I recently purchased a stability ball which is used to tone your body overall. I didn't think much of it when I bought it. I've been working out for almost a year now running and doing crunches with an 8-pound weight between my knees. The very first night I did 3 sets of 20 (a total of 60) crunches "on the ball". Supposedly this thing is supposed to extend your ab range of motion. The next day my abs were so sore it hurt to laugh. It must be working as I was never sore anymore using the weight. Today is day 4 and I'm still a little sore but not nearly as much. This thing is really working out great and I can't recommend one enough to anyone looking to strengthen and stretch--especially abs. Be sure to purchase the correct ball size for your height. These things go for about $30 a piece.