Assuming you are speaking of the Plank, there is no motion. You stay in that position and hold it for as long as possible. It really is one awesome ab workout, it'll kill after a while.
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I prefer to adopt a press-up position for the plank with the arms far apart. Seems more intense that way. And if you really want to pop a blood vessel, nothing beats cable rope crunches.
LOL as a first post let me tell u something about my ab exercises....
2 sets of 50 crunches before each workout....(3-4 workouts a week for me im a beginning)
and me and my friend have about a 25 pound weighted bag. let me tell u something about this workout-TIRING. we stand maybe 7,8 feet apart and it really works the abs, biceps, triceps, a little bit of the shoulders its actually a very effective exercise.