This exercise helps to tone and strengthen your lower back. It is important to work the back, the obliques, and the abs if you want to strengthen and tighten up the core.
Lie face-down over the balance ball, with your hips pressed against the ball and your abs lying on it. Lift your arms so that they are parallel with your body. While making a fist, bend your elbows 90 degrees and bring your elbows back so that they are touching the side of the ball (your fists should be pointing forward). Now, while extending your back (your chest should come completely off the ball), extend your arms forward. Hold for three seconds. Then return to the starting position. Do three sets of 12 - 15 reps.
To challenge yourself, extend your legs as you extend your arms!