Sissel^TM SitFit-Ergo Sit^TM:

The air-filled Ergo Sit^TM cushion is an innovative way to improve posture and promote better balance. Sitting on it encourages upright posture, which means less stress on your spine. Strengthen back and abdominal muscles while working at your computer, or put the Ergo Sit^TM on the floor to exercise your legs and feet.
New and improved design with thigh-support. Use the round side forward for active sitting and the thigh-support forward for stable sitting.