Here is a lovely example of how to do a low ab curl. Les Whitley does the movement with good control and he rolls all the way up to his shoulder blades. He also shows a slightly different variation where the legs are lowered towards the floor between low ab curls. The low back should remain relaxed in the floor if this variation is chosen.
I think that the key to knowing if you are doing this exercise well lies in the smoothness and ability to be able to slow it down. If the hips jerk off the floor or the exercise can only be done at a very rapid tempo, this means that the low abs are not engaging properly and we are doing more harm than good. Also we should be breathing out as we lift and not going turkey red or living in danger of letting one rip.
If it is hard to do this exercise, it is a sign that all is not well in our guts. And if this is the case, then doing this exercise may well cause extreme back ache! So if we can only jerk and judder our way through, then stop and leave the gym. Our back really isn’t safe. The linked blog may apparently be only for women, but scarring from hernia operations or tears can very much affect men – and cause abdominal weakness.