Why Do I Feel Tight When I Stand In A Correct Posture?

I've answered a question on the Quora platform, where people ask the general public about anything and everything, but I keep my eye out for questions on posture and ergonomics. This is the question I responded to: 

We need to question if there's such a thing as a “straight correct posture", as a misunderstanding about this could well be the cause of the tightness you're experiencing.

The overwhelming view in the public consciousness is that posture is a shape, one that you “hold", but it isn't. Posture is a balancing act, which means it's a movement. The skeleton is an extremely unstable structure, the benefit of which is fantastic mobility. The greater the stability, the less the mobility. The more you try to stabilize yourself through intentionally holding a "correct alignment" the more immobile you become, and the more tense you become.

Posture isn't a position or an alignment, it's a behaviour, which is why I much prefer the word poise, it better encompasses that.

Not only do you not need to hold a correct posture, you don't need to fight gravity in an attempt to gain good posture. As counter-intuitive as it may seem, gravity is the only reason it's possible to have good posture, and is aiding it, not opposing it. We use the earth's surface like a fish uses water, it supports us. Without going fully into Einstein's General Theory of Relativity, which supersedes Newtons laws on gravity, scientific consensus is that gravity isn't a force that pulls you down, but an effect caused by massive objects curving space time. The by-product of that is that the massive object e.g. Earth, pushes back up under your feet. You're not being pulled down, you're being pushed up, and you "surf" that upthrust by allowing yourself to subtly move through your innate balance mechanisms so you can keep re-finding the support it provides. By holding a fixed posture you interfere with that mechanism. This highlights that posture exercises are mostly unhelpful as it's about coordination, not strength. By strengthening your coordination the postural muscles will attain the functional strength they need, but strengthening muscles without consideration of coordination, your innate balancing skills,  won't guarantee an improvement of your coordination, and may encourage rigidity.

Standing on Earth is indistinguishable as an experience to being in rocket in interstellar space accelerating at 9.8ms². Why that number? Because that's 1G, Earth's Gravity, which is measured in acceleration.

You no more need to "do" good posture than a fish needs to do buoyancy, it's our naturally evolved state to take advantage of our environment.

Unfortunately a lifetime of habits, sedentary lifestyles and reacting to a "modern" environment none of us were evolved for, has left many without the coordination to stand on planet earth freely and without effort. Undoing those habits, and doing less, not more, is the key to refinding your poise.

The Alexander Technique is one of the best ways to retrain your coordination to improve your posture/poise.