Good Morning

The good morning is a glute and hamstrings movement, not a lower back movement!

You are correct partially. Yes the Glutes and the Hamstrings are involved with a anterior rotation of the hips/pelvis however because the lever arm with the most weight applied is in the upper body (Hipe to bar on the shoulders) bearing the most load, we have to look at the load being directly placed on the erector spinae of the lower back. In this case the erector spinae is in a static contracted state or isometrically contracted, but the majority of the load is directly placed onto these muscles. The hammies and the glutes do stretch and contract so yes they are used, however the NSCA defines the glutes and the hammies to be more involved when using a SMITH Machine. With a Barbell because your hips are not moving backward as much because you have to compensate for the free load of the barbell the Hammies and glutes are less involved than with a Smith Machine version. the majority of the load is placed on the lower erector spinae when the legs are slightly bent (which is recommended) , However in the case of keeping the legs straight more activation of the hammies and glutes were used. So truly depending on your technique and which machine apparatus you are using, you can change the emphasis placed dramatically on the muscles.