If you're looking for a new way to work the abs using an exercise ball, knee tucks are great for targeting balance, stability and, core strength. The arms and torso help stabilize your body as you crunch the knees in then, at the end of the movement, you get your abs involved by squeezing them as your knees come towards the chest. To get the most out of this move, try not to collapse the back as you roll the knees in.

Ball pikes are a more advanced version of the tucks and involve using the abs to pull your body into a pike position with the legs straight. This is a tough move, so you should feel very comfortable using an exercise ball before you try this version. A great way to incorporate both is to alternate a knee tuck with a pike for a total of 10-16 reps.

Do it right: Get into a pushup position with the ball under the shins/ankles (easier) or the tops of the feet (harder). Bend the knees and roll the ball in towards the chest, contracting the abs. For the harder version, keep the legs straight, contract the abs and pull the ball in a pike position until toes are on the ball.

For more, check out this Dynamic Abs Workout.