Biomch-L Readers

I have included the responses to my Kistler Bioware CoM calculation

question below. Thanks to all those who replied. From all of the

feedback I am expecting to be able to further understand and resolve my

original issue.

In appreciation,

Peter

Dr Peter Clothier

Lecturer - Sport & Exercise Science

Building 24 Level 4 Room 7

School of Biomedical & Health Sciences

University of Western Sydney

Campbelltown Campus

Locked Bag 1797

Penrith South DC NSW 1797

Australia

Ph. +61 02 46203743

Fax. +61 02 46203792

p.clothier@uws.edu.au

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Hi Peter,

I played around with this a while ago and put the spreadsheet here:

http://www.univie.ac.at/CGA/tools/ (third row from the bottom of the

table)

Integration is always a fiddle because you don't know the integration

coefficient - in this case the initial vertical velocity. Luckily, the

vertical direction is unique, in that you know that it must average zero

(otherwise the runner would start flying or sink into the track), so the

trick is:

1. Run the integration with initial velocity set to zero

2. Calculate the mean vertical velocity, Vm

3. Re-run the integration with initial velocity set to -Vm

Note that the subject's body mass is also quite critical, so needs to be

measured accurately (or fiddled!). Also, you can't know the absolute

height of the CoM so you have to estimate it (the NASA rule of thumb is

54.3% of height from the ground).

Have fun!

Chris

Dr. Chris Kirtley MB ChB, PhD

608 Dockside

44 Ferry St.

Kangaroo Point

Queensland 4169

Australia (GMT+10)

Tel. 61+7-3891 6644 x 1608

China mobile: 13418732724

Fax 3891 6900

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Peter,

You would need to ensure you have included the initial conditions

(velocity and position) before you perform the double integration

otherwise your calculations will be inaccurate. These initial conditions

have to be determined at the point of impact on the platform. I'm not

sure whether Bioware v4 allows you to include these values as I don't

use this.

Regards

Drew

Dr Drew Harrison

Senior Lecturer and

Head of Dept of Physical Education and Sports Science University of

Limerick

Tel: +353 61 202809

email drew.harrison@ul.ie

www.isbs2009.com

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Dear Peter,

One step that you have not described is that of subtracting the

bodyweight of the subject from the vertical force component before

integrating it. This may have been done, but not written in the previous

e-mail...so...

Vertical force - subtract bodyweight to get acceleration of COM -

integrate to get velocity of COM - integrate to get displacement of COM.

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Hi Peter,

I have been working on measuring COM displacement in my research. I am

not familiar with the Bioware software, but I am familiar with kinematic

and kinetic approaches for measuring COM displacement. Double

integration of the force plate signal will result in vertical COM

displacement, but only if you take into consideration the mass of the

subject and the integration constants that emerge when you perform the

integration. The following article gives a good overview of different

approaches for measuring COM displacement.

Gard SA, Miff SC, and Kuo AD. Comparison of kinematic and kinetic

methods for computing the vertical motion of the body center of mass

during walking. Human Movement Science; 2004; 22(6): 597-610.

Good luck!

Ben Roewer

MSME/MBA student

University of Delaware

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Pax!

That's true, double integration of vertical force should be related to

the vertical displacement. The problem are the numerical errors that

grow like time^2. However, since running is a cyclical motion then

perhaps a Fourier method would work? FFT the force data, replace the

frequency coefficient a(f) with -a(f)/(2 \pi f)^2 [corresponds to double

integration; f = frequency] and then take the inverse FFT.

Regards Frank Borg

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Hi Peter,

I have never used the kistler software to perform the double

integration. So, no idea what's going on with the software. However, one

thing to keep in mind is that the constants required to obtain exact

solutions from the integration steps, being the initial velocity and

position respectively, are critical to obtain realistic CM traces.

Attached is a paper that could point you in the right direction, if this

indeed is part of the problem.

Best,

Ivo

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Hi,

I don't know if it is possible for you to plot the raw data and look at

the pattern first.

The double integration is very sensitive to noise in the signal,

therefore the signal is usually conditioned/filtered first. The nature

of the forceplate instrument is that it typically have some inherent

signal noise, in our case, it's about 0.25%. So, that might have to be

resolved before you can compute for accurate CoM. I hope this helps.

Ping

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