The popular mythology around APT is that your hamstrings are stuck in a lengthened position.

The theory sounds plausible, but it only applies to a small subset of the population. For people who are stiff and inflexible, the high hamstrings are weak and the mid and lower hamstrings are actually short and weak.

If you do not restore length and strength to the entire hamstring muscle group, you will not be able to properly position your pelvis.

Ultimately you are trying to restore full function to as many muscles as possible. You should not try to train your body to LOCK into one set static position.