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.
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 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.
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.
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. 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.
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 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. Many of them, however, are specific for a certain cause of orthostatic hypotension, e.g. maintaining a proper fluid intake in dehydration.
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.
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.
Standing per se does not pose any harm. In the long term, however, complications may arise.
Leaning is a variation of standing but with some deviation from the vertical plane by support from a vertical surface such as a wall.
There may be a time where the standee pivots about the point of standing and accelerates towards the ground; this is known as "falling". Ostensibly, falling either sideways or backwards is significantly more common than falling forwards, due to the front-facing nature of human feet. It has yet to be determined whether this is also true of vertebrates with different distal limb morphology.
- Gage WH, Winter DA, Frank JS, Adkin AL (April 2004). "Kinematic and kinetic validity of the inverted pendulum model in quiet standing". Gait Posture 19 (2): 124–32. doi:10.1016/S0966-6362(03)00037-7. PMID 15013500.
- Loram ID, Maganaris CN, Lakie M (May 2004). "Paradoxical muscle movement in human standing". J. Physiol. (Lond.) 556 (Pt 3): 683–9. doi:10.1113/jphysiol.2004.062398. PMC 1664994. PMID 15047776.
- Loram ID, Maganaris CN, Lakie M (April 2005). "Human postural sway results from frequent, ballistic bias impulses by soleus and gastrocnemius". J. Physiol. (Lond.) 564 (Pt 1): 295–311. doi:10.1113/jphysiol.2004.076307. PMC 1456055. PMID 15661824.
- Hodges PW, Gurfinkel VS, Brumagne S, Smith TC, Cordo PC (June 2002). "Coexistence of stability and mobility in postural control: evidence from postural compensation for respiration". Exp Brain Res 144 (3): 293–302. doi:10.1007/s00221-002-1040-x. PMID 12021811.
- Loram ID, Lakie M (December 2002). "Direct measurement of human ankle stiffness during quiet standing: the intrinsic mechanical stiffness is insufficient for stability". J. Physiol. (Lond.) 545 (Pt 3): 1041–53. doi:10.1113/jphysiol.2002.025049. PMC 2290720. PMID 12482906.
- Standing, Line of Gravity at Joints, Postural Sway and Correction of Perturbations
- Lee SW, Chan CK, Lam TS et al. (September 2006). "Relationship between low back pain and lumbar multifidus size at different postures". Spine 31 (19): 2258–62. doi:10.1097/01.brs.0000232807.76033.33. PMID 16946664.
- Hides JA, Stokes MJ, Saide M, Jull GA, Cooper DH (January 1994). "Evidence of lumbar multifidus muscle wasting ipsilateral to symptoms in patients with acute/subacute low back pain". Spine 19 (2): 165–72. doi:10.1097/00007632-199401001-00009. PMID 8153825.
- Sasagawa, S; Ushiyama, J; Kouzaki, M; Kanehisa, H (2009-01-23). "Effect of the hip motion on the body kinematics in the sagittal plane during human quiet standing". Neurosci Lett. 450 (1): 27–31. doi:10.1016/j.neulet.2008.11.027. PMID 19027828.
- Wu, Jianhua; McKay, Sandra; Angulo-Barroso, Rosa (May 30, 2009). "Center of mass control and multi-segment coordination in children during quiet stance". Experimental Brain Research (Berlin / Heidelberg: Springer) 196 (3): 329–339. doi:10.1007/s00221-009-1852-z.
- Collins JJ, De Luca CJ (August 1994). "Random walking during quiet standing". Phys. Rev. Lett. 73 (5): 764–7. Bibcode:1994PhRvL..73..764C. doi:10.1103/PhysRevLett.73.764. PMID 10057531.
- Doyle TL, Dugan EL, Humphries B, Newton RU (2004). "Discriminating between elderly and young using a fractal dimension analysis of centre of pressure". Int J Med Sci 1 (1): 11–20. PMC 1074506. PMID 15912186.
- http://arxiv.org/pdf/physics/0411138 Random Walk and Balancing
- Yamada, N (1995). "Chaotic swaying of the upright posture". Elsevier Science B.V. 14: 711. doi:10.1016/0167-9457(95)00032-1.
- http://utopia.duth.gr/~pmarhav/12th-ICMMB/PDF12thConference/12-Blaszczyk.pdf CLINICAL ASSESSMENT OF POSTURAL STABILITY
- Masoud M, Sarig G, Brenner B, Jacob G (June 2008). "Orthostatic hypercoagulability: a novel physiological mechanism to activate the coagulation system". Hypertension 51 (6): 1545–51. doi:10.1161/HYPERTENSIONAHA.108.112003. PMID 18413485.
- Maryland Spine Center - Rehabilitation for Low Back Pain