Flutter Kicks are the perfect leg/abs exercise. Performed slowly it is great as a warm-up exercise and performed flat-out it can push your legs and abs to a pretty critical fatigue state.
What's involved: Quads, front hip flexors, lower abs, upper abs, neck muscles.
Powered by: Front hip flexors, primarily, lower abs.
Tip: Raise your chin to your chest further engaging your upper abdominal muscle group and also bringing your neck muscles into play.