I had sandwiched a quilt on the floor and developed a baker's cyst on my left knee. I had an excerise ball that I rolled on during the day and always before bedtime. One evening I couldn't get all the kinks out so decided to lay on my back on the ball. Oh it felt so good until I thought of how I was going to get off with the bad knee. Oh well I'll just roll to the right and slide to the floor. Wrong! the ball decided to dump me and on my way down I heard some cracks. When I woke up, I tried moving some of my body parts and everything seemed fine. When I tried getting up the pain was excruciating. After several x-rays, I had 2 fractured ribs and the all the cartilage on my right side was torn. It took 2 months to heal. So if you use an excerise ball, be very careful!