Step downs are a stable in my lower extremity rehab progressions and for good reason. You can isolate/bias one leg without any "cheating" using the other leg like on squats, deadlifts, or lunge variations. Furthermore, depending on which direction (forward, sideways, backwards) the step down is, you can preferentially bias either the glutes or the quads, or work around people's mobility constraints (ankle dorsiflexion). It all comes down to simple biomechanics and lever arms - if you know your biomechanics you know MOVEMENT. If you want to:
➡️Target the knee --> anterior step downs
➡️Target the hip --> lateral or posterior step downs
⬇️Posterior (posterior) Step Downs. With knee patients, I will typically start here as the demand is less on the knee and more on the hip. The trunk will flex forward naturally to keep your center of mass on the box, thus utilizing the glutes/hip more in addition to shortening the lever arm on the knee. For hip patients, this is a good starting place as well.
➡️Lateral (sideways) Step Downs. Due to the step down occurring more in the frontal plane than the sagittal plane, this exercise will place more demand on the hip ABductors. The big keys here are to ensure your hips stay level and do no drop. Imagine wearing a belt and keeping it straight the entire time. Furthermore, do not reach to the ground. Lower yourself. Love using this for both hip and knee patients.
Anterior (forward) Step Downs. Opposite of the posterior step down, the trunk has to stay more upright to keep the center of mass on the box. Furthermore, there will be more anterior knee translation as well as ankle dorsiflexion required with this variation. Thus, this is a more advanced progression for knee patients. And YES, I want the knee to translate forward past the toes here as we need to build tissue resilience and load tolerance. For someone with ankle mobility deficits, I often program in some sort of anterior reaches (like an anterior step down) to load the ankle into dorsiflexion after mobilizing it.