sorry i've been to long in replying.
snapper has a video somewhere of her doing wide stance squats (also VERY wide stance squats) well below parallel. if they aren't quite ATG they are close. hope she doesn't mind my saying that just hitting depth on her powerlifting squats (rather than going well below parallel) was something that she had to work on. back before the days of her squat suit...
i am starting to see that the hip flexors are involved for recruitment rather than stretch. i've been contemplating this while doing my (stick assisted) natural GHR. working hard on keeping the tension / activation through the full ROM (glute pump ftw!!) and recruitment does (to my body at least) kind of feel like a stretch (e.g., on the way back up).
found this a while back and simply couldn't believe my eyes:
his feet are together so hit butt has to sit back behind his midfoot. his shins must remain upright because of the wall. he can't lean forwards very much because of the wall.
and yet he doesn't fall backwards on his ass.
how is that possible?
(of course his bottom position is a little soft - but the exercise isn't supposed to be weighted. his shoulders also slump forwards a little which helps shift his gravity forwards. he is also a dude so his gravity is a bit higher - more in the shoulders than the hips). but still...
he is 'hanging on' with his hip flexors, isn't he?
it does make sense to me that the hip flexors are particularly involved in an upright torso position. squats are a bit precarious for me (because my center of balance is a bit back behind my midfoot). seems to me that i feel it in my hip flexors when i'm doing my squats 'more right' (in the sense of sitting into a tight bottom position with good lumbar arch and upright torso). i feel it in my hip flexors less when my bottom position is softer (lumbar arch slacks a little and / or i lean forwards a little to try and deload my pathetic little legs).
i noticed that the kettlebell squat dude (in one of the youtube demo vids i posted earlier) said some stuff about 'stretching out the hip flexors'. so... seems that it is a popular misconception that the hip flexors are stretched in squats.
thanks very much for pointing out that that isn't (can't anatomically be) the case.