This is a classic abdominal exercise that for some reason seems to have fallen out of the mainstream in favor or exercises such as the standard crunch. As is often the case, what "most people" are doing is not necessarily the best course of action, as an exercise like Air Bicycles is much more effective than a standard crunch or leg raise type of movement.

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Air Bicycles

The idea behind this exercise is to try to touch the opposite knee to the opposite elbow on every repetition. Success Tip: Do not pull on your head! You are supposed to be exercising your muscles, not straining them unnecessarily. 

Your upper back should be off of the floor during the entire exercise, and your legs should also never rest on the floor. By keeping your upper back and legs elevated during the entire movement, you will get amazing abdominal and core muscle stimulation. At the same time, you will gain  "dynamic strength" by moving many different muscles while others remain in a static contraction throughout the movement. 

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