Toe Taps are a fantastic ab exercise. Lie on your back, keeping your lower back on the floor. Start with your knees in tabletop position. Gently lower one leg to the floor, tap and return to tabletop. Repeat with the other leg. The straighter your leg when it taps the harder the exercise. If you feel your back coming up, don’t extend legs as far, and place your hands under your lower back.