The abdominal crunch is a classic core-strength exercise. See how it's done.
Dr. Laskowski: The abdominal crunch is one of the most common exercises used to train the abdominal muscles. The abdominal crunch targets the abdominal muscles, which are part of your core muscle group. The stronger your core muscles, the easier it is to do most physical activities and sports.
Nicole Krupa: There are many ways to do an abdominal crunch, but proper technique will give you the most benefit. To isolate your abdominal muscles, lie on your back with your knees slightly bent. Make sure your feet are planted firmly on the floor and about hip-width-distance apart. Keep your knees comfortably apart. Fold your arms on your chest and tighten your abdominal muscles. Raise your head and shoulders off of the floor. Hold for three deep breaths, then return to starting position. You'll feel tension in the muscles in your abdomen.
When you're doing abdominal crunches, don't clasp your hands behind your head. This puts your neck at risk of injury and prevents you from effectively isolating the abdominal muscles. Keep your movements smooth and controlled. Visualize your bellybutton moving up toward your pelvis. Do as many repetitions as you can with good form. Stop when your form begins to suffer.
Remember, for best results, fold your arms on your chest and let your abdominal muscles do the work.