"fit enough" is a pretty subjective term. A "bad back" also covers a multitude of conditions, from a bit of a twinge to spinal surgery, rods, etc. I can imagine it is entirely possible to find that your back or bottom (either from an injury or poor posture) is uncomfortable or sore after sitting on a park bench for four hours, but that would not stop you operating an exit door or impinge your daily activities in any significant way.