Definitions

postural position

Standing (position)

Standing is a human position in which the body is constantly in an orthostatic state.

Although quiet standing appears to be static, modern instrumentation shows it to be a process of rocking from the ankle in the sagittal plane. The sway of quiet standing is often likened to the motion of an inverted pendulum.

Control

Standing posture relies on dynamic rather than static balance. The human center of mass is in front of the ankle, and unlike 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.

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 (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.

Nervous system

According to current theory, however, 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

Lately, attention has focused on the role of the core muscles in maintaining stability. The core muscles are deep muscle layers that lay 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

Although the pendulum model is a good approximation, analysis of postural sway shows much more variation than is seen in a physical pendulum. 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 isn't 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 means 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 doesn't 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. Many of them, however, are specific for a certain cause of orthostatic hypotension, e.g. maintaining a proper fluid intake in dehydration.

Long-term complications

Standing per se doesn't pose any harm. In the long term, however, complications may arise.

Good posture

Good posture or "neutral spine" refers to the "three natural curves [that] are present in a healthy spine.

External links

References

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