What is Correct Posture?

The following three pictures are expressing what is regarded as the correct posture, and all these pictures are based on a common view in which straightening up one's back along the vertical axis is justified. As far as I know, almost all of experts engaging in medical-and-fitness-related industry appear to have almost same opinions, and I have an opinion that goes against their widely accepted opinion.

While most experts think like above, they also know that the correct alignment of spine should have four arches as you can see below. Pointing up the importance of standing along vertical axis can hinder a person to make correct S-shape on his spine as he will try to straighten his back. Since thoracic spine draws an arch rising backward, trying to straighten one's back will quite go against favorable result; a person who is directed to straighten his back will rather try to stick out his chest, which will make a reverse curve of correct one on his back.    

Here are examples of correct postures that can be defined to be so from an athletic viewpoint. Their spines have well curved shape with thoracic spine making backward curve and lumbar spine making forward curve. (center is Cristiano Ronaldo)

So what is the key element to make this posture?  A certain muscle is the answer of it - a muscle called psoas major muscle. Psoas major muscles work to bend the hip joint. So if your psoas major muscles contract in the situation of your legs being fixed to the ground, those muscles will cause your pelvis to tilt forward. Black people have bigger psoas muscles than other races, especially compared to Mongoloid race; Mongoloid race particularly has small psoas muscles. That is the reason why black people have hip-up posture.

Comparison of Psoas Major Muscle: Left is Jamaican sprinter and right is Japanese sprinter.