If you look carefully at the anatomy of the human body, you will see that your abdominal muscles have nothing to do with your neck muscles. Your neck should not be in pain to strengthen your abdominal muscles. Simple as that.
Your head can weigh up to 17 pounds or more. You should not be straining your neck by holding up your head to gain abdominal strength by exercising. Many people either avoid abdominal work due to the fact that it hurts their neck to perform the exercises or they feel it is normal and safe to have neck pain during abdominal exercising.
Here are some tips.
1. Your body follows your eyeballs. So, if you are staring at the ceiling and bending at your waist, your neck is going to go towards the ceiling instead of flowing naturally in the bend with your body.
2. Drop a pencil on the floor. When you pick it up your head natural flows with the body as your head and body aim the same directing. Your eyes are looking at the pencil as your body bends you in the natural position to pick it up. Now think of bending your body to pick up the pencil as you try to keep your head and eyes facing the wall in front of you. It feels wrong and unnatural.
When you perform abdominal exercises with your head focused keeping the neck straight and facing the ceiling it is not a normal flowing movement and cause stress on the neck. No one needs extra stress on muscles, joints and bones.
The best way to exercise is in the natural flowing movement the body was made to move in. Mimicking everyday movement so we exercise in functional movements.
Now try doing a abdominal crunch with your head gently tucked towards your chest, lifting your shoulder slightly off the ground as you have your eyes focus towards your navel. Gently lift your head and shoulders in this curve, engage your abdominals and do slow crunches. Don't lower your head back down to the floor or straighten your neck out, until you have to rest after several crunches.
Unless you have an injured neck or limitations, getting in this proper form will keep neck pain away. If you continue to have neck pain always in abdominal work even when in a proper position, you may need to find standing or ball exercise to strengthen the abdominals and avoid floor abs. It is okay to have exercises that do not work for your body. We are all different, with different fitness, healthy and injury (or lack of) history of our own bodies we are working with.
Be good to you!!
Take care of your body and make it last!