Standing

Standing, also referred to as orthostasis, is a human position in which the body is held in an upright ("orthostatic") position and supported only by the feet.

Although seemingly static, the body rocks slightly back and forth from the ankle in the sagittal plane. The sway of quiet standing is often likened to the motion of an inverted pendulum.

Standing at attention is a military standing posture, as is stand at ease, but these terms are also used in military-style organisations and in some professions which involve standing, such as modeling. At ease refers to the classic military position of standing with legs slightly apart, not in as formal or regimented a pose as standing at attention. In modeling, model at ease refers to the model standing with one leg straight, with the majority of the weight on it, and the other leg tucked over and slightly around.

Standing posture relies on dynamic rather than static balance. The human center of mass is in front of the ankle, and unlike in tetrapods, the base of support is narrow, consisting of only two feet. A static pose would cause humans to fall forward onto the face. In addition, there are constant external perturbations, such as breezes, and internal perturbations that come from respiration. Erect posture requires adjustment and correction. There are many mechanisms in the body that are suggested to control this, e.g. a spring action in muscles, higher control from the nervous system or core muscles. Humans begin to stand between 8 and 12 months of age.

Traditionally, such correction was explained by the spring action of the muscles, a local mechanism taking place without the intervention of the central nervous system (CNS). Recent studies, however, show that this spring action by itself is insufficient to prevent a forward fall. Also, human sway is too complicated to be adequately explained by spring action.

According to current theory, the nervous system continually and unconsciously monitors our direction and velocity. The vertical body axis alternates between tilting forward and backward. Before each tilt reaches the tipover point the nervous system counters with a signal to reverse direction. Sway also occurs in the hip and there is a slight winding and unwinding of the lower back.

...
Wikipedia