If you want to build up a good mid section, with strong lower back muscles and abdominals that can take a punch or two, you're better off doing an exercise like the plank rather than crunches. Better trainers have been saying this for a few years already, and a study from Pennsylvania State University, which will soon be published in the Journal of Strength and Conditioning Research, now proves them right.
There are two disadvantages to old-fashioned sit-ups and crunches. In the long term they are likely to lead to back problems because of the pressure they create on the discs between the vertebrae, and, more to the point, they are not very effective. The muscles in your midsection area are not so much meant for moving your torso, but above all to keep your torso stable when your spine is subject to tension.
So the better exercises for your lower back and abdomen, as you can read in nearly every Men's Health magazine, are ones where the muscles in your mid-section have to work so that your vertebrae don't move. That's why the plank is a better exercise than sit-ups or crunches, and the side-plank is better than the oblique-crunch.
The better exercises don't require you to isolate muscle groups. They involve all muscle groups in your midsection [what magazines and trainers like to call your core], including muscles in your hips, upper back and shoulders. To use sports scientists' jargon, these are integration core exercises.
The researchers attached electrodes to the muscles of 20 test subjects, and then got them to carry out 16 core exercises. The electrodes enabled the researchers to measure how intensively the exercises activated the subjects' muscles. The more electrical activity in a muscle, the harder it's working.
The figure and photos below show that the core muscles have to work harder during a plank [below right; foreground in graph] than during a crunch [below left; background in the graph].
AD = anterior deltoid, RA = rectus abdominus, EO = external abdominal oblique, TE = thoracic erector spinae [the muscle that runs along your spine from your skull to your hip bone], LE = lumbar erector spinae [lower back muscle], GM = gluteus maximus.
The figure and photos below show a comparison between the superman [below left; background in graph] and the pointer [below right; foreground in graph].
The superman is an isolation exercise, as is the crunch. You lie on the ground and lift your legs and chest by contracting your gluts and lower back muscles. The pointer is more similar to the plank. You bring your body into a position in which your core muscles have to work hard to keep your vertebrae stable. The figure below shows which of the two is the better exercise.