Raised leg circles is a fast, easy way to harden your abs, work your front hip flexors and strengthen your neck muscles. Best of all, you can do it any time, anywhere (well, anywhere you can lie on the floor without drawing undue attention, that is).
What's involved: In order of load, from highest to lowest, lower abs, lateral abs, front hip flexors, upper abs, quads.
Powered by: Lower abs and front hip flexors primarily.
Tip: Raise your chin to your chest further engaging the upper abdominal wall (for a complete abs exercise) and also strengthening your neck muscles (makes you harder to knock out in a fight).