The trick is to tell her not to move. You are standing up and you pick her up and lower her on your cock. At this point, some girls want to work it, and that's what kills your back. they start bucking against your cock and try to lower themselves up and down on you. This is the worse thing for your back. I always tell them not to move, and instead I lift their butt and up and down on my cock. Its like doing curls, and after a while, they will get in a rhythm with you. that way, you control the movements, and give your biceps the work out and not your lower back. Give it a try lah. I recommend SYTs around the 45kg weight range. Once you go over 60kg in weight, cannot already. Unless u are a body builder. LOL.