Primary Muscle Group: Core Muscles
Muscle groups worked in this exercise: Rectus Abdominus, Internal and External Obliques.
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: This is done exactly as the Oblique Twist. The only difference (which works your abs even more) is that when you crunch and twist up, lift the opposite knee towards your opposite shoulder. Release the leg back to the floor at the end of the crunch.