Bored with usual crunch? This is another variety of crunch using cable machine where you can add resistance
a. Lie on the bench without arching your lower back.
b. Bend your knees to 90 degrees
c. By gripping the handles, set your arms to 90 degrees
a. Slowly contract your abdominals, bringing your shoulder blades off from the bench
b. Exhale as you come up and keep your neck straight, chin up and pull the cable
c. Hold at the top of the movement for few seconds, keep breathing
BTW, since it is quite difficult, I can only do this crunch for 12 reps each set. But the result is better than 50 sit ups.