exactly that. after 4 hrs of walking I put the gymnastic ball on the treadmill to sit. Getting used to it may take a while but I think its the optimal solution,

exactly that. after 4 hrs of walking I put the gymnastic ball on the treadmill to sit. Getting used to it may take a while but I think its the optimal solution, as you will never be in a very static position. Regarding limited space, maybe you can bring the ball somewhere else while walking, I mean you switch once a day.