CGA FAQ: Hitting the force-plate

Dear all CGA members,

Here at Speising, Vienna-Austria we will get a complete new 
Motion Analysis system with the beginning of February.

We also get two force plates. 
>From my experience from the AKH hospital I would choose a 
position of the second force plate right behind the first plate  (no 
gap) but shifted by the half width of the force plate to the lateral 
side. Our force plate size will be about 50*50 cm.
This would be our configuration for the children with CP.

Also we plan to have an other position for adult of the second force 
plate: The same shift but 8 to 10 cm gap between the plate in the 
walking direction.

My question to you is what positions you would recommend.

We still have the option to change the plan because we do not 
have started to build the foundation.

Best regards,


Mag. Andreas Kranzl
Waehringer Guertel 135/23
A-1180 Wien, Austria
Phone: +43 1 47 82 555
Mobil: 0676 5129787
Orthopaedisches Spital Speising
Labor fuer Gang- und Bewegungsanalyse
Speisinger Straße 109
1130 Wien
Tel: ++43-1-80182-276

Dear Andreas, and others interested, I looked into force plate positioning recently when we moved into our new lab. By a lucky accident, a lab opened up just before ours in a local hospital, and I sent my students to do a project there collecting normal adult data. You can see this lab and its force plates at: I was unfortunatley away in Thailand at the time of the data capture (it's a hard life over here in Asia!), but what I noticed immediately I saw the results <> was that the subjects were not walking naturally. They all had a short stride length and reduced walking velocity (note teh small A2 power bursts). At first I thought this was simply due to the lab dimensions (a very short walkway), but I realised later that a much more important factor was the positioning of the plates. Since they were placed together, the subjects (whether subconsciously or consciously - I don't know) targetted the plates so as to land cleanly on each with successive steps (you can actually see the subject looking at the plates in the picture). Of course, my students should have spotted this and told the subjects not to look at the plates, but I think in practice it would be very difficult for the subjects not to be influenced by the plate positions. Of course, the vast majority of patients we see in the lab are children with CP, so you could argue that this problem does not matter so much, but in my view it does, because even older children will be influenced by the plates. In my lab, therefore, I have very deliberately seperated the plates. Moreover, I have a portable Kistler plate of width only 30 cm (one of only two of its type in the world, which I managed to kidnap from Christian Calame!), and which was originally custom made for stair-climbing. I have mounted this in the modular (MERO) floor that I have, such that it can be placed in two positions - longitudinally for adults with a typically shortened stride and tranverse for children with very short stride lengths. You can see these two arrangements at: In my view, this small Kistler plate is a Godsend, and I wonder why manufacturers of force plates still make plates so big (40 or 50 x 50 cm) when most of us are doing gait analysis on children with very short strides most of the time. I'd especially like to hear from Kistler and AMTI why this is. Best wishes (and by the way, it would be nice to see some discussion on the list, which has been strangly one-way lately!) Chris -- Dr. Chris Kirtley MD PhD Dept. of Rehabilitation Sciences The Hong Kong Polytechnic University
Thank you for the picture of the lab with the force plates visible.  It was interesting to see that not only was the subject focused on the force plate, so was everyone in the room!  At the Kinetics lab in Grand Rapids, MI, USA, the force plates are hidden under a matching carpet. We have the subjects start at the same location for three trial runs, and mark where they land close to where the plates can move to each time.  Then, the subject leaves the room for the physical therapy evaluation, and the forceplates are moved into position, and the carpet replaced. The subject, therefor, does not even know what we are looking for when we decide if it was a "good trial" or not.  It does take extra time, since we have to process each trial at the time, but the blind results are worth it.  From working with children with CP, I know that even a three year old will "stomp" on the forceplate each time if they think they should! Kristin Dart, MSPT
I have a couple of comments on the force plate position discussion that might
be of interest:

Since I installed Gait Labs for about 15 years (working until a few years ago
for Oxford Metrics in the US) I've had the chance to set up a lot of different
configurations (in 25+ labs) over the years.  While I've not been involved in
the day to day use after the lab has started up, I have been involved in
making sure that the customer could use the lab, and that the environment and
equipment worked.

