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Legs on Stability Ball CrunchMain Muscle: Abdominals Mechanics Type: Isolation Lie on your back and place your knees on a stability ball. Knees should be bent at a 90-degree angle and feet should be 3-5 inches apart on the ball. Using the lower core, pull the stability ball slightly off the ground until your knees are just over your hips. This movement will engage the lower abs during the exercise. Lift your shoulders a couple inches off the ground as you crunch and keep the chin off the chest. Elbows should be wide. Push the lower back into the floor as you crunch and control any movement with the stability ball. Focus on proper breathing and do not hold your breath.