Original request from https://www.reddit.com/r/posturepals/comments/18hmlym/i_was_not_able_to_keep_my_arm_straight/
The green line is our reference. We measure position of parts of the body to this line / plane. It's always plumb (vertical) and always it's always set to the first blue and white marker at the front of your ankle.
Next one up is the front of your knee. In your case, too forward from reference, which means that your ankles and your knees are slightly released (not in full extension).
One up again is your pelvis. The front blue marker at waist height is your iliac (anterior superior iliac spine) - front of your pelvis. The green marker is your sacrum - back of your pelvis. You can see that the pelvis is rotated forward and down, as shown by the blue arrow. Look at your waist band on your trekkies. It should be water level.
Let's call the rotation of the pelvis forwards a "counter clockwise rotation" (as seen on this image). And also, let's call the pelvis the "lower torso". So now, we have a rotation anti clockwise in your pelvis. Let's move on.
The two top most blue markers are your bottom and top sternum. The yellow line is your sternum bone. Although the top of the sternum placed reasonably well on the reference, the bottom one is pushed too far forwards. You can also see the sternum bone is sloping backwards at the top. Now, the sternum bone is the front of your ribcage. And the back of your ribcage is where the straight bit of the red curve is - thats your spine. That means, your ribcage is also rotated, but this time, it's rather "clockwise". And again, we could call your ribcage the "upper torso". So now, we have a rotation clockwise of upper torso.
Well, and your middle torso, which consists of your abdomen cavity, internal organs and spine at the back is connected to the top and bottom torso via the spine. Which is highly flexible part of the body with many joints.
So, imagine the upper torso rotates one way (clockwise), and the bottom torso rotates the other way (anticlockwise). If you connect the two with a flexible spine, the spine shape will result in the arch as shown by the red curve.
The brown line is a "broom stick test". You place a straight edge (broomstick typically) as shown, in such a way it touches the bottom of your sacrum at one end and between your shoudlerblades at the other. What you measure is the distance in the apex of the lowerback arch, and also how close is the back of your head (where one would have a pony tail if long hair was worn) to the stick.
So I just described what we currently see. Your current state of affairs.
What we would want (for proper functioning of the system) is to have all the blue and white markers on the green line.
We would also want the red curve to completely flatten on the broomstick. And we would want the pony tail to go forwards from the broomstick. About as much as there is the space of the apex.
You would also want to flip the rotations in both, the ribcage and the pelvis.
Then on the front and back view, you can see you place your feet too wide at the front, and you habitually prefer one foot ahead of the other.
As I said in the previous comment, it's a bit of scoliosis associated with the shortening in your torso (the arch in your lowerback). Pelvis is laterally tilted (down on the right), left arm is retracted further back (although both arms are already too far backanyways ) hence the left shoulder higher. This is due to equilibrium, you are compensating the right bottom part of your body being shifted forwards with your left arm counterbalancing the mass backwards.
Everything I just described above is a result of how you move the different parts of your body in relation to each other. You can learn to move them differently, and therefore end up with a different shaped posture, that would promote correct functioning of the system.
Let me know if there are any questions.