An abdominal crunch is the contraction of the
rectus abdominis muscle. The
rectus abdominis concentrically contracts to move the spine forward moving the
thorax toward the pelvis.
Basically the motion of moving your lower ribs of your rib cage towards the
front part of your hip bone in accordion-like fashion.
To properly perform a crunch you need to contract your abdominals by folding
your abdominal muscles (like an accordion) while you keep your stomach drawn
in. Yes, your abdominal muscles will be tense when you contract them.
You are not pushing your stomach out and most definitely are not pretend you
are going to the toilet.
That is called the Valsalva maneuver and it involves forcefully exhaling with
a closed glottis. The only acceptable time to perform a Valsalva maneuver is
when you are lifting extremely heavy weights.
Here is the Ultimate Abdominal Exercise Secret for Anyone Confused
Just kidding about the secret thing.
A simple kinesiological tip to get the best abdominal contraction which not
many people know about is to contract the glutes isometrically during all lying
supine (facing up) abdominal exercises.
If you squeeze your butt, you will inhibit the hip flexors which are the
muscles which raise your knee like marching as well as flex your trunk like full
Keeping your butt squeezed will allow your abdominals to fully contract
without any help from your hip flexors.