I tried this at home and had similar results. For the first week I thought, "hey, this is great!" Then one day I was particularly tired and could barely stay

I tried this at home and had similar results. For the first week I thought, "hey, this is great!" Then one day I was particularly tired and could barely stay on the thing. My upper back actually started spasming from the strain. I kept the ball but don't use it as a chair any more, though it's still fun to roll around on sometimes.