The crunch is a basic abdominal exercise that builds strength in the abs. It is a good starting point for more advances exercises. The crunch exercise will train you to engage your abdominal muscles. A proper crunch should use your ab muscles, not momentum, so it should be done smoothly and in control. Start with 10 reps and work up to three sets of 15.
Here's how :
Lie face up on your back with your knees bent . Place your fingertips behind your ears so you do not pull on your neck.
Squeeze your ab muscles in by drawing your belly button to your spine.
Exhale and slowly crunch your shoulders up off the floor.
Hold for 2 seconds. Inhale and lower yourself slowly back to the mat.
Start dong this three days a week, then you can work up to five days because the abdominal muscles recover quickly.
Have at least one day of rest between ab workouts for the first month to allow for proper recovery.