Primary Muscle Group: Core Muscles
Muscle groups worked in this exercise: Upper and Lower Abdominals
Preparation: Start by sitting on the ball with your knees bent and feet flat on the floor. Slowly walk your feet forward as you slide your torso down the ball. Your trunk should be parallel to the floor. Just let your hips clear the ball and let your lower back press into the ball; cross your arms over your chest.
Breathing: Exhale as you lift. Inhale as you relax back to starting point.
Execution: Crunch as you have for the previous exercises, only this time as you lift your torso, lift and extend one leg. Come back to neutral with both feet on the floor following each crunch. Don't forget to alternate your legs!
Comments: Since you are off balance in this stance, you are forced to use your core muscles even more to steady your body on the ball.