The Plank: your new favorite workout
Unlike a traditional sit-up, the plank works not only your external abdominal muscles, but also internal abdominal muscles - often referred to as your core stabilizers. Strong internal abdominal muscles assist in better posture, improved balance, a healthy back and, of course, a flatter stomach.
As with any exercises, it's important to begin with a strong understanding of the fundamentals. In this case, the fundamentals are the basic plank and the iso-plank, also known as the modified plank. To do a basic plank start on your hands and knees. Engage your abdominal and glute muscles. Align your wrists under your shoulders and your knees under your hips. One at a time, extend your legs so that your body weight is supported by your hands and feet. You are now in the plank position. Keep your abdominal and glute muscles tight. By keeping your abdominal muscles tight, you avoid movement of your hips towards the floor. Also, be sure that your spine stays straight all the way through your neck. A common tendency is to drop the head and hips to relax the abdominal muscles. This places stress on your back and diminishes the returns of your abdominal workout. Hold in the plank position for as long as you can maintain proper form. Rest by returning to your hands and knees. Repeat the exercise.
The iso-plank can be just as effective for your abdominal muscles, while placing less emphasis on upper body strength. To do an iso-plank lie on your stomach. Align your elbows under your shoulders. Engage your abdominal and glute muscles to pull your body off the ground so that you are resting on your elbows and toes. To avoid stress on your back keep your neck and hips in line with your spine. Hold the iso-plank for as long as you can maintain proper form. Repeat the exercise.
Movements can be added to the plank and iso-plank to make the positions more difficult, to challenge abdominal stabilizing muscles and to use a wider range of muscles. My personal favorite plank variation exercise combines the plank and the iso-plank to engage core stabilizers and increase upper body strength and agility. In this abdominal exercise you move from the plank position to the iso-plank position and back as quickly as possible. Always continue to maintain proper form in compound exercises.
Concentrate on keeping your abdominal and glute muscles engaged. As the name of this abdominal exercise suggests, your body should have a straight alignment. Don't let your hips drop (or rise) and keep your neck in-line with your spine. Good form is the key to a successful plank workout.
Resources:
Len Kravitz, Ph.D, Super Abs Resource Manual, c 1998,
http://www.unm.edu/~lkravitz/Article%20folder/abdominal.html 
