Hi Greygates I have found some interesting info off a Horse Q&A related website:
Many back problems develop because of pain lower down the hind legs and cannot be cured without solving the primary problem. Asymmetry of muscle mass is not necessarily cause for alarm (unless accompanied by lameness) but illustrates an underlying rotated pelvis, which he may have acquired when young and has since stabilised.
Veterinary advice is vital to decide if the problem is acute or chronic and if the pelvic area has suffered any fracture or merely muscle or ligament strain. Physiotherapy will help muscle strain and can then build muscle back up correctly once inflammation has gone down. However, physiotherapy might not have been a priority in the first instance - box rest may have been a more suitable treatment for him.
Ensure that your physiotherapist works under the guidance ofa vet and only once a diagnosis has been made. Also, check he or she is a member of the ACPAT (Association of Chartered Physiotherapists for Animal Treatment, (tel: 01962 844 390).
An osteopath or chiropractor might help in other ways, depending on the problem.
I was either told or read somewhere that it would take the strength of about ten men half an hour of heaving and pushing to push a rotated pelvis back into place. I think what actually happens is that the chiro or physio comes along and manipulates the muscle which is in spasm which in turn holds the ligaments, tendons and bones in place. Once the muscles are loosened off it is possible for some degree of manipulation of the bones but the way the horse is worked following manipulation is crucial to its correct way of going, and thus the problem remedying itself. I might be wrong, but that is my understanding of the process. x
