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Use a box or aerobic step approximately 6" tall.  Begin facing the box with a slight bend in your knees and your weight on the balls of your feet.  Start with one foot on the box and the opposite foot on the ground.  Moving both feet at the same time, switch positions with your feet as quickly as possible while maintaining control.  Start the next repetition with the opposite foot on the box.  Coordinate your arms to move with your legs as if you were running.