Sway velocity | COP displacement | Ellipse area
The clinical examination of balance control, while relatively simple and cost-effective, offers only a subjective assessment of potential deficits. In response to the limitations regarding objectivity and reliability in clinical testing, instrumented testing involving force and pressure plates has enabled the quantification of postural control, this technique is called Posturography.
The most common measure used in posturography is the center of pressure (COP). This is the location of the point of application of the ground reaction vector on the support’s surface. Through COP measurement it is possible to quantify the small corrections that are performed to oppose the destabilizing effect of gravity, meanig that we can quantify and characterize postural stability. It can be performed statically and dynamically and under a variety of sensory conditions, such as:
Different stance position;
Different type of surfaces;
Presence of conflicting visual input;
Absence of visual input.
Choosing the right test conditions and the right COP resultant parameters to analyse is fundamental for conducting a valuable balance assessment. (Heamawatanachai et al., 2021; Quijoux et al., 2021; Visser et al., 2008).
The table below shows all PhysioSensing assessment protocols and their respective stance positions and test conditions.
TABLE - PhysioSensing Assessment Protocols
As an example, the Balance Error Scoring System requires the subject to maintain balance in three different stances (narrow stance, unilateral stance, and tandem stance), with eyes closed, while standing on a firm surface and on an unstable surface (foam pad).
But how can we measure balance? And what is the meaning of the resultant parameters?
We can start by examining the stabilograms that display the COP displacement in the mediolateral and anteroposterior directions. What we see in this type of graphic are the small corrections that are performed to oppose the destabilizing forces acting on the body.
There are numerous parameters related to COP, It's helpful to be acquainted with the most studied and commonly used ones in clinical settings. So, let us start by discussing the folllowing parameters:
1. COP displacement (mm)
Description: Total length of the center of pressure path. The COP path is the series of data points traced out by the movement of the COP.
Clinical Meaning: The total length of COP displacement has been considered one of the most reliable parameters to distinguish between different age groups and different conditions of balance dysfunction (Duarte & Freitas, 2010). Higher values of COP displacement represent increased body sway, in other words, an increased unsteadiness (Quijoux et al., 2021). We can also analyze COP displacement in the mediolateral plane and the anteroposterior plane since there is some research on how balance control in these two planes behave differently in face of diverse conditions. As an example, body sway range in the mediolateral plane has been reported to be the best predictor for fall risk (Raymakers et al., 2005; Visser et al., 2008).
2. Ellipse area (95%)
Description: Area of the prediction ellipse with 95% of the COP values, calculated with the principal component analysis method (mm²).
Clinical Meaning: The Ellipse Area is a different method used in posturography to quantify postural sway. Although being widely used and considered as a “traditional” parameter, it is important to have in mind that comparisons between different protocols are not reliable unless the calculation method is the same, as well as the sampling frequency and trial duration (Schubert & Kirchner, 2014). The interpretation of this parameter is quite simple, a large area signifies higher levels of postural sway, studies have shown larger ellipse area in the elderly with increased risk of falling, and as for Parkinson’s Disease, ellipse area was considered one of the most reliable parameters to assess postural control (Merlo et al., 2012; Terra et al., 2020).
We can further analyze this method by looking into the rotational angle of the ellipse, which shows us the principal sway direction, this can be valuable information since it allows the characterization of postural instability (Quijoux et al., 2021).
3. Sway velocity
Description: Distance travelled by center of pressure divided by test time (mm/s or °/s).
Clinical Meaning: The mean velocity of the COP, in the likeness of the COP displacement, is one of the most widely used and reliable posturographic parameters, specifically in the anteroposterior plane. Studies have shown it to be influenced by age-related postural alterations and to be a good predictor of fall risk (Howcroft et al., 2017; Quijoux et al., 2021). In Parkinson’s Disease, sway velocity is considered reliable for assessing postural imbalance, specially, during Romberg condition with eyes closed (Terra et al., 2020). In a cross-sectional study with the aimed of assessing how static posturography techniques could quantify balance dysfunction in Multiple Sclerosis patients, it was found that mean sway velocity could detect differences between healthy subjects and Multiple Sclerosis patients with normal cerebellar function and Romberg Test (Inojosa et al., 2020).
