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Use a box or aerobic step approximately 6" tall.  Begin standing on the box with your feet together, a slight bend in your knees, and your weight on the balls of your feet.  Moving both feet at the same time, hop off the box so that both feet are on the floor.  You should be in a position where you are straddling the box.  As soon as your feet hit the floor hop back up onto the box.  Perform this pattern as quickly as possible for the designated time.  You should aim to get off and on the box at a rate of 2 per second.  That means if you have 8 seconds of work, you should hop off and back on the box at least 16 times.