Barbell Step-Up

Rest a barbell across your upper back the way you would for a squat, and stand facing a box or bench roughly knee-height or lower. Place one whole foot flat on the box, then drive through that heel to stand fully upright on top of it, letting your trailing leg follow up rather than pushing off the ground to help. Keep your torso upright through the drive instead of leaning forward to launch yourself up. Step back down under control with the same leg that led, resetting both feet on the floor before the next rep. The barbell load makes the working leg do essentially all of the work, since there's little room for your other leg to assist.