What happens when you do crunches fast?

Answer:
No control and you can get an ab cramp. I've just experienced one, sucks. Especially when your holdinG anything in.

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Oh!!! my back hurts. I would start slow and build up.

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You don't use the muscles, you use the joints and tissues instead, you also increase your risk of injuring your back or pulling a muscle. If you want a more intense crunch, instead of having your feet resting on the floor at a 45 degree angle, rest them up on a couch while you lay on the floor so they are at a 90 degree angle. It takes a little extra effort.

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Not much. Only less of a muscle contraction. Slow down it works better.

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You run out of breath!

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