Healthy posture is dynamic, all muscles interrelated and constantly changing length, even at rest. The relationship between opposing muscles is kept lively by myotatic reflexes – a muscle is stretched by its partner and responds by shortening (stretch reflex), and the process goes on.
Of course, there are stress responses that disturb posture – protective grasping reflexes that create fixity in muscles and disrupt healthy posture. If these are often repeated, they can prevent the movement needed for healthy posture.
Constant muscle movement is essential for proprioception – the sense of where our parts are in relationship to each other. Nerves in muscle spindles and joints must be stimulated by movement in order for proprioception to function. We all know the feeling of having an arm fall asleep and not being able to tell where it is.
Nerve impulses from the motor cortex which create movement can travel between bodies. This is known as kinesthetic empathy. I’m sure you’ve experienced people whose touch was pleasant or unpleasant for you. This is because your body will imitate the muscular action of a body it comes in contact with. Kinesthetic empathy gives infants a sense of the movement patterns of caregivers who pick them up or carry them – it helps infants to learn to move and can also have a negative effect if the parent has bad posture. We imitate our parents through touch and vision.
Alexander obviously had very good use, having grown up on a farm and mostly kept out of school because of health problems. When he was teaching breathing and began to use his hands to guide his students and his words were ineffective, his use and hands must have been already remarkable to get the results he sought. I spent hundreds of hours in training to use my body and my hands before I could give a student good hands-on guidance. There is no way to teach what Alexander taught without very good hands-on work.