Lay on the floor on your back. Place your hands on your abs. Squeeze your abs tight towards the floor. Bring your head and shoulders up off the floor. Tuck your knees above your hips and lift your feet off the floor. Reach a single leg out straight. Hold this "hollow" position for the desired amount of time. Make sure your lower back stays firmly connected with the floor. If it comes off, stop, reset, and see if you can get the correct posture back.

Core - Abdominals