I also use an office chair but it hugs my lower back and adjusts where and how I need it to. That said, I found that my stiffness came from two things: not moving enough AND hunching my shoulders as I sewed. I bought a Gypsy Sit-Upon (a round purple thing that looks like a flattened playground ball) and that has been immensely helpful. My lower back muscles were talking to me for two or three days, but that was because all the micromovements of my hip and entire back muscles were developing. It's important to move during the process of quilting - getting up to iron, cut etc. It eliminates your hips and lower back locking up because of relative immobility.