Procedure: Keep your arms straight and bend your knees, pulling the ball towards your chest. Then straighten your legs out without letting your back sag. If this is too difficult begin with “walk-outs,” where you simply walk your arms out until your feet are on the ball and walk back.
Where: You should feel this in your abs and thighs.
Make it harder: Bring the ball in with two legs and push out with one. Use one leg to bring the ball in and out.