MN/PE5 help?

Hi. I was wondering about the correct form in this exercise. The legs are supposed to be raised untill they're at an 90 degree angle, of course, but what about the lower back? I can't get my feet up that high without letting my lower back lose contact with the stall bars. Is this acceptable or do I have tight hamstrings?

For 1/2 HLL, you ideally want both.

 

Lift the legs to 90 with neutral back, then tense the abs to bring hips off the bars.

Your shoulders should be as relaxed into the hang as possible, and for bonus points, ensure your feet come as low under the stall bars as possible (locked legs are non-negotiable)-- the ideal is  pointed toes touch only the bottom of the bar you're hanging from. This indicates you are lifting from compression of the abs/hip flexors rather than curling around the movement by yanking down with the lats