Answer:

It is usually recommended when doing the Abdominal Crunch to not anchor your feet under anything as it activates the hip flexors.

While this is very true, here is a variation where you DO need to anchor your feet under something.

• The best place to do this exercise is on a flat bench though any bar set a few feet off the ground will work, e.g. the Smith machine. The underside of a chair or table will work if you are exercising at home.
• Lie on your back with your head on the bench end that is away from the bar.
• Your legs should be bent like you are in a seated position (hips and knees both bent at 90 degrees) with your toes hooked underneath the bar that is resting on the bench press racks.

• You may need to adjust your position on the bench to achieve the proper position: knees and hips both bent 90 degrees.
• Hook your toes under the bar and try push the bar lightly up with your feet while crunching. You may want to place some weight on the bar to keep it from moving.

• While this action does is activate the hip flexors, it also activates the lower abs isometrically. When you crunch up, you activate the upper abs and the lower abs to some degree.
• Add that activation to what you are getting from the isometric contraction you have already put on the lower abs and you will get a greater total contraction.