If anyone has a horse with this condition I would be very interested in hearing from you.
My pony has in it one hindlimb and apart from limping at walk, it doesn't seem to bother her.
Dont know if this is helpful but my last horse was diagnosed with fibrotic myopathy of the iliopsoas muscle (there are in fact a few of these muscles ie. psoas, iliopsoas, they are deep muslces and apparenlty the only way to exam these are through a rectal examination) My boy received treatment at Hartpury college, treadmill with string attached from his tail going to the bridle and a girl standing to one side pushing his "weak" quarter over. He also had a weight "glued" onto the front "dorsal" part of his hoof... anyway I can give you more info send you video clip of his treatment (sad old me did this for reference!!) but unsure if this is the condition you need info. on so will not ramble on anymore!!
My mare has it in a different muscel, but yes it would be helpful to hear about the treatment your horse received as it is the same condition (just slightly different location).
Thanks!