It’s not often I see someone doing box step-ups in the gym, but I think they’re an excellent exercise in certain training situations. I’m finding them to be really be effective in my muscular endurance phases where I’m doing high reps of 50 reps per set or more,

1. The focus of strength training in the gym is not to work your cardio system but to inflict as much hurt and burn on your leg’s muscle fibers so they get stronger and bigger and can tolerate more lactic acid etc.  Since the goal in the endurance phase is to do high reps with light or moderate weight, I noticed that with a single leg exercise like box step ups I can work on one leg at a time and put more focus and intensity on it then when squatting or leg press. I also notice that my heart rate goes through the roof when doing high rep sets with dual leg exercises like squats. I felt like the limiting factor in doing more reps is not muscle failure but cardio exhaustion, which isn’t my goal when training my leg muscles. With box step-ups my heart rate still gets up plenty high, but it’s manageable at 85% MHR, and I can do many more reps and get more of a focused burn on my legs. I’m able to do it for a while to the point where the 30lbs dumbbells start getting hard to hold onto.
2. I can isolate one leg at a time and get it to really burn more then any other way.
3. I think the movement is much more sports specific training. I think that is important in the endurance phase and during the early and main season that any muscular endurance work tat is done
4. It feels much less stressful on my knees then lunges, lunges feel weird to do right now, also leg extensions are often times bad for you knees.

As much as I like this excellent single leg exercise, I find that I like using squats and dual leg multi-joint exercises for the hypertrophy and strength phases of my leg training, because it feels safer and more stable with both legs, and I can push more weight all together with both legs then with leg alone. I don’t think it would be smart or safe to do heavy weights with single leg exercises.