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Liduin Meershoek
09-30-1998, 11:15 PM
Dear collegues,

Here is a summary of the replies on my question about sand on the force
plate. Sorry for my late reply but I was abroad for a while and I wanted to
trace some references first. Thanks for all reactions.

Liduin Meershoek
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ORIGINAL MESSAGE

Dear collegues,

I want to do GRF measurements of jumping horses using a force plate. For
several reasons I want to cover the force plate with sand. Does anybody
know if this influences my measurements? I understand that the GRF on sand
will be different from the GRF on other surfaces. However I wonder whether
the GRF measurements are still accurate: do they represent the force
between sand and hoof, or does the sand influence these measurements?
Furthermore, will this be any different when I cover the force plate with a
rubber mat.

Thanks,
Liduin Meershoek
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SUMMARY OF REPLIES

It is possible to cover the plate with sand. The difference between the
force plate measurement and the force acting on the hoof should be found in
accelerations of the sand, and equals the vector sum of mass*acceleration
of all sand particles that are above the force platform. Compared to a
horse this difference should be neglectable, it can be measured using
simultaneous measurements with a instrumented horse shoe and a force plate
or can be estimated using a model of the accelerations of the sand.
Furthermore the damping of the force plate's natural frequency will be
improved - which is good. There should be no or little oscilation effects.
Of course there are differences between measurements with sand on the plate
and without sand, in fact one of the reason to put sand on the plate, since
that is the normal surface to jump on.
When the point of application is used care should be taken to correct for
the additional height above the plate.

Finally there is some positive experience with a force plate covered with
snow:
During the 1988 Olympics in Calgary there was a force plate in the
take off area of the ski jump. I have been told that Dr. Paavo Komi
himself cut the snow around the force plate, risking his life for
science. Qualitatively, there was no difference between the trials
before and after the cut was made. Also, after the cut was made,
the slab of snow covering the force plate did not slide down the hill
during the competition (fortunately!), suggesting that the horizontal
stresses in the snow were small, or at least small compared to the
shear stress between snow and force plate.

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REPLIES

We have looked at the compliance of various types of sand and it does make
a difference to the readings that you get from the force plate. Check out a
couple of papers (Barrett, Neal & Pen (1998). Journal of Science and
Medicine in Sport 1:1-??;
Proceedings of 2nd Conference on Mathematics and Computers in Sport - Bond
University edited by Neville de Mestre (neville_de_mestre@bond.edu.au)

Cheers,

Rob

Robert Neal, PhD
Department of Human Movement Studies
The University of Queensland
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Vet Med Univ in Vienna may have experience with this activity. I
think that have a plate in a cutting stable.

Motion Analysis Corporation
Daniel India, Vice President
3617 Westwind Blvd
Santa Rosa, CA 95403 USA
HQ Tel: 707-579-6500 Direct 847-945-1411
HQ Fax 707-526-0629 Direct 847-945-1442
www.motionanalysis.com
Dan.India@motionanalysis.com
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Greetings!

Actually, you can't cover your force plate with sand for the same reason
you can't have the horse jump onto hard concrete. The sand is there to
dissipate the GRF so the horse isn't injured. Because the energy is
dissipated by interactions between particles of sand, a buried forceplate
will see little or no pressure at all.

I recommend trying to instrument the horse's hoofs. Certainly this is a
technically difficult task, but there are surely creative solutions to be
found. You could also weigh the horse & rider and use an accelerometer to
calculate impact forces based on conservation of momentum. Find a hungry
engineering student and offer her/him a grant! :-)

Regards,
-- Jeff
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From: "Nichols-Ketchum, Martha"

I know if you use a rubber mat, you are measuring force from rubber to
force plate. The rubber should attenuate the impact from the horse. I
don't know exactly how sand will affect measurements. I would assume
that the thinner the layer (of rubber or sand), the more likely to have
representative forces.
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You could do a simple experiment to determine the effects of sand over the
force plate - have a horse stand on the force plate directly, and then with
the plate
covered by sand. The differences measured with static loads may be
different than
for dynamic loads, but the static experiment would identify any large effects
or may provide some confidence in the measurements. Out of curiosity - what
magnitude of forces do you expect - are you measuring take-off or landing
forces?

John Hipp, PhD
Director of Research
Baylor College of Medicine
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Anything you put on top of the forceplate will deform as the horse steps
on it and thereby alter the gait forces. In our lab we have seen
significant changes in human gait by wearing tennis shoes. As you
know tennis shoes are made to absorb shock so the effects are more
pronounced. If you want to see a horses gait on sand, then use the
sand, but you will not be seeing the gait on a hard surface unless the
force plate is a hard surface. If the purpose of the rubber mat is just to
protect the forceplate I would use a piece of hard flexible plastic intead
of rubber, also you will want to use a thin piece. Basically, anything that
will compress between the horses hoof and the forceplate will increase
the stopping distance of the horses hoof and will therefore decrease
df/dt, at the onset of the impact of the horse hoof to the floor, but the full
force of the horse resting on the forceplate will be the same.

George Davey
gdavey@stu.uomhs.edu
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Hello Liduin

You may want to contact the Motion Analysis customers at the Veterinary
Medicine University (VMU) in Vienna. The person to contact is Dr. Christian
Peham (Christian.Peham@vu-wien.ac.at) They have done a lot of horse
research, and may be able to help you. Are you using video technology to
measure the kinematics, or are you purely measuring forces ?

