After years of doing sit-ups every night before bed, I was pretty certain I could hold my own in any ab challenge. I was wrong. I made this discovery after a run-in with a large stability ball at my gym. I thought I might as well give it a try, and although I only managed a few measly crunches, I could hardly laugh the next day. I can honestly say that doing my crunches on the stability ball helped me discover muscles I never knew I had.

A stability ball adds something to your workout that regular crunches or sit-ups can't -- instability. You need to use all your core muscles to help balance on this non-flat surface, which is why it offers such a great workout. It can be used to tone your abs, thighs or arms, and it can also be used as a posture-perfecting chair for your home office. And it's not that expensive either, and trust me, it's a worthwhile investment.

Click here to see examples of great workouts you can do with a stability ball, and tell us in the comments how you get on the ball.