The center of pressure (COP) is the point on a surface where the total sum of the resultant forces can act with the same magnitude of the force which is distribuited on the surface of an object. Measuring the COP has been used in biomechanics as a way to measure the postural balance in humans. There's been created variables or parameters related to the COP, in order to measure postural balance (e.g. COP Velocity, COP Displacement, COP Trayectory). Some studies have used one or more of this parameters with the purpose of investigate postural balance. But Which of this COP parameters is more usefull to reflect the COP behavior? Are there better parameters than others or maybe each parameter is better measuring balance in an specific task or context?
I think it all comes down to matching your outcome measure to your research question. For instance, if you want to look at variability of a dataset, perhaps the standard deviation of the COP or even a nonlinear approach such as approximate entropy to look at the randomness within a sample.
Another example is using fractal dimension, which looks at the 'predictability' of a given path, where one would assume a more complex path represents either an altered postural control strategy, or perhaps dysfunction of the sensorimotor system.
I suggest reading this article by Prieto et al, 1996, regarding various COP outcomes, and it includes the calculations for each: http://www.ncbi.nlm.nih.gov/pubmed/9214811
Within my lab, we use time-to-boundary, which is a derived signal that looks at the direction and velocity of the COP signal with respect to the borders of the foot. See Hertel et al, 2006: http://www.ncbi.nlm.nih.gov/pubmed/16760569 There are other outcomes like this, and this would represent the amount of time it would take for an individual's COP to pass outside the borders of their foot (base of support), thus the assumption is made they would lose balance or be in a compromised position.
The 95% confidence ellipse is also a popular outcome, representing the locations of the COP, or another measure of COP area, if you want to look at the area of COP displacement.
Ultimately, my first point will help you best. Figure out which outcome will tell you most about your population or your balance task, and then use it.
I hope this helps,
I always liked the 95% confidence ellipse mentioned by Christopher Burcal.
Check out the publications by Dr. Liliam Oliveira of the Federal University of Rio de Janeiro too.
Best wishes
I have previously stated that all force platform measures correlate poorly with stability (falls risk) therefore they should not be used to infer stability. Sway measures indicate both behavior humans use to maintain stable equilibrium but also behavior to redistribute plantar load to prevent plantar skin ulceration, and it is impossible to know the amplitude of each component. This is explained in the following working paper available on Researchgate:
Working Paper: Letter to editor regarding Zech, A., Argubi-Wollesen, A., & Rahlf, A. (2015). Minimalist, standard and no footwear on static and dynamic postural stability following jump landing. European Journal of Sports and Exercise Science, 15(4), 279-285.
Martina Mancini and Fay Horak have led on use of body worn devices (acceleromaters) to quantify Jerk and RMS relating to rate of change and magnitude of accelerations from the CoM.
Mancini et al. ISway: a sensitive, valid and reliable measure of postural control. JNER. 2012, 9:59
We have successfully used these outcomes in Newcastle also but adapted to examine change over a 2min standing balance test:
Del Din et al. Time.dependent changes in postural control in early Parkinson's disease: what are we missing? Med Biol Eng Comput (2016) 54:401–410.
And we have also examine in older adults in community testing under different balance tests:
Godfrey at al. iCap: Instrumented assessment of physical capability. Maturitas. 2015, 82, 116-122.
Allan Godrey is inciorrect. The reference cited (Mancini et al) merely shows this method is capable of destinguishing those with untreated early Parkinson's from normals, which also can be done reliably via normal clinical neurological exam performed by a physician. It is incapable of determining relative stability between healthy humans under differing environmental conditions such as footwear versus barefoot or types of footwear.
Dear Garzon,
There are a lot of different methods, as stated before, addressing different types of analysis.
From my experience and lots of reviews a have gathered lots of methods, but I haven't seen something specific for a type of pathology.
Also considering the stochastic variance of COP you might use the reference below. There might interest you another method which calculates the so called indices of stability. If you are interested you should contact me.
If none of this is of interest than you can go ahead using any other method described by other colleagues, there are very various.
References:
http://www.delsys.com/Attachments_pdf/nmrc/files/2011/02/057.pdf
With consideration.
I suggest reading this article by Ho Seon Choi et al., (2018) Design of an Artificial Neural Network Algorithm for a Low-Cost Insole Sensor to Estimate the Ground Reaction Force (GRF) and Calibrate the Center of Pressure (CoP) DOI: 10.3390/s18124349, regarding various COP outcomes and you will definitely understand much better.
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