back. Once they reach forward have them reach down in-between their front legs, essentially bowing. This helps arch the back and stretch from head to tail.