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There are a several factors that improve when your posture gets better.

First of all you look better, when you have a straight spine your body is not deformed. The tension in the muscles and skin normalises, your skin looks more tight and toned.

Your facial expression normalises, we communicate with our faces it is mostly how other people understand our emotions.

If you have exces tension in the muscles in your face, when your face is in a neutral resting position is not going to relax fully. You are going to convey negative emotions at rest. Really you can read the pain on most people faces.

Your voice also changes, since your upper body is well aligned, you are going to speak louder at a normal pitch. So you are going to sound confident.

There is nothing worse than having to repeat yourself all the time because people around you don't understand you, do to your weak voice.

Also your body functions better internally, so you feel more at ease, it is easier to relax and pay attention to what matters most in the present.

Various aches and pains might go away, and if you are in a constant low amount of pain you are in a constant fight or flight. Your body instinctually triggers your survival instincts do to the chronic injury you are suffering from, makeing it hard to be at ease.

There might be some literal truth to the expression of being laid-back, there are some studies that have shown that striking various postures stimulate your body to produce various hormones. Like for example if you make yourself big, by spreading your legs and lifting your arms you are going to have more testosterone in your body prior to striking the pose.

So I would imagine that if you have bad posture you walk around hunched over with rounded shoulders constantly you are probably not going to feel so great about yourself.

Another thing that changes is the way you move. Well aligned joints move gracefully, I used to be a little of a cluts for most of my life. I used to drop things and knock things over often, all that has changed when my posture improved.

My movements have becomed more fluent and more precise. Even the way I walk changed.

Nothing exudes more confidence than someone that is walking around straight and proudly, you can feel that energy from miles.