Crunches, How much motion is necessary to make them effective?

Your body will take it from where it wants. Weight train 2–3 times a week, and cardio 3–5 times a week. Alternate the days, and your body will shape up where it wants. When you do cardio it will take the fat from wherever, and you will begin to see toned muscle.
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Hey, if you’re feeling it in your stomach, it’s working against your muscles. You’ll notice eventually you’ll be able to move up more and more each time. Measure your work out based on the pain and how your abs look and feel. That’s what matters right?

Yup, Your doin fine already. The more you do it, the more farther you’ll go up, but it’s still working now :]

If you’re feeling it, it’s working. As you get stronger, you should be able to sit up further. Make sure you keep your back straight. Also, if you can handle it, try doing situps on decline bench while holding a weight to your chest. Once this becomes easy, you can increase the weight and you can add “turns” – where as you sit up you rotate your body in either direction. This will work your obliques. If you’re doing this 5 times a week with a 25+ lb weight on your chest you should have an 8 pack.

If you feel the resistance you are working the muscle. The better you get the further you can go. Don’t kill yourself, just keep challenging yourself a little more as you can handle it.