Proof: plank is better for your core than crunches
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.