Vertical Leg Crunch.

Lie on your back, and extend your legs straight up in the air, feet crossed at the ankles, with a slight bend in the knees. Place your hands loosely behind your head. Contract your abdominals, and slowly raise your head, shoulders, and upper back about 30 degrees off the floor. Hold, then slowly lower. Keep your legs still. (This move can also be done with your lower legs resting on a chair or bed.)