I use step ups on boxes of various heights as my main quad exercise, and have never had any lower ( or upper ) back trouble. I perform the step ups by starting with both feet ON the box, lifting the non working leg up and back and then, keeping all the weight on the ball and heel of my foot ( you can make sure you are doing this by actually lifting up your toes ), squatting back down until the non working foot TOUCHES the floor,at which point I use the quad of the working leg to power back to the start position. It is important that the non working leg DOES NOT take any weight when it contacts the floor, it is used as a depth gauge and for balance only. In fact, I make a point of just tapping the floor with my big toe before starting the ascent.In reality, I suppose this movement could be called a “reverse box step down” or a “reverse box lunge” depending on the height of the box and the depth to which they are performed. My personal favourites are doing them on a low box and shooting the non working leg way back, while squatting down until my ass contacts the heel of my working leg.
I can see how regular step ups could be seen as harmful to the lower back, especially if the trainee lets the weight shift from the working leg to the non working leg. I started performing my version of the step up in the manner just described to avoid such a situation. BTW, I alternate between repping out one leg at a time and going one for one each leg, and continually swap between BB’s and DB’s, just to add further variety to the movement.Using DB’s gives a decent forearm workout at the same time, with the BB balance is more challenging and, if I feel extra keen, I can add chains to it. This movement has given me far more results in quad development than squats ever did.Hope this gives you something to think about.