...Bend your knees in a plank! First we'll go over proper plank form again:

Essentially, you are holding yourself up with your toes and your elbows (or hands).  But it's really your core holding all of your weight tightly.  Your elbows  (or hands) are right underneath your shoulders (not in front or behind),  and your toes are hip width apart or slightly narrower.

When on elbows, keep your hands separated, and palms faced up - so that all tension or weight being held is in your abdominals and not being lost in the hands as they depend on one another. As you exhale, close your ribcage toward your hipbone, and press your bellybutton up to your spine.  Hold this as you breathe normally and sustain / deepen the contraction in the core.

Your hips will be in line with your shoulders and your toes, and your lower back will be neutral.  An arched lower back indicates that the abdominals are not contracting properly to suspend the spine straight.

Now, as you keep your hips in line, held by the core, slowly bend the knees.  Feel the lower abs grip that closure. Keeping the abs tight and the hips still, pull the legs straight again.  Alternate with 2 counts of each for a full 30-40 seconds.

You will feel a burn zipping up the lower rectus abdominis, the strip of muscle that runs from your ribcage to your pelvis. Internal and external obliques work as stabilizers.

When your nutrition is clean and your cardio is on point, this results in a very strong and very hot core!

*photo courtesy of: http://photos.demandstudios.com/195/206/fotolia_4232116_XS.jpg