How to calculate sway area rate for center of pressure
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Hello, i want to calculate sway area (mm2 /s) which is defined as the area enclosed by the center-of-pressure trajectory per unit time. Some useful paper (find below). Please find attached the CoPx and CoPy data.
Hufschmidt A, Dichgans J, Mauritz H, Hufschmidt M (1980) Some methods and parameters of body sway quantifcation and their neurological applications. Arch Psyc Nervenkr 228:135–150
Prieto TE, Myklebust JB, Hofman RG, Lovett EG, Myklebust BM (1996) Measures of postural steadiness: differences between healthy young and elderly adults. IEEE Trans Biomed Eng 43:956–966
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Accepted Answer
William Rose
on 8 Sep 2022
Since you already have the data in Excel. there is no need for Matlab. Do the calculation in Excel, using equation 19 of Prieto et al., 1996.
When I opened the file you attached, I received a warning message: "External links have beeen disabled", and the data do not look like COP data. There are no headers and the first row is at time=0.006. Therefore I have not analyzed the data in the spreadsheet you uploaded.
I am attaching an Excel file that demonstrates the calculation of Sway Area using equation 19 of Prieto et al., 1996.
Good luck.
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William Rose
on 18 Jul 2023
@Mario Yes it is possible, and not difficult. I am traveling now and have only a cell phone. More later.
William Rose
on 19 Jul 2023
This uses a two-column array data which has the x,y coordinates of the center of pressure. In this example, the COP path consists of 100 points around the unit circle.
data=[cos(2*pi*(0:100)/100);sin(2*pi*(0:100)/100)]';
%plot(data(:,1),data(:,2),'-r.') %view the path
n=length(data);
pL1=0;
for i=2:n
pL1=pL1+sqrt((data(i,1)-data(i-1,1))^2+(data(i,2)-data(i-1,2))^2);
end
fprintf('Path length 1=%.3f.\n',pL1)
The length is 2*pi, as expected for this path.*
In Matlab it is a point of pride to eliminate for loops. You can compute the path length without a for loop:
pL2=sum(((diff(data(:,1)).^2+diff(data(:,2)).^2).^0.5));
fprintf('Path length 2=%.3f.\n',pL2)
The pL2 code does not use n, so you can eliminate the line n=length(data); also.
Try it. Good luck.
*Not exactly, since the path is a 100-gon, not a perfect circle.
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