Isolated lower abs exercise?

Leg lifts, you can do these on the floor or on a bench. What you do is you start out body flattened out and you lift your legs until your body makes a 90 degreee angle and make sure you keep your legs straight then proceed to return to starting position with controlled movement . When your on a bench your going to have to hold on to something for stability and if your on the floor you can simultaneously reach up and try to touch your toes while you are lifting your legs.