it's a special way of running. You lay on your back and move your legs so that you make the same movements as when you run. It is usually practiced by people who do not want to admit that they are lazy.
Aubrey: Amy what are you doing?
Amy: Im doing horizontal running.
Aubrey: Just admit that you are lazy.
Amy: NOOOOOOOOOOOOOOOOOOOOOOOOOOO
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Some thing Fat Amy does in the first Pitch Perfect
āFat Amy what are you doing?ā -Abory
āIām horizontal running.ā -Fat Amy *laying on the ground and moving her legs in a bicycle motion*
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