These comments are based on the assumption that you're setting up a lab with
some sort of 3D photogrammetry system, at least one force plate and possibly
an EMG system.  Opinions expressed are my own.

Force plate position is often one of the big questions when the lab is
designed and built.  For most people, given the responsibility of starting a
lab, this is the first time that they've had to make this sort of discussion
and it always seems to cause a lot of soul-searching.

In my experience the best position for two plates is to place them adjacent to
one another (separated by a gap of about 1mm) in the direction of progression
of the subject down the lab walkway.  The lab walkway should almost always be
in the center of the lab and the force plates in the center of the walkway.
This makes for an easy installation and provides the lab users with the best
chance of getting the subject to produce a clean strike with one leg on one of
the plates within the first two of three walks.  If you have more than two
plates then space them out a little further along the walkway and your chances
of getting a clean strike improve somewhat.

Offsetting a pair of plates slightly to either side of the walkway centerline
seems to make it slightly harder to get a good strike as the subject seems to
miss both plates a little more often - mainly because the subject does not
always lead with the same leg as the prior trial.

The walkway is always best placed down the center of the viewable part of the
room.  I have seen a couple of  labs where the walkway was placed diagonally
from corner to corner and, while this increases the length of the walkway very
slightly, it can make camera placement very difficult as you're restricted in
the positions of your 3D system cameras at either end of the walkway.  It's
not a good trade.

One point to remember as you add more plates to a walkway is that, if you are
using a 3D photogrammetry system, then you really need to be able to capture
3D data to complete the gait cycle both before the first plate, and after the
last plate.  This means that you will need a larger data capture volume (4-5
metres long) to produce accurate data unless you use smaller force plates.
But using smaller force plates reduces that chance that you will get a good
foot strike...

I do agree that it is important that the plates are either color matched to
the floor or disguised in some way - I have seen people put contact paper
(normally used to line kitchen shelves) over the plates to hide them.  I have
seen several labs that have put carpet over the plates - this seems to work
well but the carpet must be very carefully applied to avoid getting any glue
around (and in) the edges of the force plate.  At least one lab has had to
pull the carpet up and lay it again after the carpet "settled" slightly in one
direction and started applying a constant force to the plate in one axis!

It's also very important that area immediately adjacent to the plate be
solidly constructed so that it doesn't have any "give" in it, or sound
differently to the rest of the lab floor.  This reduces the chance of any
subtle external cues to the subject that something is in the floor.  Good
construction also prevents the subject from catching their foot on the edge of
the force plate.  You have to balance the visibility of the force plate
against the fact that the whole test will go very much faster if the people
running the tests can tell immediately that the subject either got a clean
force plate strike or missed...

Some labs have solved the position question by installing movable plates -
this has always seemed an expensive solution and I'm not certain that the lab
users actually move the plates around that often in real, day to day use.
Possibly in part because most 3D photogrammetry systems appear to assume that
the plates are fixed to the lab and require considerably reconfiguration if
the plates are moved after system setup.

Edmund Cramp,
Motion Lab Systems, Inc.
4326 Pine Park Drive, Baton Rouge, LA  70809  USA
+1 225 928-4248 (voice, 2 lines) and +1 225 928-0261 (fax) Note - New Area
Code effective April 1999!
My email address is <>  - web site is

Dear cga subscribers,

        I have some strange ideas on how to avoid subjects awaring the force

        1) A pair of frequency-blocking goggles.  Subject under testing
wears this pair of goggles and do the walking as natural as he or she can.
One cannot see the force plate whose surface is coated with a
selective-light-frequency-absorbing material.  A simpler way is to make the
force plate colored differently from all else objects while the goggle block
this colour.  Safety on risk of falling should be observed.

