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Posturography

What is posturography?
How dos it work?
How to choose a posturographic device?

Browse this reference page to find the answers to these and other frequently asked questions regarding posturography.

Posturography

Highlights

It can be static or dynamic
It measures the postural control system performance

Clinical Standards

Formulated in 2013 by the ISPGR

Instrument characteristics

They are defined as:

  • accuracy =
    trueness + precision
  • resolution

Not all posturographic instruments are created equal!

What is posturography?

Posturography in general is a non-invasive specialized clinical assessment technique used to quantify how well a person is able to control posture and balance. As eloquently stated by Visser et al. [1] "the term posturography refers - literally - to the description of posture, which we interpret as the rather static relative position of different body parts with respect to each other. Most frequently, however, posturography techniques are used to investigate the active and passive regulation of balance under a variety of conditions."

The control of posture and balance, i.e. postural control, necessitates a complex interaction of sensory, motor and central nervous system processes. The sensory aspect involves vestibular, visual, proprioceptive and somatosensory inputs. The motor aspects include not only the principal skeletal muscles but to a lesser extent almost every muscle in the body. The nervous system aspects include not only the vestibular nuceli but the spine, cerebellum, and practically all the central and peripheral nervous system. Essential elements of most posturography techniques include the ability to actively manipulate posture or balance, and evaluate the subject's response to such interventions [1].

Posturography is used not only in pathological (abnormal) conditions (particularly in the diagnosis of balance disorders and in physical therapy and postural re-education); it is also used in normal healthy conditions such as in physical education and sports training.

Although semantically the term posturography refers to any Test of Balance, even observation based tests, it is most commonly used to refer to the investigation of postural control performed using devices capable of providing objective quantifiable measurements.

[1] Visser JE, Carpenter MG, van der Kooij H, Bloem BR. "The clinical utility of posturography." Clin Neurophysiol. 2008 Nov;119(11):2424-36.

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Types of posturography

Depending on the conditions in which the subject's balance is tested, it is common practice to classify posturography techniques in one of two major categories [2]:

If the posturographic technique involves automated measurement and analysis using a computer, the adjective Computerized is sometime used. This results in the definition of the terms Computerized Static Posturography and Computerized Dynamic Posturography sometimes used in the literature.

Incidentally, unlike what some of our competitors wrongly claim, according to the definition the testing on a foam cushion as is done with the CAPS® system is Computerized Dynamic Posturography (CDP). Furthermore, on systems like the CAPS® it is possible to "emulate" the tilting platform approach used in other systems by using, instead of a foam cushion, a wobble board to perturb the subject's balance, especially one with adjustable "wobble", such as those with an inflatable bottom.
However, the foam provides a better perturbation than a tilting/wobbling platform. As stated by Girardi et al. [5] and later by Amin et al. [6] "The BalanceTrak 500 [Vestibular Technologies' previous version of the CAPS®] has several advantages over its predecessors. First, the foam used for testing is a medium that more accurately simulates conditions that may be encountered in daily life. It simulates thick, plush carpeting; rough, uneven terrain (encountered on hiking trails and in the rough on golf courses); and even certain types of heavily padded shoes. Second, unlike a platform, which tilts only forwards and backwards, the foam also assesses a patient's ability to maintain balance in the lateral plane. The ability to assess an individual's response to lateral perturbations is especially important because many falls occur laterally."

Some systems allow clinicians to "force" an external perturbation to the balance in a predictable or unpredictable way. For instance the moving platform models from NeuroCom® (not their fixed force platform ones) allow the platform to translate or tilt forward/back in a controlled (and predictable) way to perform several tests. This seems at first a great idea, capable of providing useful information. However, there are some issues with using predictable perturbations in general, and the moving/tilting platform in particular, that have been pointed out by several investigators (see [1] for an overview):

[2] Furman JMR, Baloh RW, Barin K, Hain TC, Herdman S, Horst RK, et al. "Assessment: posturography. Report of the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology." Neurology 1993 Jun;43(6):1261-4.
[3] Kuo AD, Speers RA, Peterka RJ, Horak FB. "Effect of altered sensory conditions on multivariate descriptors of human postural sway." Exp Brain Res 1998 Sep;122(2):185-95.
[4] Peterka RJ. "Sensorimotor integration in human postural control." J Neurophysiol 2002 Sep;88(3):1097-118.
[5] Girardi M, Konrad HR, Amin M, Hughes LF. "Predicting Fall Risk in an Elderly Population: Computer Dynamic Posturography Versus Electronystagmography Test Results." Laryngoscope. 2001
Sep;111(9):1528-32. [6] Amin M, Girardi M, Konrad HR, Hughes L."A Comparison of Electronystagmography Results with Posturography Findings from the BalanceTrak 500." Otol Neurotol. 2002 Jul;23(4):488-93.
[7] Keshner EA, Allum JH, Pfaltz CR. "Postural coactivation and adaptation in the sway stabilizing responses of normals and patients with bilateral vestibular deficit." Exp Brain Res 1987;69(1):77-92.
[8] Hansen PD, Woollacott MH, Debu B. "Postural responses to changing task conditions." Exp Brain Res 1988;73(3):627-36.
[9] Bloem BR, van Vugt JP, Beckley DJ, Remler MP, Roos RA. "Habituation of lower leg stretch responses in Parkinson's disease." Electroencephalogr Clin Neurophysiol 1998 Feb;109(1):73-7.
[10] Bernstein J, Burkard R. "Test Order Effects of Computerized Dynamic Posturography and Calorics", Am J Audiol; 2009 Jun; 18(1):34-44.
[11] Wrisley DM, Stephens MJ, Mosley S, Wojnowski A, Duffy J, Burkard R. "Learning effects of repetitive administrations of the sensory organization test in healthy young adults." Arch Phys Med Rehabil. 2007 Aug;88(8):1049-54.
[12] Evans MK, Krebs DE. Posturography does not test vestibulospinal function.Otolaryngol Head Neck Surg 1999;120(2):164-73. Feb.

