On RP, i tend to stick my butt out/arch my back. He's the first horse I can remember ever doing this on and I"m curious about what it is about his way of moving or riding that makes me want to do this. As a result of this, i often find that I am sitting forward more than I should be.
With MM, sitting up and back and on my pockets is no problem. But with her I can get into a bit of a chair seat if i'm not careful (so almost the opposite of RP). I also try to ride her with 'legs off' which means my leg looks looser.
On both I tend to have an unruly right elbow, so that's just purely on me