        2)  Mask the force plates.  In the old days, it's masked by carpet
with same color as the ground.  Modern technology and architecture made it
possible by in-building sensors under the pile with the surface reveals no
visible difference.

        3)  Stealth.  From a book on espionage, a clothing using optical
technology: 'instant optics folding procession', which harmonized with the
surroundings to minimize the visible recognision.  If such tech exists, we
may probably use it on covering the plates.

        4)  Software that offsets the 'noise' from such influence.  Advances
in gait analysis machine may render such technical problem obsolete.

        5)  Back to basics.  Study the optimum separation of two force
plates to ensure such interference is minimal, which tallies with what Dr.
Kirtley did.


Thomas Chan Kam Kee
PolyU Physio Year 3

Chan Kam Kee, Thomas
Physiotherapy Year 3 Student

The following comments are based on day to day experience over 16 years using two forceplates with a video-based 3-D motion measurement system in a children's orthopaedic hospital. 1.  For children, or other people who take short steps, I agree with the idea of placing the force platforms one after the other in the path of progression, without any large space between them.  I do NOT agree with the idea of offsetting one of the forceplates to one side, because I think it is important to record data for both right-left and left-right double-support phases, whenever possible.  If one of  the forceplates is offset to one side, the tendency is to collect only one or the other double-support phase. 2.  For people who take longer steps, the outputs of the two forceplates can readily be combined by a computer program to make the two adjacent forceplates look like one big forceplate.  Active people who take long steps can be measured one foot at a time on this "big forceplate."   After years of measuring people, many of whom found it difficult to walk, I came to regard simplicity of test procedures FOR THE TEST SUBJECT as a primary goal.  Leaving the forceplates in the same place but using them differently for different subjects seemed to provide the simplest test procedures for the test subjects.  Force data are very important for meaningful test results, but the quality of force data is difficult to assess by watching the test.  Consequently, I always start any data processing by plotting out and examining the force and center-of-pressure data to insure that the feet were totally on the forceplates.  If not, the trial is discarded without any further time being spent on it. Multiple trials are used to get the subject to step on the forceplates.  If the subject does not step on the plates, he or she is started in a different place along the walkway until successful contact is made.   I concur with other people's comments about the importance of avoiding targeting, that is, the tendency of a subject to alter accustomed gait in order to try to step on a forceplate.  Measurements of some other variable, such as progressional velocity or acceleration, during the full period before, during and after contact with the forceplate can be used to detect such gait alterations. Best wishes for a fruitful laboratory!      Larry Lamoreux Larry W. Lamoreux, Ph.D. GAIT DIMENSION P.O. Box 1186 Lafayette, CA  94549-1186 USA Telephone:  925-283-7718 FAX:  925-283-5919 Email:
I'd like to add a small contribution in the discussion as how to hit a force
plate in the walkway. Some times, we use the following strategy, in order to
obtain a maximally "natural"gait pattern, and minimizing the time to achieve
a succesful trial.
A self sticking little figure is put on the walkway, at approximately one
stride before the force plate. The child is asked to hit this figure (make
it a game), with his - let's say - left foot,  and walk on untill the end of
the walkway. This step is very likely not natural (due to the targeting),
but the following with the same foot, that hits the "hidden" force plate,
usually is. The position of the figure can be adjusted in between trials,
depending on the subject's actual stride length.
It can - and must - be questioned what the wash-out period of a targeted
step is, but some children will get exhausted by many trails, so no
representative step will be achieved at all.

