How to Ride A Horse Upside Down

What a beautiful sunny day!! Perfect for a ride...Mark came with us on his birthday bike and we had an hour or so out an about on the Moss behind the stables - Merlin and I did some trot and we were even brave enough to canter a little, though the bravery did not last too long...I've been having trouble with my saddle slipping and sure enough, when I was coming down a bit of an incline my saddle went sideways and I went with it - I was lucky that Mark managed to grab my arm before I ended right under Merlin - and Merlin, bless him, did not turn a hair despite me hanging off his neck going down a hill with his saddle round his belly - most horses would probably have run for the hills. I think its to do with my left side being so weak because of my RA, but I have a tendency to run the saddle to one side despite it being girthed properly - its definitely not Merlin's fault and I have to occasionally shift it over a touch to stop it turning turtle; it has been properly fitted and all, I think its just rider discrepancy!!