This exercise is used to isolate the muscles of the lower back and can be useful if a lower back injury has occurred.
Begin by adjusting the seat height and the back pad height on the back extension machine. Next, pick a weight that you can handle comfortably. Now, sit on the machine with your upper back against the back pad. Some machines have a seat belt to help secure you into the machine; others have handles to hold on to. You should be able to perform this exercise with your arms crossed in front of your chest, if you need to hold onto something you are using too much weight. Begin the movement by pushing back against the back pad until your back is extended; every machine will have a different range of motion but just push back as far as you can. Next, return the weight to the starting position; keep the weight under control and do not allow the weight to slam back into the stack. Repeat this movement until you complete your set.
This movement is good if you have a lower back injury and need to strengthen the injured muscles. It is useful in a core workout, because the lower back is often neglected.