MATLAB codes/ Butterworth filter and Spline interpolation

[Farzaneh Yazdani]

Hello everyone,

In the process of analyzing raw gait kinematic data that we collected with the use of a qualisys system (8 cameras, 120 Hz) I need to firstly eliminate noise
in the signals by a fourth order, zero phase shift, Butterworth filter with a cut-off frequency of 6 Hz and then time-normalizing data with Spline interpolation. Is there any Matlab code on how to do these functions?

Thanks in advance for your attention.

Farzaneh Yazdani
PhD Candidate, SUMS school of rehabilitation sciences
Center for human motion sciences research

[Ganesh B]

Re: MATLAB codes/ Butterworth filter and Spline interpolation

Hi,
There are built in commands available in MATLAB such "butter" and "spline" which can be used for Butterworth filtering and spline fitting respectively. Also you can check curve fitting app available in MATLAB to experiment with different fitting options-spline, Fourier, polynomial etc. You can generate code for the same in app window->File menu-> generate code option. MATLAB versions 2014 onward have these options for sure.
I would like to know,how did you decide your cut-off frequency from raw gait data?

[Jason Friedman]

Re: MATLAB codes/ Butterworth filter and Spline interpolation

I have a lecture with matlab examples of filtering using the Butterworth filter and using spline interpolation, they can be found here:

http://www.curiousjason.com/presentations.shtml

Look in the lecture "Introduction to human movement analysis"

As the previous poster wrote, you should consider carefully what cutoff frequency you are using for your data.

Jason Friedman
Tel Aviv University

[Farzaneh Yazdani]

Re: MATLAB codes/ Butterworth filter and Spline interpolation

Dear Ganesh
thanks a lot for your reply.
The optimal cutoff frequency in gait is actually 6*natural frequency. This just happens to be about 1 Hz for freely selected cadence. (determined by David Winter using a residual analysis of gait kinematics filtered at various frequencies)

[Farzaneh Yazdani]

Re: MATLAB codes/ Butterworth filter and Spline interpolation

Dear Jason,
thanks a lot, i will check the your suggested lecture.

regards

[Hyun-Seob, Lee]

Re: MATLAB codes/ Butterworth filter and Spline interpolation

Dear Farzaneh Yazdani

A long time ago I had been wrote a program for filter and interpolation by Matlab.
If you would like to receive my program, please let me know your e-mail address.

[James Dunne]

Re: MATLAB codes/ Butterworth filter and Spline interpolation

MOtoNMS is an excellent open source package to use. They have scripts for zero-lag filtering.

If you use their work, make sure to reference their publication.