View Full Version : Force Plate and balance: summary

Jose Haroldo Cavalcante
03-13-1996, 11:42 PM
Last week I send the following message to the list. Now I am sending a summary
of the answers I got. Many thanks to everyone who wrote. It was really helpful!


J. Haroldo

Jose Haroldo de Assis Cavalcante SMHS 501 "A"
Engineer 70.330-150

Motion Laboratory Brasilia-DF

(55) (61) 319-1440 (voice)
(55) (61) 319-1447 (fax)
E-mail: haroldo@lab01.sarah.br

*** This is my original letter:

>Hello all,
>We are doing some balance research using a force plate (AMTI ORS6-5).
>The subject stands as quietly as possible on the force plate and the center
>of pressure (CP) position is calculated from the signals collected from it.
>I would like to present some questions:
>1- What to do if the subject has to use some kind of support (like crutches or
>a walker) to be able to stand on the plate? They might be in contact with
>the plate or not?
>2- What are the most used/interesting parameters of CP to analyse? We here are
>mainly concerned on the speed and radius.
>Thank you in advance for your collaboration. I will post a summary if it is
>of interest.
> J. Haroldo

*** These are the answers:


From: MX%"H.Naesborg@mai.ku.dk" 6-MAR-1996 08:46
Hello Jose,

I'm very interested in the responses you get from the biomech-l. especially
regarding question #2
>2- What are the most used/interesting parameters of CP to analyse? We here are
>mainly concerned on the speed and radius.

I'm about to begin an experiment where we will use variation in the centerof
pressure as one parameter. I hope you will post the responses on biomech-l.

Kind regards

Jesper Bencke


From: MX%"Josef.Kollmitzer@AKH-WIEN.AC.AT" 6-MAR-1996 08:50
Organization: Institut fuer Medizinische Computerwissenschaften

please send me summary on force plate and balance topic
We do have AMTI force plate in use for gait analysis
and want to go in ballance study
What software are you working on?
Thank you for reply

josef kollmitzer
PMR - Vienna - Austria


From: MX%"ariel1@ix.netcom.com" 6-MAR-1996 08:52

Surf the net at:


From: MX%"ikirtley@info.curtin.edu.au" 6-MAR-1996 09:00


My advice to you would be to read David Winter's recent book, "ABC:
Anatomy, Biomechanics, Control".

You need to understand the difference between COP and Centre of Gravity
(COG). They are not the same, and COP is not a measure of sway.

You can think of COP as the control signal used to control COG, just as EMG
is the control signal used to control joint moment. There may be a casual
relationship between each pair of variables, but they are not
causally-linked. For example, the elderly have reduced sway (COG) but have
increased COP motion - they are working overtime to keep their COG within
reduced limits of stability.

Hope this helps,


__________________________________________________ __________________
Dr. Chris Kirtley MB ChB, PhD c.kirtley@info.curtin.edu.au
Lecturer, Bio-engineering --_ / \
/ \
School of Physiotherapy, Perth #_.---._/
Curtin University of Technology, V
GPO Box U1987,
Perth 6001, Tel +61 9 351 3649
Western Australia. Fax +61 9 351 3636

WWW: http://www.curtin.edu.au/curtin/dept/physio/pt/staff/kirtley/
Internet Relay Chat: irc.curtin.edu.au (port 6667) "bio-engineering"
NetPhone: IP address 34311.180.105
__________________________________________________ __________________


From: MX%"T.Bach@latrobe.edu.au" 6-MAR-1996 09:12


In response to the second of your questions, we have done some studies on
the validity and validity of force platform measures of postural steadiness
and have found that horizontal force variability, rather than CP
variability is a more reliable and valid measure. You might read the

Goldie, P.A., T.M. Bach and O.M. Evans (1989). Force platform measures for
evaluating postural control: Reliability and Validity. Archives of Physical
Medicine and Rehabilitation 70:510-517.

Goldie, P.A., O.M. Evans and T.M. Bach (1992). Steadiness in one legged
stance: Development of a reliable force platform testing procedure.
Archives of Physical Medicine and Rehabilitation 73:348-54.

Goldie, P. A., Evans, O. M., Bach, T. M. (1994) Postural control following
inversion injuries of the ankle. Archives of Physical Medicine and
Rehabilitation 75:969-975.

Tim. Bach

Timothy M. Bach, PhD Phone: +61 3 9285 5311
Senior Lecturer in Biomechanics FAX: +61 3 9285 5184
School of Human Biosciences E-mail: T.Bach@LaTrobe.edu.au
La Trobe University
607 Swanston Street
Locked Bag 12, Carlton South, VIC, 3053


From: MX%"ELemaire@rohcg.on.ca" 6-MAR-1996 10:18


1- What to do if the subject has to use some kind of support
(like crutches or a walker) to be able to stand on the plate?
They might be in contact with the plate or not?

