Humans begin to stand between 8 and 12 months of age. Spring action
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 (C.N.S.). 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. Nervous system
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.An analogy would be a ball that volleys back and forth between two players without touching the ground. The muscle exertion required to maintain an aligned standing posture is crucial but minimal. Electromyography has detected slight activity in the muscles of the calves, hips and lower back. Core muscles
The core muscles play a role in maintaining stability. The core muscles are deep muscle layers that lie close to the spine and provide structural support. The transverse abdominals wrap around the spine and function as a compression corset. The multifidi are intersegmental muscles. Dysfunction in the core muscles has been implicated in back pain. Expansion of pendulum model
Some investigators have replaced the ankle inverted pendulum analogy with a model of double linked pendulums involving both hip and ankle sway.
Many of them, however, are specific for a certain cause of orthostatic hypotension, e.g. maintaining a proper fluid intake in dehydration. Orthostatic hypercoagulability
Prolonged still standing significantly activates the coagulation cascade, called orthostatic hypercoagulability. Overall, it causes an increase in transcapillary hydrostatic pressure. As a result, approximately 12% of blood plasma volume crosses into the extravascular compartment. This plasma shift causes an increase in the concentration of coagulation factors and other proteins of coagulation, in turn causing hypercoagulability. Orthostatic tremor
Characterized by fast (12–18 Hz) rhythmic muscle contractions that occur in the legs and trunk immediately after standing. No other clinical signs or symptoms are present and the shaking ceases when the patient sits or is lifted off the ground. The high frequency of the tremor often makes the tremor look like rippling of leg muscles while standing. Long-term complications
Standing per se does not pose any harm. In the long term, however, complications may arise. See also
At attention
Agonoclita, a former Christian group that said prayers standing
Human position
Prostration
Standing desk
Bowing (social)

References
External links

Dizziness-and-balance.com – Description
apta.org – The Secret of Good Posture