Original post: https://www.reddit.com/r/Posture/comments/1go2it7/how_do_i_fix_my_posture_pls_help/
Hmm, I feel your frustration. Been there, done that.
Let me offer you some more insights from a person with a trained eye.
I have downloaded your images, compared them and imposed few lines on them. Hopefully, it will help you to understand better what is going on. Check the visuals here: https://goodposture.studio/assessment/many_arm_2503
For starters, there are few facts we can state.
1/ Camera is in the exact same position between images (probably on a tripod). That's a very good start
2/ You have moved your stance between images 1 and 2. I'm assuming, you would have gone to and from the camera between the snapshots. Either way, between images 1 and 2, your feet are not placed on the exact same spot (on 2nd image, you land forward to image 1). That makes things a little harder to see as we have to compensate for this shift.
3/ Green reference line is set as vertical as possible (using the architrave of the door and the drawer cupboard behind you). So the green line is stationer in relation to the room, not your body.
4/ It's a shame we don't see your feet. If we had your feet, we cold place the green line according to your stance and that would make things easier to see. Next time you make effort taking postural images, make sure to include feet - it's important.
Righto, so now, since we established the references, we can start actually measuring your posture (or the changes between 1 and 2). To do that, we use some bony references and in your case, we can also use your clothing, as it's fitting and wouldn't have changed between images.
Consider image 1:
Yellow line - that's your sternum. I have matched it's top and bottom. Notice the bottom ending pretty much at the top of the bra strap, just under your breast.
Blue line - (we call it a "belt" line) and essentially, it's a line between the top of your sacrum at the back and the anterior superior iliac spine at the front. For our purposes, we actually just use the stretchy belt, specifically the bottom seem of it. Notice the slope of it - as you correctly pointed out it indicates your APT.
Red curve - I just traced part of your spine on the image 1. Notice, that to reference it, I have matched the bottom of this curve to the end of the blue line.
Now consider the changes on image 2:
The yellow line, the blue line and the red curve have NOT been changed in any way from image 1, except they have been transposed forward (to accommodate for the change of your position in space we talked about earlier).
What do we see?
Blue line hasn't changed at all. So that means, the APT on image 2 is there exactly the same way as on image 1.
Yellow line on image 2 has been matched to the top of the sternum, and as you can see at the bottom - the lower sternum has now travelled forwards about an inch. Which corresponds to your observation of the rib flare - that's spot on.
There also has been a change at the red curve. The bottom reference is the same on both images, but on image 2, you can see that the top part of your spine has travelled about an inch forward - which of course corresponds to the travel of the bottom sternum forward.
To conclude: between being "relaxed - 1st" and being in "good posture - 2nd", there has been no change in the APT, but you did increase the rotation of your ribcage (which is something you don't want to be doing).
Turns out, we know why that is, and we also know how to change it, but that would be for a much longer explanation.