>> Some previous work I have seen required that either the
walker be modified, a wide walker was provided, or a railing
was build over the edges of the force plate to support the
assistive device (i.e., the railing did not touch the force

2- What are the most used/interesting parameters of CP to
analyse? We here are mainly concerned on the speed and

>> We use the Ariel motion analysis system. The software
has the features you require already built in. You may want
to check with this company - maybe you won't have to buy a
complete system to get what you need.

Ed Lemaire, MSc
The Rehabilitation Centre
Ottawa, Ontario, Canada
613-737-7350 x5592


From: MX%"krishna@gait52.mgh.harvard.edu" 6-MAR-1996 13:08

Hello Mr. J. Haroldo

I presume that the subject would lean on the crutch/walker to help
him/her balance during the test. I think it is important for the crutch
supports also to be on the force plate. This would enable you to
understand how the entire system - the subject with the crutch,

my tuppence:
COP, is simply the point on the support surface, through which the
resultant ground reaction forces act. The ground reaction forces are
the resultant of the forces acting on the body segments (due to
muscles,gravity) The ground reaction forces are generated by subject
in order to stabilize himself with respect to the support.

My background:
My PhD thesis is - to predict 'preventive stepping'
during quiet standing of vestibular hypofunction pataients

Padmanabhan Srikrishna
PhD Student, Mechanical Eng., MIT
Biomotion lab. MGH. Boston.


From: MX%"paul@gaitlab1.uwaterloo.ca" 6-MAR-1996 13:33

Hi Jose,

> 1- (...)

We've done experiments that are concerned with this same problem (even =
have the same force plate(s)). For those studies, we instrumented the =
crutch or walker. I put triaxial strain gauges on the feet of the =
support device, and we looked at this data in conjunction with the 6 =
channels of force plate data. The support device could be on or off the =
forceplate, as long as your analysis programs know about the difference. =
The instrumentation gives you a very good idea of the stability of the =
subject, and just how much he or she depends on the support device. =
Knowing the 'base of support', it is fairly easy to determine the risk =
associated with subjects walking.

> 2- What are the most used/interesting parameters of CP to analyze? We =
here are
> mainly concerned on the speed and radius.

That depends on what you are looking for, and what 'story' you want =
your results to tell. By itself (especially with just one plate for both =
feet), CofP doesn't have a lot to say, although Fourier analysis of the =
data can show some enlightening things. We usually determine the =
subjects centre of mass (CofM) and compare that with the CofP. The CofM =
is where you really are, the CofP is the mechanism that controls the =
position of the CofM. The difference between the two is a measure of the =
body's 'servo error' as it dynamically tries to maintain balance for =
quiet standing. For large perturbations in balance, other factors enter =
into the equations.
Currently we're investigating some of the issues of quiet balance, =
please contact me if you would like more details.



From: MX%"ad432@dial.pipex.com" 7-MAR-1996 14:25

Dear Jose,

Concerning your 1st question:
It depends of what you are interested of. If for example the subject is
using crutches and you want to look at the loading of the upper body then
you need individual force plates (just a quick thought). However since you
are interested probably only for the subject as is, then I suggest these NOT
to be in contact with the platform.

Concerning your 2nd question:
As a time series you might be interested of the 0th ... 2nd derivative
patterns, the minimum - maximum values, mean and SD. You might be interestes
to see the frequency domain patterns as well. A point on the last bit: It
has been shown that the CP signal is not stationary (Carroll, 1993, JoB, 26,
409-416; Schumann et al, 1995, JB, 28, 603-607). This is more evident in
patients with several neuro-muscular instabilities, and therefore the
Time-Frequency domain approach is more appropriate.
I am currently doing some relevant work on that topic and it would be
usefull to "exchange" some ideas.

Good luck and hope this helps.


Please mention my name on the subject because I am sharing this account.

The Manchester Metropolitan University
Crewe + Alsager Faculty
Biomechanics research group
Emberton Bungalow, Hassall Road,
Alsager ST7 2HL
The UK


From: MX%"RDH00636@umes-bird.umd.edu" 8-MAR-1996 16:17

We are conducting a study with similar force plates. I beleive they
are the same manufacturer as the ones you are using. We are
excluding any assistive devices in our study. It involves both uni
and bilateral stances with eyes open and closed. We will look at
total sway envelope, mean velocities, anterior-posterior forces, and
medial-lateral force amplitudes. There are many parameters being
currently studied according to our literature review. Let us know if
we can be of help.

Richard D. Horsey