Lower back and sitting head to knee

[FlatBack]

When I am in the sitting head to knee pose with either both legs out or one leg, I have difficulty keeping a straight back because my lower back doesn't want to move. It seems that I haven't developed muscles enough to pull my lower stomach down and turn my pelvis forward. I think this is a common condition many people have. Are there any poses that I can practice to help this?

[Hubert]

Might be due to tight hamstrings and hips, not necessarily the lack of abdominal muscle strenght.

[InnerAthlete]

It is a misnomer that the spine is straight. The spine is neutral, yes. And so I'll assume the difficulty is in keeping the spine neutral. Forward bends, when the student is not able to tip the pelvis forward, can be bad news for the low back as it forces the sacrum to flow opposite the spine rather than with it. It is for this reason that height under the sitting bones, in the form of a foamy pad or folded blanket(s) is mandated.

It is thus far more critical for the student to maintain the length in the front (anterior) spine even when that means the student can not come down as far toward the extended leg(s).
It is the tension of the hamstrings which attach at the iscial tuberosities (sitting bones) that may inhibit the forward tilt of the spine in paschimotanasana and janu sirsasana, the poses I believe you are referencing.

So the "remedy" if we even choose to look at it that way, would be to continue working the opening of the hamstrings (which requires the student to find the full contraction of the quadriceps), take height under the sitting bones for forward and/or microbend the knees, and work the length of the front body as a primary while letting go of the Ego's desire to "fold all the way".

There are specific methods for working with props in order to find the length in the front body but they are best learned in person from a qualified teacher