Peterson Step-Up

Stand on top of a step or box with one foot, letting your other leg hang free in front of you. Bend the standing knee to lower yourself down slowly until your free foot's heel just taps the floor, then drive back up to standing on the box through that same working leg. The floor-touching leg stays passive and light on contact; it's there to mark the bottom, not to help you back up. What makes this different from a regular step-up is the emphasis on the slow, controlled lowering rather than the drive upward, which is where most of the useful work on your quad and knee control actually happens. Keep your knee tracking in line with your toes rather than caving inward.