Processing marker trajectories and force plate data

[Maryam Hajizadeh]

Dear All,

I want to do kinematic and kinetic analysis during walking ,and I have marker trajectories. I have some question for writing my MATLAB code:

1- Our VICON system has missed some marker trajectories during the test, while we have used helen hayes marker set. I do not know how I should fill these gaps at the first step.

2- The frequency of our cameras is 100HZ, while the frequency of our Force plate is 1000Hz. I do not know what I should do in order to use these data for kinetic analysis.

3- As I have noticed some filtering procedures must be done on marker trajectory data to smooth their path. I do not know how I should smooth my data.


Would you please help me with my questions? It would be too nice if you help me whether there are MATLAB functions for these procedures.


Regards,
Maryam

[Will Robertson]

Re: Processing marker trajectories and force plate data

Hi Maryam,

These questions would probably be best answered by your supervisor or others in your lab.
I'm not sure how much detail you need; the short answers to these would be:

(1) We've used interpolation for this recently but my students' matlab code isn't good enough to share yet I recommend find the missing points with the "find" function and then using "interp1" with the neighbouring points to fill in the blanks. The only "tricky" part is finding the correct neighbouring points if there is more than one missing point in a row.

(2) Consider using "resample" in the signal processing toolbox but if that's not available you could low pass filter (using "butter" then "filtfilt") and then decimate with "data(1:10:end)". Decimating on its own would be risky but perhaps fine.

(3) Answered in the previous point.

Hope this helps,
Will

[Edmund Cramp]

Re: Processing marker trajectories and force plate data

1- Our VICON system has missed some marker trajectories during the test, while we have used helen hayes marker set. I do not know how I should fill these gaps at the first step.

2- The frequency of our cameras is 100HZ, while the frequency of our Force plate is 1000Hz. I do not know what I should do in order to use these data for kinetic analysis.

3- As I have noticed some filtering procedures must be done on marker trajectory data to smooth their path. I do not know how I should smooth my data.

The Helen Hayes software filtered and interpolated the marker data to smooth the trajectories and fill in any gaps in the trajectories - this was part of the Gait Parameter Calculations program and was the first step performed in processing the trajectory data. The Helen Hayes laboratory used hardware filters set at 6Hz on the raw force plate date prior to analog data sampling. The documentation supplied with the Helen Hayes Software does not specify the actual filter frequencies used by the software but at the time that it was written, it's probably a safe assumption that a Butterworth filter at 6Hz was used on both force plate and trajectory data.

Motion Lab Systems sells a general purpose C3D file editing program (C3Deditor) that can automatically interpolate gaps in 3D trajectories and filter both 3D markers and analog data, as well as perform almost any other C3D editing task but you could certainly write a MATLAB routine to do this if you wish.