Kind regards, Jaap Harlaar

Jaap Harlaar, PhD
Dept. Rehabilitation Medicine
University Hospital "Vrije Universiteit"
pob. 7057
1007 MB  Amsterdam
The Netherlands
Tel. +31 20 44 40 773
Fax. +31 20 44 40 787

We have taken a slightly different approach. We have to larger force
platforms (120x60 cm) made by AMTI. They are placed side by side with a 1 cm
separation, in the center of the gait lab walkway. We found this works for
most of our adult and children cases. For adults that have a very long step
or for running we sometimes have to preposition the subject at the start
point to make them step on the plate.
You can see images of this setup at
and with a video of our forceline visualization system on
To determine "goodness" of data we use
a gait consistency test as described by R. Selectar et all.
A. Esquenazi, MD- Director
Gait & Motion Analysis Laboratory
Regional Amputee Rehabilitation Center
MossRehab Hospital/Albert Einstein Medical Center
1200 West Tabor Rd. Philadelphia PA 19141 USA
Phone:  215 456 9470
Fax:    215 456 9631

A Member of the Jefferson Healthcare Network

We sometimes find that subjects either target the plates 
or put on their "clinic walk" for the test. Even well disguised 
plates are noticed and in some respects the subtler the outlines, the 
more 'interested' the subject is - everyone likes to play the 'lets 
not step on the pavement cracks' game! What we do see in the majority 
of these cases though is that the subject uses their 'natural' walk 
when returning up the walkway to the start point. So, in these cases 
we fake the test and get them on the way back! If your cameras are 
not set up for successful recording in both directions then try
starting the test from the wrong end.
Jeremy Linskell, Clinical Engineer
Manager, Gait Analysis Laboratory 
Co-Ordinator, Electronic Assistive Tehcnology Service
Dundee Limb Fitting Centre
Dundee, DD5 1AG, Scotland 
tel +1382-730104, fax +1382-480194

Dear all,
For about a year I have avoided getting into discussion on this forum but
force plates have drawn me out of my shell.  Full marks to Andreas for
asking the question.

There seems to be a general feeling that two plates are better than one and
although this may seem self evident, I feel that some fundamental issues
are involved.  This view has been partly vindicated by the comments from
Chris on normal gait being modified to match platform spacing.  Even if
spacing is organised to prevent this, there may still be some modification
in order to correctly contact the first platform.

Concerning the comments from Kirstin Dart about using a carpet to cover the
plates I have two reservations.  In the first instance it is quite possible
for shear forces to be transmitted between the platforms and the
surroundings by the carpet.  Are there any studies of the possible errors
that might be introduced?  More importantly, even with a platform visible,
it can often be difficult to be confident that a foot has landed cleanly on
a platform or that double contact has been avoided.  It must be almost
impossible to be sure the information is correct with a carpet obscuring
the platforms.

Returning to the fundamental reasons for using more than one platform, I
will accept that increasing the number of platforms increases your chance
of gathering useful information, but I have heard it argued that you need
two platforms so as to gather double stance forces and thus get a proper
understanding of the full gait cycle.  Considering the layout being
proposed by Andreas and two of the three featured in the Dundee site, you
will find that you only gather one of the double stance periods, either
left foot to right foot or the reverse.  Walking over the system in the
opposite direction still produces the same transition!  Thus, if you really
want to gather double stance forces completely, three platforms are
required.  This will compound the question of positioning and, I suggest,
make clinical working a nightmare!

I feel confident that both AMTI, Bertec, Kistler and any other manufacturer
would answer Chris by saying that they will produce platforms the size and
shape that are requested (I know of several labs with special size
platforms).  The real question is:  does the gait analysis community have a
real view on the ideal shape and size and is it well thought out?  I
suspect the answer is a resounding NO.  Perhaps this area is ripe for some
fundamental work.  Has any such work been done?

That's all folks!

Richard Major

I seems about time that the force plate manufacturers get into the

One of the most profound and practical answers so far came from Edmund
Cramp. He mentioned almost everything which should be mentioned
regarding force plate placement.

