On your toes place your elbows on the ball in front of you. Your toe should be behind you and on a the floor so most of the pressure is on your forearms raise one foot off of the floor into the air. The key to this exercise is to try and hold these contractions as long as you can without rest. if you feel it in your lower back you are not squeezing your buttocks, sucking in your stomach, and or rounding your back enough...Try to mimic the picture as much as possible. Notice how the back is not arched. Try as best you can to rotate your hips forward and ROUND your back to compensate your back muscles from being overworked! Tips
Focus on the core. Squeeze your buttocks as tight as you can and REALLY suck in your stomach and tighten as much as possible.