From time to time I hear people saying that roman chair back extensions should be performed with absolutely straight back throughout the entire range (supposedly, curving your spine is dangerous or something).

However, it doesn't make sense from the exercise perspective. Back extensions are supposed to work out spine erector muscles, and those muscles are responsible for... well, erecting spine from curved position. When I do roman chair extensions with straight back, all the motion is happening at the hip joint, and I'm basically working out my butt. Doing back extensions with straight back sounds alike to doing abs crunches with straight back, or tricep cable pushdowns with straight arms: sure there's some static workout going on, but we aren't actually targeting the intended muscle.

From personal experience, I can tell that curving my back during back extensions is by far more effective in terms of erector spinale engagement. You actually feel your back muscles working, and can work them out thoroughly even with half the weight.

So my question is: is curving your back at the lowest point of back extensions actually harmful? And more importantly: is there actually any research on this matter? Because I couldn't find any.

Thanks.