Regarding the sizes: yes it is in fact possible to make almost any size
of force plate. In fact we have made force plates for squirrels for an
institute in Germany. While the size 40x60cm is mainly used for gait
analysis (one adjacent to the other, the long way) and a wide variety of
special applications the larger dimensions are often used in sports
because they are easier to target.

In the early days many Japanese institutes bought super large plates (up
to 2 meters) which they mounted parallel for medio-lateral stride
separation. While this seems to work well with the way the Japanese walk
it has never been used a lot in the rest of the world. The disadvantage
of very large plates is certainly the difficulty of handling and the
lower natural frequency.

Today there is a tendency for smaller plates for a variety of
applications including gait (see Chris Kirtleys remarks about the
"Godsend small plate"). Since force plates have become less expensive it
has become more affordable to have three of them and measure a full gait
cycle including both double stance phases.

Some remarks: 
*       A publication on the subject "Visual guidance to force plate
does not influence ground reaction force variability" by Grabiner
was published in Journal of Biomechanics Vol 28, No 9, pp. 1115-1117,
1995. I cannot comment it.
*       A carpet which is not too hard (not like linoleum) may be layed
over the force plate without a gap. The force shunt will be minimal. The
example mentioned by Edmund Cramp where a lab had to take the carpet out
after it started applying shear forces could not have happened with a
piezoelectric force plate.

Best Regards
Christian Calame
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

Regarding Christian Calame's comments about force-plates becoming  'more affordable' and also there being a tendency for smaller plates, has anyone ever considered or used 'micro-plates', maybe 30cmx15cm or smaller,  placed together in say a 4x4 matrix in the expectation that each foot would  hit more than one plate, but that the plates would be small enough that both  feet would not hit the same plate simultaneously during double support.  Would a system like this be affordable? - I can imagine it would certainly make clinical  life a lot easier! Ben Heller Medical Physics Royal Hallamshire Hospital Sheffield
As some of you know, my main concern these days is trunk accelerometry. I
have followed the force plate discussion, however, and being amateurishly
ignorant here, I hope I am apologized for a possibly naive idea. I wonder
if a force plate for gait analysis might be constructed as a central
support type with one corner being cut off at 45 degrees to be able to
adjoin two plates both in the line of progression and meidiolaterally like
                                |                |
                            ____|                |
                                        |          \             |
                            |              \ ____|       
                                |                |
(Sorry about the oblique line) From a safe distance, it seems to me that
some of the alignments/small/large plate problems might  find  compromises
this way possibly resulting in a larger rate of successful trials. But no
doubt there are mountains of technical problems which I do not see from
here. I am sure you can put me right.
Best regards from wintry Norway

Rolf Moe-Nilssen, MS, PT, Research fellow, Division of Physiotherapy Science,
Department of Public Health and Primary Health Care, Faculty of Medicine, 
University of Bergen, Ulriksdal 8c, N-5009 Bergen, Norway, 
email: voice:+47 55 58 61 70, fax:+47 55 58 61 30 

For what its worth, our group at Surrey,UK has developed a 3.3metres long
twin walkway which gives simultaneous 3D GRF, temporal and spatial data,
each plate 400mm wide.  This avoids most of the positioning problems
encountered by the conventional force plate approach.It also gives multiple
bilateral foot contact rather then just one on each traverse.  If there are
problems for a particular patient, then walking across them allows them to
be used exactly as the smaller traditional plates.

More information on our R&D by email or from our web page.

A couple of questions appear to be coming up quite often - my experience here
relates directly to the construction of Clinical Gait Analysis labs so I'll
address them from this point of view, using Richard Majors message as a
starting point:

> There seems to be a general feeling that two plates are better than one and
> although this may seem self evident, I feel that some fundamental issues
> are involved.  This view has been partly vindicated by the comments from
> Chris on normal gait being modified to match platform spacing.  Even if
> spacing is organised to prevent this, there may still be some modification
> in order to correctly contact the first platform.

