The fascia and ligaments that support an upright posture function as the source of the action of posture. If the tissues that make up a particular fascial sheet or ligament are sufficiently stretched, the tissues will generate nerve impulses to contract appropriate muscles to relieve the stretch. Because the major postural fascia is paired on either side of the body, the contraction of muscles to relieve the fascial structure on one side of the body is the stretch of the fascial structure on the other side, and action initiated by fascial stretch can continue back and forth without any conscious involvement.
For example, if we tip forward, at some point the stretch of ligaments along the rear of the spine can trigger action in the extensor muscles (behind the spine) to pull the spine backward; if we tip backward, the stretch of ligaments along the front of the spine can trigger action in the psoas muscles (along the front of the spine) to pull the body forward. In this way, the action of the extensor and psoas muscles is reciprocal.
What we perceive as a steady upright posture actually consists of reciprocal innervation (nervous activation) of the various muscle pairs that hold us upright, in the three directions of movement permitted by the structure of the vertebrae of the spine.
The top of the spine has the maximum turn-left, turn-right motion; the mid-spine primarily permits a side-to-side motion; and the lower back mostly allows a forward-and-back motion. All three of these motions are carried by gravity down the spine and into the sacrum, and from the sacrum into the pelvis and legs, and all three of these motions will generate reciprocal innervation in a normal upright posture.
The body is already in a stretch at any given moment, even if the stretch is as subtle as the stretch of the fascia of the abdomen and lower back in the movement of breath. If we relax the activity of the stretch that is already in existence, the reciprocal innervation of paired muscle groups comes forward to inform the sense of location in consciousness, and the sense of location in consciousness leads the balance of the body in the stretch that is already in existence.
Anything that enters our consciousness can affect the sense of location in consciousness, and through the balance of the body anything that enters consciousness becomes part of the stretch that we experience.