I have a naturally hollow lower back. When I was a child, learning to ride in Germany, and I heard, "Sit up!," I would just hollow it more. And I see the same problem in lots of the equitation riders I teach: Trying to sit straight, they hollow their backs and "perch," with their buttocks pushed out behind them. When I ride now, I think about stretching up rather than sitting up. And I think about tucking my tailbone under me as I'm stretching, so I don't hollow my back and push my derriere out.
Sitting straight is not to be confused with stiffness, though-I don't stiffen my back even when I'm using my lower back a lot in conjunction with my stomach muscles. There's still an elasticity; I think of allowing my back to swing with the movement of the horse's back.
This article originally appeared in the October, 1999 issue of Practical Horseman magazine.