Step ups and step downs are both amazing exercises for prehab and rehab purposes alike. But in order to truly MAXIMIZE YOUR GAINS in the gym or clinic, YOU NEED TO PERFORM THEM CORERCTLY!! Yes, stepping up and down off an extremely tall box looks impressive and all, but can you actually control it?
Unless your goal is to get some cardio, you need to really focus on FORM and CONTROL in the step up and down. Firstly, don't blaze through it; focus on an extremely SLOW and CONTROLLED descent (eccentric control). Your knee should never collapse and be medial to the ankle. You should feel the burn in your glutes and quads as you try and stabilize your pelvis during the descent. Developing eccentric control is crucial, especially for QUADRICEPS TENDINOPATHY (P)REHAB.
Secondly, only use a box height that you can control. You should be able to control the descent throughout the entire range of motion. Most of the time, you'll find that you can control the descent initially, but as the quad demand increases (as knee flexion angle increases), you'll lose the control of the descent. If you can't control the end range of the descent, you're using a box that is too high!
Finally, don't cheat and push off with the opposite leg! The leg ON THE BOX should be doing ALL THE WORK, both raising (concentric) and lowering (eccentric). Don't push off the ground with that back leg!