Now that could be your problem! If you're sat on the back of the seat ( which I think you'd need to be for you're thighs not to touch the tank sides ) then you're probably having to push against the slope of the seat and therefore put tension into your arms. I personally couldn't imagine riding a bike from the back of the seat but often see scooter riders sat right back with straight arms and legs, seems totally unnatural to me.
At the weekend, I'm going to try lifting my pegs up to the top and see if that helps.that will lift my thighs into the tank, but make it a pain to get feet off the pegs and onto the floor in a hurry, so increases the risk of dropping the bike.
