This is the most popular exercise for the oblique muscles.
Lie down on the floor. Bend your legs at the knees, and place your feet firmly on the floor.
Place your arms behind the head or cross them on thr top of your chest.
As you inhale, arch your back stretching your abs muscles slightly.
Keep your neck stationary.
As you reach maximum contraction, while exhaling, lift up the upper body as if you are crunching inside trying to bring your shoulders closer to your waist.
Don't move your shoulders.
Reach maximum contraction of the abs, then, exhaling short breaths, bring your left elbow to your right leg, then right elbow to the left leg.
Slowly return to the starting position.
Don't bring your elbows forward and don't hold your arms behind your head. Thus, it will be easier to perform the exercise.
To keep your neck stationary, imagine that you are holding a small ball with your chin.
If the exercise is too easy, your can hold a barbell plate or a dumbbell over your head.
To get a better foothold you can place them against the wall or behind a machine.
If you have back pain, you should be careful doing this exercise.