Thumb draw problems

I have been thinking of using a "horse bow" on occasion (I am a field archer) instead of my usual barebow recurve. I have noticed that I find it requires far more effort to draw the same bow with the Asiatic thumb draw method as it does with the standard Mediterranean 3 finger draw. I think this may be due to the fact that for the Asian method, the forearm is rotated anti-clockwise (for a right handed archer), whereas in the Med. style there is no rotation. For me, this rotation leads to an opening and raising of the draw-hand shoulder. I am loathe to persist with his as I fear it may well lead to injury. I would welcome comments/advice.