The step-down is a variation of the step-up that focuses on control through the eccentric motion which is often absent in step-ups, and eliminates any assistance from the down leg in the concentric phase.
 
From a standing position on a box, lower yourself under control by bending one leg in a squatting motion until the free foot lightly touches the floor, then stand again.
 
Assistance from the arms by holding onto a rack or similar can be added to allow a longer range of motion before you have the strength to do it independently rather than limiting the range of motion.
 
Purpose
The step-down provides unilateral leg strength for better strength balance and to reduce weaknesses that limit bilateral leg strength, improves hip stability, and stresses knee extension in the most mechanically difficult range. Performing from the top down without support of the down leg at the bottom forces the working leg to be totally responsible for the entire range of motion, improving strength and stability development.
 
Variations
The step-down can be loaded in a number of ways: barbell on the back, barbell in the clean rack position, one or two dumbbells or kettlebells on the shoulder(s) or hanging in the arm(s), barbell or dumbbell(s) overhead, or sandbag on the shoulders, chest or Zercher position.
 
Programming
The step-down is most often used as more of a prehab or rehab exercise than a primary leg strength exercise because loading is relatively limited both by the lack of assistance from the other leg and the additional difficulty in balancing. Reps from 5-15 can be used depending on the goal, tempo and ability to add loading. Perform the exercise near the end of a workout after more technical and speed-oriented work.