"Posture" is a minefield.
We have so many definitions about what it is, what it means and how to make it better that it is rare that two people in discussion are actually talking about the same thing.
In "normal standing" with the feet flat on the ground roughly hip width apart, one easy way to think of perfect posture is the configuration of the body parts in such a way that it occupies it maximum possible vertical height.
From the side, this would generally be characterised by an imaginary plumb line passing through the centre of the ear hole, shoulder, hip and ankle.
A brilliant definition that refers to the functionality of this perfect static posture is from the late movement expert Moshe Feldenkrais. I'm paraphrasing when I say it is the position from which one can move in any direction with minimum effort and without some sort of preparatory movement.
By preparatory movement, think of being slouched in a soft comfy chair and having to rearrange your limbs before getting yourself up. Perfect seated posture would allow you to transition smoothly from sitting to standing. Any other posture requires a movement before your ultimate intended movement.
The sitting experience has brought us to an example of posture in a position that isn't normal standing.
Given that we're not standing all that often, fixating on perfect standing posture seems a waste of time and energy.
Especially in sports, we are constantly on the move.
Movements can be viewed as an infinite number of static postures. High quality movements are made up of infinite numbers of static positions.
While it is not practical to deduce the perfect bodily configuration with exact angles and ratios for muscular activity of various body parts for every position that the body can get into, from my coaching experience there is one key practical point worth knowing:
The closer somebody stands to perfect posture without any special attention, the more likely it is that they will be able to move with high movement quality.
Notice that I said "without any special attention". Perfect posture is governed by the reflexive systems. Intentionally trying to have better posture by standing tall, pulling your shoulders back, your belly in or whatever is a preparatory movement in itself. It is merely another compensation to deal with.
If you want better posture and better quality movement (a must for anybody looking to take their sports performance up a few notches), your energy will be better spent doing activities that block your reflexes from doing their things - in this case, keeping you upright with minimal energy expenditure.
Keep your eye out for the next piece of this article in which I'll go into some useful markers of high quality posture and movement and how you can free yourself up to have better posture and movement.
P.S. perfect posture doesn't exist but the notion of perfection is useful so it does.