In a clinical gait test (often CP subjects in the USA) it's a reasonable
assumption that the subject does not walk efficiently.  This tends to mean
that they get tired relatively quickly so the person conducting the test
doesn't have the luxury for continually repeating the tests until they get the
results that they want.
Thus having more than one plate improves the chance of getting one good strike
on one leg.  If chances of getting a good strike on each side in the same run
(for a CP subject) are so small that they can be ignored - at least from a
test protocol view.

My advice has always been, "If you get a L/R strike then great - but if you
try to walk your subject until you get a L/R strike then it's doubtful that
you'll have a representative walk because the subject may well be tired by the
time that they generate the walk that you're trying to get them to do."

Targeting of the plate is an issue regardless of the number of plates - this
is best dealt with at the time the data is collected by careful observation of
the subject.

Likewise the "Doctor Walk" - the best defense for both issues is to watch the
subject carefully and talk to them and/or their parents/guardians/siblings.

Concerning the comments from Kirstin Dart about using a carpet to cover the
plates I have two reservations.  In the first instance it is quite possible
for shear forces to be transmitted between the platforms and the
surroundings by the carpet.  Are there any studies of the possible errors
that might be introduced?  More importantly, even with a platform visible,
it can often be difficult to be confident that a foot has landed cleanly on
a platform or that double contact has been avoided.  It must be almost
impossible to be sure the information is correct with a carpet obscuring
the platforms.

I believe that there is a study on carpet vs. uncarpeted plate data and it
found little difference.  I have always thought that carpeted plates would
modify the data but I've never seen any evidence of this happening and have
not been able to distinguish carpeted force data from uncarpeted data.  I
would have thought that the Force Plate manufacturers would be able to provide
some input on this...

The issue of plate visibility in carpeted plates is a good one.  In practice
the users know where the plates are and seem to be able to work around this -
and of course the issue of targeting goes away.

Edmund Cramp,
Motion Lab Systems, Inc.
4326 Pine Park Drive, Baton Rouge, LA  70809  USA
+1 225 928-4248 (voice, 2 lines) and +1 225 928-0261 (fax) Note - New
Area Code effective April 1999!
My email address is <>  - web site is

I have two comments concerning the current discussion of Force Platform

1. Using a math.

In our lab, we have been using a thin rubber carpet to avoid targeting
for many years. Although we have not done any proper analysis, it seems
unlikely to me that the carpet is transmitting any substantial forces,
since the plate (piezoelectric) is extremely rigid. Before any forces
are transmitted, the plate must move. If this occurs you are in trouble
even without a carpet, since you no longer measure the forces between
the foot and top plate (in this case you also measure inertial forces
due to motion of the top plate).

2. Detection of unclean force plate hits when a carpet is used.

For the staff to ensure a clean hit on the force plate we are using
small white markers on the carpet in the position of the force plate
corners. Such markers are also placed along the walkway at evenly
distributed positions. The walkway looks like this from above:

                  Force plate corners
                        v     v
.     .     .     .     .     .     .     .     .     .

.     .     .     .     .     .     .     .     .     .
                        ^     ^
                  Force plate corners

In this way the patient is guided as far as walking direction is
concerned, but is not targeting the plate. The staff, on the other hand
is well aware of which of the markers corresponds to the plate.
(In Uppsala we also have a system where the force vectors are overlayed
in real time onto video images of the walking person. By looking at a
video recordning it it is easily verified if the hit was a good one.)

