In regard to sore rear-end muscles, normally, I would say that the trainer is the one that needs to build up strength in his horse by working him in soft going, up hills, etc., but there may be a pathological problem in play here. I have had good luck injecting a 2% iodine in almond oil infiltrated in the upper rear end muscles over the years. Also, if you have access to Rubeola Virus Immunomodulator. It is rather pricey, but has worked wonders for my race horses in the past:
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