Do you have questions about this topic? Or want to know more about balance system?
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Ana Souto
Meet Ana, a physiotherapist with a master's degree in human physiology, currently specializing in neurobiology. Her professional journey has led her to gain extensive expertise in both neurology and sports physiotherapy.
Ana currently serves as the clinical specialist at PhysioSensing, a cutting-edge Balance Assessment and training device. Leveraging her strong foundation in scientific research and evidence-based practices, Ana creates customized assessment and training plans. Her approach is firmly rooted in the latest scientific findings, ensuring that PhysioSensing users receive the most effective and up-to-date care.
In addition to her role in designing tailored programs, Ana plays a pivotal role in guiding new clients through the learning process of using PhysioSensing. She also provides advanced training and support to existing customers seeking to further deepen their clinical practice knowledge and stay on top of the latest scientific advancements.
References
Duarte, M., & Freitas, S. M. S. F. (2010). Revision of posturography based on force plate for balance evaluation. Revista Brasileira De Fisioterapia (Sao Carlos (Sao Paulo, Brazil)), 14(3), 183–192.
Heamawatanachai, S., Wiriyasakunphan, W., Srisupornkornkool, K., & Jorrakate, C. (2021). Test-Retest Reliability of Low-Cost Posturography for Assessing Postural Stability Control Performance during Standing. Journal of Aging Research, 2021, 1–11. https://doi.org/10.1155/2021/9233453
Howcroft, J., Lemaire, E. D., Kofman, J., & McIlroy, W. E. (2017). Elderly fall risk prediction using static posturography. PLOS ONE, 12(2), e0172398. https://doi.org/10.1371/journal.pone.0172398
Inojosa, H., Schriefer, D., Klöditz, A., Trentzsch, K., & Ziemssen, T. (2020). Balance Testing in Multiple Sclerosis-Improving Neurological Assessment With Static Posturography? Frontiers in Neurology, 11, 135. https://doi.org/10.3389/fneur.2020.00135
Merlo, A., Zemp, D., Zanda, E., Rocchi, S., Meroni, F., Tettamanti, M., Recchia, A., Lucca, U., & Quadri, P. (2012). Postural stability and history of falls in cognitively able older adults: The Canton Ticino study. Gait & Posture, 36(4), 662–666. https://doi.org/10.1016/j.gaitpost.2012.06.016
Quijoux, F., Nicolaï, A., Chairi, I., Bargiotas, I., Ricard, D., Yelnik, A., Oudre, L., Bertin‐Hugault, F., Vidal, P., Vayatis, N., Buffat, S., & Audiffren, J. (2021). A review of center of pressure (COP) variables to quantify standing balance in elderly people: Algorithms and open‐access code*. Physiological Reports, 9(22). https://doi.org/10.14814/phy2.15067
Raymakers, J. A., Samson, M. M., & Verhaar, H. J. J. (2005). The assessment of body sway and the choice of the stability parameter(s). Gait & Posture, 21(1), 48–58. https://doi.org/10.1016/j.gaitpost.2003.11.006
Schubert, P., & Kirchner, M. (2014). Ellipse area calculations and their applicability in posturography. Gait & Posture, 39(1), 518–522. https://doi.org/10.1016/j.gaitpost.2013.09.001
Terra, M. B., Da Silva, R. A., Bueno, M. E. B., Ferraz, H. B., & Smaili, S. M. (2020). Center of pressure-based balance evaluation in individuals with Parkinson’s disease: A reliability study. Physiotherapy Theory and Practice, 36(7), 826–833. https://doi.org/10.1080/09593985.2018.1508261
Visser, J. E., Carpenter, M. G., Van Der Kooij, H., & Bloem, B. R. (2008). The clinical utility of posturography. Clinical Neurophysiology, 119(11), 2424–2436. https://doi.org/10.1016/j.clinph.2008.07.220