Perhaps the first real 'core exercise' you will ever do is the plank.
While it may look very simple and a matter of simply holding a position, the
actual form for the floor plank is far more complex. In order to perform the
plank properly you must isolate the transverse abdominis. In order to do so,
you must keep your hips from rotating to the anterior. You can do this by
squeezing (isometrically contracting) your glute muscles which will tilt
your pelvis to the posterior and allow you to strengthen your core
safely and effectively.
The floor plank can be considered a 'gateway exercise' as once you master
it there are very many progressions which increase the difficulty whether it
be extra resistance or stabilization. The next progression from the standard
floor plank would be to simply lift one of your feet from the ground while
you maintain the exact same position.
Start: Lie on a comfortable surface such as a yoga mat or the floor
facing down. Spread your body out as much as you can with both arms on the floor
under you. Put your weight on the balls of your feet.
Begin the motion: Lift your hips up from the floor. Once your body is
about parallel with the floor engage your core by drawing in your abdominal wall
at the belly button. Squeeze your glutes to make sure you do not anteriorly tilt
your pelvis and hold the position for as long as you can while maintaining form.
When you're finished, slowly lower yourself back to the ground.
Breathing: Unlike most dynamic resistance training exercises you should
not have any unusual breathing pattern when you do the plank. Even though you
have your core drawn in, you should strive to maintain your normal breathing
pattern as if you were sitting on the couch.