Regards,

Barry

Barry O'Flynn
European Sales & Support
Motion Analysis Corporation
London, U.K.
Tel: + 44 181 7470396
Fax: + 44 181 7428608
Cell: + 44 46 7372084
Email: barry.oflynn@motionanalysis.com
Web: www.motionanalysis.com
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Dear Liduin,

Covering the forceplate with any material changes the depth of the az
dimension of the plate (or the distance from the top of the plate to the
origin in the vertical direction. If this distance is different from what
you have input into the software you will get centre of pressure data that
is a little wacky. Ultimately affecting the joint moments. You can cover
the forceplate with something solid (artifical turf) because unlike sand it
doesn't displace. Meaning that you can measure its thickness and add this
to the 'az' dimension of the plate. You'll need to recalibrate the
forceplate to determine the exact dimension of az and the torques.

I've attached a zip file which you should take a look at.

Hope this helps.

Cheers.

Mick
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Using a free body diagram, you can see that the difference
between the force plate measurement and the force acting on the
hoof is exactly the vector sum of mass*acceleration of all sand
particles that are above the force platform. All sand particles
should be included, also those that are not under the hoof and also
those that are not in contact with the plate because they are in free
fall (their acceleration is -g vertical while it was zero before the
horse jumped on it, so they cause an error). The weight of the sand
was already compensated for when you reset the charge amplifiers.

Based on this, you can probably come up with an upper bound for
the error you are making. The first sand particles that are hit
by the hoof undergo a large acceleration, but this is only the
surface layer. After an initial downward acceleration, the layer will
follow the hoof in its downward motion and have an upward acceleration.
This happens successively to deeper layers.
Some of the sand will be moving upwards, and some of it will be
in free fall. Those contributions are harder to estimate. Also
don't forget the horizontal accelerations of the sand. A
simple model, based on your knowledge of the velocity-time data of
the hoof, may get you closer to an answer.

You could also do an experiment where you drop a rigid object with
a known mass and shaped like a hoof. Choose the drop height to
give a realistic landing velocity and choose the mass to get realistic
force magnitude. Instrument the object with an accelerometer, so
that you know the total force on it, which can be compared to the
force plate signal plus weight.

My gut feeling is that the error is small. Also for the rubber
mat. Rubber is elastic, so it needs to stretch before you get a
force. The force plate is stiff enough that the rubber will not
have to stretch when horizontal forces are applied to the plate.

During the 1988 Olympics in Calgary there was a force plate in the
take off area of the ski jump. I have been told that Dr. Paavo Komi
himself cut the snow around the force plate, risking his life for
science. Qualitatively, there was no difference between the trials
before and after the cut was made. Also, after the cut was made,
the slab of snow covering the force plate did not slide down the hill
during the competition (fortunately!), suggesting that the horizontal
stresses in the snow were small, or at least small compared to the
shear stress between snow and force plate.

Regards,

Ton van den Bogert
Dept. of Biomedical Engineering
The Cleveland Clinic Foundation
bogert@bme.ri.ccf.org
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Luidin,

Thoughts regarding the surfaces with which you wish to cover your
force plate:

SAND: I don't how accurate this will be, since I imagine that sand
very "forgiving" (i.e., dissipates applied force), probably too
forgiving to obtain accurate measurements. You might be able to
create an algorithm that would account for this.

RUBBER MAT: much more feasible with regards to obtaining consistent
force data - the only thing that needs to be done with the mat,
depending on its thickness and density, is zero the plate before
you start measuring your data (which is, of course, something that
you would need to do with sand as well).

So in response to your questions:

* I don't think the GRF measurements on sand would still be accurate - the
sand would probably influence these measurements.

* Depending on the thickness and density of the rubber mat, this is
probably the better bet for consistent GRF mesurements.

Good luck - I'd be interested in a summary of the replies that you get

-Joe
================================================== =================
Joseph Seay, M.S.
Gait Lab, Geriatric Research Phone: 336-713-8541
Sticht Center on Aging Fax: 336-713-8547
PO Box 57201 - Medical Ctr. Blvd E-mail: seayjf@wfu.edu
Winston-Salem, NC 27157
================================================== =================
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Hi, Whilst I cannot contribute to your knowledge I would be inteested in
the response you receive to your query. Wouuld you be prepared to share
these with moi?

John Sharp
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Dear Sir

I don't know for shure whether you are using a Kistler force plate or

* You will need a rubber mat anyway to ensure that the gap around
the force plate does not fill with sand.
* Statically the force measured is correct.
* The damping of the force plate's natural frequency will be
improved - which is good.
* Since sand is not a rubbery material there should be no be
little osciallation effects.
* The only error we can think of is by the mass of the mass of
sand which is moved when stepping on it - compared to a horse this
should be neglectable.
* We have customers who use sand and other materials to test
running shoes in a very similar setup.

We don't see any problem with your application.

Best Regards
Christian Calame
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Mr. Christian Calame, Product Manager Biomechanics
Kistler Instrumente AG Winterthur, P.O.Box 304,
CH-8408 Winterthur, Switzerland
Tel: +41 52 224 11 11, Fax: +41 52 224 14 14
E-Mail: cl@kistler.ch, http://www.kistler.ch/biomech
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