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Begin facing a box 18" tall.  This is the maximum height you should use.  It is alright to use a shorter box but do not use a taller one.  Step with one leg onto the box and explode vertically using the leg you stepped up with.  Jump as high as possible off of this one leg by aggressively extending the hip, knee, and ankle while swinging the arms upward.  Land on the box using both legs to absorb the landing by bending at the knees and hips.  Step down from the box and repeat using the same leg until you have completed the designated number of repetitions.  Once you have completed all the reps on one leg, switch and perform the same number of reps using the opposite leg.