Want to seriously challenge your core? Try this move from Alison Sweeney, host of The Biggest Loser. Adding this type of jumping jack to the standard plank engages your abs even more.
Here's how to do it: Start in the "up" part of a push-up position, with your shoulders over your wrists and feet together, and your body in a straight line from head to toe. Jump your legs out wide and then back together. Repeat for 1 minute, eventually building up to 5 minutes.
Try this move: Plank Jacks