Get in pushup position, then rest the tops of your feet on a Swiss ball . Keeping your legs as straight as possible, bend your hips and try to pull your feet toward your chest so that the ball rolls forward . Hold at the top for three to four seconds, then slowly roll back to the starting position. Continue rolling the ball backward (letting it move up your legs) until your body forms a straight line about 30 degrees to the floor . That's one rep. Perform two to three sets of 10 repetitions, resting 45-60 seconds between each set. Save this exercise for the end of your workout.
While the "pike" action at your hips works the abs through a wider range of motion than you're probably used to, extending your body at the end of the movement hits even more muscle. Your chest and shoulders play a big part, too, balancing your upper body as you safely stretch the limits of your ab development.