This is an exercise that I love that is great for your core! If you do it properly you'll feel it in your lower abs, back, and under your arms. Start in a kneeling position with your arms on the ball. Slowly roll out until your body is straight from your shoulders down to your knees, keeping your hips down and in line. Hold in the extended position for a few seconds and then roll back in. Try to do 3 sets of 15-20.