Regards - Håkan

* Håkan Lanshammar    Systems and Control Group, Uppsala University   *
* P.O. Box 27, S-751 03 Uppsala, SWEDEN                               *
* E-mail:, Tel: +46-18-471 30 33, Fax: +46-18-50 36 11*
* WWW:                   *

I'd like to back up the recommendations in Edmund Cramp's posting. We have 4 platforms, in line with a minimal gap between plates 1 and 2,
4" between 2 and 3, and 6" between 3 and 4. The plates are covered with linoleum matching the surrounding floor. Compared to my
experience in my former lab which had 2 unmasked plates that we were constantly repositioning according to the subject, we have been very
happy with this arrangement. In my opinion, the masking solves the issue of targeting. We still get kids who try to target (the gaps around
the plates are still visible), but they have to look so closely to see if they are hitting the force plates, it makes the attempt obvious to detect,
and generally it is then easy enough to distract these patients to make them stop trying to target. The masking does not prevent an
experienced observer from detecting whether or not a clean strike is obtained. We use a sheet to keep track of left and right strikes and which
plate(s) they occurred on. Using 4 platforms has dramatically reduced the number of trials needed for clean strikes, we don't insist on getting
sequential left and right data, but more often than not it happens anyway, and this then becomes the preferred data. For subjects with
extremely small stride lengths, we put a piece of tape down the center of the walkway and ask them to stay to one side of it, this allows us to
collect data from one side only. Finally, we have not needed to set specific starting points for subjects, as other labs do. We have found that
there is enough natural variability and a high enough probability of obtaining a clean strike not to have to give this instruction, and I think
the less instructions needed, the more natural the walk.
Bruce MacWilliams, Ph.D.
Co-Director, Movement Analysis Laboratory
Shriners Hospitals for Children Intermountain Unit
Fairfax Rd. at Virginia St.
Salt Lake City, UT 84103
Phone:  801-536-3800
Fax:       801-536-3782
Research Assistant Professor
Dept. of Orthopedics  
University  of Utah School of Medicine

Interested to see so many replies on the subject of forceplate
positioning - seems to be a burning issue! The replies are all filed on
the FAQ, by the way at:

I personally cannot understand why some labs have suggested larger
plates. Surely this will reduce the chances of getting a clean strike,
especially in children?

I guess the problem of what is the best size plate for the maximum
chance of a clean strike, given a certain step length, should be
solvable by an optimization method - any mathematicians out there want
to give it a try?

As far as modular plates made from small cells is concerned, I'm working
with an electronic engineer in the UK on just that idea. He has some
nice rigid piezoelectric sensors 1 cm square which are capable of
measuirng shear as well as load - no moments, but I guess we can work
those out from the centre of pressure if the cells are small enough.

Dr. Chris Kirtley MD PhD
Dept. of Rehabilitation Sciences
The Hong Kong Polytechnic University

The following paper may be of interest.  I regret I couldn't get the
table to print properly!
Mike Whittle

East Coast Clinical Gait Analysis Meeting, Rochester, Minnesota, 1993.