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How does it work?

The role of the postural control system is to control the movement of the Center of Mass (CoM) of the body and to minimize its undesired motions. In other words, its role is to maintain the control of the Central Nervous System (CNS) over the CoM. The movement of the CoM is what is called sway, and posturography works by measuring sway.

It is important to clarify that sway is not the movement of body parts, but the movement of the CoM. A person can appear to be moving a lot, but the location of the CoM can be practically immobile. An example of this situation is a person walking on a narrow support like a balance beam or a tight rope: the body, especially the arms, move a lot, but they do so to minimize the motion of the CoM and maintain it over the very narrow support area . The opposite can also be true: a person might appear perfectly still, but in fact is unable to minimize the movements of the CoM caused by the internal movements of blood, other tissues and structures generated by the heart contractions, respiration, digestion and other physiological processes or caused by external perturbations. In this case the sway is large, although the external apparent movements of the body are small or even imperceptible. An example is a person with akinesia or bradykinesia.
The most important consequence is that it is inherently impossible to measure the sway of a person by relying only on sensors placed on a specific location of the body because in this way only the motion of that specific location can be quantified and not the movement of the overall body CoM. Similarly, it is not possible to quantify sway by measuring the tilt of the support surface as some claim, because one can tilt the support without moving the CoM (think of a person standing on a skateboard). It is also makes inherently difficult if not impossible to evaluate postural control by observing a person.

Unfortunately accurately measuring or even estimating the position of the CoM and the sway is practically impossible: it requires measurement of not only the external motion of the skeletal segments of the body (e.g. arms, legs, trunk, head) but also the movement of the soft tissues such as muscles and blood. However, it is possible to quite accurately measure the position and movements of the Center of Pressure (CoP), which is the point of application on the body of the Ground Reaction Force. The CoP and its motions are related to the position of the vertical projection of the CoM, and in fact in static conditions they are the same point. In biomechanical terms, the CoP displacement is the controlling variable of balance, reflecting the net neuromuscular responses generated by the CNS to maintain control over the CoM, the controlled variable [13]. For this reason, posturography approximates true sway, i.e. the movements of the CoM, with the movements of the CoP.

During a computerized posturographic test, as the subject makes small movements the sensitive detectors in the force platform underneath the subject detect them and transmit this time-varying information in real time to a computer. Essential elements of most posturography techniques include the ability to actively manipulate posture or balance, and evaluate the subject's response to such interventions [1]. Due to the complex interactions among the sensory, motor, and central processes involved in posture and balance, testing in different combinations of visual and support surface stimuli are necessary to differentiate among the many defects and impairments which may affect a subject's postural control system. Thus, posturography test protocols quantify the ability of a subject to maintain balance in static and perturbed conditions, usually coupled with the ability to test the subject either with or without visual references (eyes open or closed) or with an environment that gives conflicting visual information. By performing a combination of static and dynamic posturography in different sensory conditions it is possible to quantify how much a subject's postural control system relies on visual, proprioceptive and peripheral vestibular information to maintain balance.

[13] Winter DA, Patla AE, Ishac M, Gage WH. Motor mechanisms of balance during quiet standing. J Electromyogr Kinesiol 2003 Feb;13(1):49-56.

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Clinical Standards

The movement of the CoP, that we shall from now on conventionally call sway as it is commonly done, can be extremely small, sometimes less than 2 mm [14], and a change in the amount of sway of less than 20% can be significant [15]. This means that a change of less than 0.4 mm can be significant. Furthermore, the sway is an amplitude of a movement or a displacement, hence it requires two extreme points to be determined. This means that each CoP point should be determined with an error of less than 0.2 mm. Therefore, it is important that the instruments used have accuracy, precision and resolution sufficient to detect such small changes.

In 2013, the International Standardization Committee for Clinical Stabilometry of the International Society for Posture and Gait Research (ISPGR) recommended for the CoP measurements an accuracy of 0.1 mm and a precision and resolution of 0.05 mm [16]. Statistically, this means that 95% of the time the error in determining the CoP position should be 0.1 mm + 1.96*0.05 mm or about the 0.2 mm previously indicated. Since its introduction to the market in 2002, well before the ISPGR standards were developed, our CAPS® systems have satisfied these requirements [17], because we recognized from the beginning the importance of determining the CoP with sufficient accuracy and precision.

