The step-up is an exercise for the legs and buttocks. See how it's done.
Dr. Laskowski: A step-up is a simple body resistance exercise that works muscles in the legs and buttocks. A step-up targets the quadriceps, here, and hamstrings, here, as well as the gluteal muscles in the buttocks. This is a good general lower body conditioning exercise.
Nicole Krupa: To do a step-up, you'll need a small step stool. Or you can stand in front of the stairs. Pushing primarily through your lead foot, lift your body up onto the step. Then step backward to the starting position.
When you're doing step-ups, keep your back straight and your abdominal muscles nice and tight. Make sure your foot is planted entirely on the step. It's fine to start with a low step height, as well. For this exercise, your form is more important than the step height. When you step up, alternate your lead foot each time. Do as many repetitions as you can, depending on your fitness level, and stop when you're fatigued or your form begins to suffer.
Remember, for best results, keep your back straight and plant your foot entirely on the step. Keep your movements smooth and controlled.