Our standing posture gives us many advantages in the animal world. Our ability to run is perhaps the most outstanding of these advantages. The extension of the human body against gravity in standing is very quickly adapted to movement. A slight release of the head’s weight in the intended direction to movement allows us to use our entire system of extensor musculature in a manner that leads to efficient movement.
Let’s analyze our verticality. As stated above, the weight of the head is poised atop the spine, prevented from falling forward by muscles at the rear of the neck. The pull of gravity on the toppling weight of the head stimulates a stretch response in neck and back muscles, giving them the tone they use to extend the spine against gravity, and to maintain the spine in conditions propitious for supporting the ribcage and the internal organs against the pull of gravity.
The head’s condition is constantly altered in response to sensory input. The inner ear canals send information to the brain about linear and angular acceleration. The brain takes these signals, along with input from the eyes about our relative position in the seen world, and input from sensors on the skin — cutaneous sensors, in the muscles and joints (muscle spindles and golgi receptors) — and uses them to make constant adjustments to muscle tone to balance us and to prepare conditions for possible movement. For any possible movement, we can first map a change in the poise of the head, which prepares the spinal musculature to support the movement of a limb to move a finger. This pathway of movement, through the hierarchy of postural reflexes beginning with how the head is poised atop the spine, recapitulates the path of fetal development of movement. There is an organizing of the whole system off of which specific muscular action is constructed. This is posture – the conditions of the whole being are used to prepare for actions in relation to the perceived world, and the whole being adapts in action to changes in the planned course of action based on constant input during action.