SLDL's and good mornings are NOT lower back exercises.
At no point in my comment did I mention Stiff Legged DeadLift's, I mentioned Romanian DeadLifts. As I recently said in another thread, they are two similar yet subtly different exercises.
SLDL's keep the legs completely perpendicular to the floor, hence the name STIFF, with no movement of the hips to the rear and acting only as a pivot. Weight is shifted toward the balls of the feet.
RDL's, the hips are pushed as far back as possible without loss of balance. Weight is on your heels.
Good mornings
ARE a lower back exercise that isometrically contract the erector spinae muscle in order to maintain correct posture. No they are not producing the primary forces involded in moving the weight, the glutes and hamstrings are responsible there.
"Spine health isn't about making your back muscles stronger or more flexible", "It's about training them to maintain the strength they have over long periods of time." Dr Stuart McGill, Professor of Spinal Biomechanics and a leading reseacher in the field.
Just out of curiousity, how many of those low back machines exist and are available to the public in a commerical setting?