If your horse is out of work, or has a history of back pain, consider strengthening the core/stabilisation muscles. This is helpful to the horse, just as it is to us. Often if a horse has an injury, weakness, or pain response, the deep stabilisation muscles ‘turn off’ and need to be reactivated and strengthened. Think of the difference in your back pain when you have a strong core! The exercises that have shown significant improvement to a horses stabilisation muscles are these simple carrot stretches :
1. nose to chest (nose close to sternum)
2. nose to knee (nose at level of knee)
3. nose to fetlock (nose at level of fetlock)
4. neck extension (chin as far forward as possible)
5. nose to girth (both sides)
6. nose to flank (chin heading toward point of hip both sides), and
7. nose to hind knee.
These exercises performed at 5 repetitions, held for 5 seconds, 5 days a week for 3 months SIGNIFICANTLY improved the stabilising muscles in horses that were not being worked. This could be a real life saver for some of those weak backed horses. 
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Ref . (Stubbs N.C., Kaiser L.J., Haulptman J., and Clayton H.M., (2011), Dynamic mocilisation exercises icrease cross sectional area of musculus multifidus), Equine Veterinary Journal,43(5) 522-529