[14] Browne J, O'Hare N. "Development of a Quality Control Procedure for Force Platforms." Physiol Meas. 2000; 21(4):515-24.
[15] Moghadam M, Ashayeri H, Salavati M, Sarafzadeh J, Taghipoor KD, Saeedi A, Salehi R. "Reliability of center of pressure measures of postural stability in healthy older adults: effects of postural task difficulty and cognitive load." Gait Posture. 2011;33(4);651-5.
[16] Scoppa F, Capra R, Gallamini M, Shiffer R. "Clinical stabilometry standardization: Basic definitions - acquisition interval - sampling frequency." Gait Posture. 2013;37:290-2.
[17] Pagnacco G, Carrick FR, Wright CHG, Oggero E. "In-situ verification of accuracy, precision and resolution of force and balance platforms." Biomed Sci Instrum 2014;50:171-8.

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Not all posturography devices are created equal

Several published research investigations have shown that force platforms designed for gait analysis and often used for posturography do not satisfy the ISPGR standards. These devices have, even after a very advanced non-linear in-situ recalibration possible only in research environments, errors in the determination of the location of the CoP ranging from almost 1 mm (accuracy of 0.5 mm, precision of 0.2 mm) to over 4 mm (accuracy of 2.0 mm, precision of 1.2 mm) [18]. Analyzing an AMTI OR6-6-1000 and a Nintendo Wii™ Balance Board, Bartlett and colleagues [19] reported at best an accuracy of 1.6 mm with a precision of 1.7 mm for the Wii device and an accuracy of 1.2 mm with a precision of 1.1 mm for the AMTI instrument. NeuroCom® states in their technical literature [20] that the errors can be "+/- 1% FS", i.e. 1% of half the dimensions of the platform which means over 2 mm.
AMTI introduced the "Optima HPS" line of force platform they claim has, thanks to a sophisticated calibration performed at their facilities, typical CoP errors of about 0.2 mm. However, this might be true when the platform is calibrated and verified in the same position, but mounting it in a different location is likely to increase the errors: the platform will deform differently because of the slightly different characteristics and geometry of the surface on which the platform is positioned/mounted.
This problem does not exist on a system like the CAPS® because of the way it was designed: since it is supported by only 3 points, and there is only one plane through 3 points in space, it is "self-leveling" and does not deform differently when positioned in different locations.

As explained earlier, what constitutes a significant change in a person's sway can be smaller than 0.4 mm. Furthermore, this conclusion was reached in studies performed using force platforms designed for gait analysis that, as we have just seen, have errors much larger than that, so in reality a significant change in sway is perhaps much smaller than 0.4 mm.

This has extremely important consequences to the clinical use of posturography. If the instruments used are not accurate and precise enough, it is impossible to detect small changes that could be clinically significant.
Perhaps an example can be useful to clarify this point: if a scale has an accuracy and precision such that the error can be as much as 2 kg, one cannot say that a diet is working because the weight of a person changed 1 kg, as this change could simply be the error of the measurement (it is within the margin of error of the instrument). Only if the change is greater then the margin of error of the instrument, in this example 2 kg, can one be sure it is real with a certain confidence.
In research this issue can be somewhat ameliorated by performing multiple repetitions of the tests on several subjects and using statistical analysis (statistics are designed to reduce random "noise" such as measurement errors caused by the instrumentation). But in clinical practice, statistics cannot help, because each subject is considered individually.
For a great example of how a poor posturography instrument can be unable to detect changes in a person's balance, we suggest the paper by Bernstein and Burkard [10], where the authors reported they could not detect changes in the results of the Sensory Organization Test (SOT) battery of posturography tests even after bi-lateral and bi-thermal caloric stimulation of the vestibular system.

[18] Cappello A, Bagalà F, Cedraro A, Chiari L. "Non-linear re-calibration of force platforms." Gait Posture 2011;33:724-6.
[19] Bartlett HL, Ting LH, Bingham JT. Accuracy of force and center of pressure measures of the Wii Balance Board. Gait Posture 2014;39(1):224-8.
[20] NeuroCom Balance Manager Systems - Instructions for Use. Rev. E. Natus Medical Incorporated, 2012.

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Some comments on the validation of posturography instruments

Many often ask about the clinical validation of our instruments. As with any measuring instruments, the validation should be done by determining the metrological characteristics of the instrument, i.e. the accuracy, precision and resolution.
So called "clinical validation" can validate a test or a protocol, not a measurement instrument. One does not validate a scale or a stadiometer (height gage) by using clinical subjects, just as one does not validate a measuring tape by measuring random objects. The "validation" must be done by well established metrological procedures that make it possible, by comparing the readings of the measurement device in question with high precision references, to determine the accuracy, precision and resolution of such instrument.
The right question to ask regarding a posturography instrument (or any measurement instrument) is therefore not about its clinical validation, but about its metrological characteristics. If these are sufficient enough for the task at hand, the results will be the same no matter the brand, make or model of the device used; if they are not, then the results obtained with different instruments might not be comparable.

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