Neither model is accepted as more than an approximation. Analysis of postural sway shows much more variation than is seen in a physical pendulum or even a pair of coupled pendulums. Furthermore, quiet standing involves activity in all joints, not just the ankles or hips.In the past the variation was attributed to random effects. A more recent interpretation is that sway has a fractal structure. A fractal pattern consists of a motif repeated at varying levels of magnification. The levels are related by a ratio called the fractal dimension. It is believed that the fractal pattern offers a range of fine and gross control tuning. Fractal dimension is altered in some motor dysfunctions. In other words, the body cannot compensate well enough for imbalances. Pathology
Although standing per se is not dangerous, there are pathologies associated with it. One short term condition is orthostatic hypotension, and long term conditions are sore feet, stiff legs and low back pain. Orthostatic hypotension
Orthostatic hypotension is characterized by an unusually low blood pressure when the patient is standing up. It can cause dizziness, lightheadedness, headache, blurred or dimmed vision and fainting, because the brain does not get sufficient blood supply. This, in turn, is caused by gravity, pulling the blood into the lower part of the body. Normally, the body compensates, but in the presence of other factors, e.g. hypovolemia, diseases and medications, this response may not be sufficient. There are medications to treat hypotension. In addition, there are many lifestyle advices.
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.