Michael W. Whittle* and Karen J. Ferris#
* The University of Tennessee at Chattanooga
# Siskin Hospital for Physical Rehabilitation, Chattanooga, Tennessee
With the recent growth in clinical gait analysis, there has been a move
towards improving the appearance of gait laboratories (Gage, 1983),
which in many cases includes carpeting the walkway, including the force
platforms. The assumption is generally made that this does not affect
the data obtained, but there do not appear to have been any publications
on this topic. The present paper describes a study to test the
hypothesis that the presence of carpeting on a gait analysis walkway
does not make a significant difference to a range of commonly measured
gait variables.
The study was performed using a 12m walkway equipped with two Bertec
4060H force platforms and a five camera Vicon system. Ten adult
volunteer subjects (5 men, 5 women) with no known disorders of the
locomotor system participated in the study. The ages of the subjects
were evenly spread across the range from 25 to 62 years. The standard
protocol for the Vicon Clinical Manager software (Oxford Metrics, 1992)
was performed with the subjects walking barefoot at a self-selected
comfortable speed. Three walks were performed with the floor and force
platforms covered by carpet tiles and three walks with them uncarpeted.
Five of the subjects were tested first walking on the carpet, then
walking on the bare floor. The other five subjects walked first on the
bare floor, followed by the carpet. The carpet tiles consisted of 6mm of
a dense looped pile and 2mm of stiff latex backing.
Three categories of variable were chosen for analysis: the general gait
parameters, 11 measurements of sagittal plane joint angle, and six
components of the ground reaction force. The results from each leg, for
all three walks under a given condition (carpeted or uncarpeted), were
averaged and compared, using the paired t-test.
No statistically significant differences were seen between the carpeted
and uncarpeted conditions for either the general gait parameters or the
components of force. However, significant differences were seen between
the two conditions for five measurements of sagittal plane joint angle
(Table 1).
It might be expected that carpeting the force platforms would modify the
ground reaction forces, so it is surprising that the only differences
noted between the two conditions were in the joint angles. The observed
differences were small, all being less than 1.5 degrees. It is possible
that these results were due to random variation, since if enough
variables are examined, it is likely that one or more will show
significant differences. Further studies would be needed to confirm
whether there are any real differences between the test conditions for
these variables. Whether or not these differences are real, however,
none of them was sufficiently large to be considered significant
Since carpet is slightly resilient, it might be expected to reduce the
magnitude of the heelstrike transient, by increasing the time to peak
deceleration of the foot following initial ground contact. Although
detailed measurement of the heelstrike was not attempted in the present
study, it was noted that most of the subjects had a more marked
heelstrike when walking on the bare floor than when walking on the
The present study suggests that while there may be slight changes in
gait analysis variables as a result of carpeting the walkway, the
changes are unlikely to affect the results of clinical gait analysis.
1.  Gage JR. (1983) Gait analysis for decision-making in cerebral palsy.
Bulletin of the Hospital for Joint Diseases Orthopaedic Institute
2.  Oxford Metrics (1992) Vicon Clinical Manager Users Guide. Oxford:
Oxford Metrics.

We are considering renewing the floor covering here to improve the
appearance and also to reduce reflections of the current linoleum,
which can cause us problems with our video and online movement
analysis systems. 

I am concerned with the idea of using carpet on the force plates as I
feel this may well affect the frequency response of the forces. If
anyone knows/uses low reflective flooring in their lab (or very hard
carpet!) which works well I would be really interested to know where
they got it.

Has anyone reported formally on different frequency responses, or rate
of loading during heel strike transient using different materials?

Jim Richards

Jim Richards
Lecturer in Biomechanics
Department of Rehabilitation
University of Salford

from BIOMCH-L, April 1999

Grabiner MD, Feuerbach JW, Lundin TM & Davis BL (1995) Visual
guidance to force plates does not influence ground reaction force
variability. J Biomech 28 (9): 1115-1117.

Abendroth-Smith, J. (1996) Research Quarterly 67 (1) 97-101.

In 1979 we published the following paper for the exact
purpose/concern that you have raised:
Seliktar, R., Yekutiel, M. and Bar, A., "Gait Consistency Test
Based on the Impulse Momentum Theorem," Prosthetics and Orthotics
International 3(2), 91-98, 1979 (British Journal).
The general premise was that you allow your subject to target
himself to the force plates and screen the tests for consistency (cyclic
gait or non cyclic gait) rather than try to make your subject walk and hit
the plates in an unbiased way. I find this approach much better especially
with individuals with neuromuscular problems.
The "consistency test" is a relatively simple one, based on
impulse-momentum consideration, and can be easily implemented
in a gait lab routine. As with many simple things, I don't think that our
technique was widely adopted, but it's there to try never the less.
Good luck,

Hirokawa S.Normal gait characteristics under temporal and
distance constraints  J
Biomed Eng 1989;1

Harrison A.J., Folland J.P., (1997) Investigation of gait
protocols for plantar pressure measurement of non-pathological subjects using
a dynamic pedobarograph. Gait and Posture Vol 6, pp. 50-55.

The work focused primarily on pressure responses to variations
in gait controls rather than force, but I expect there would be a
similar response with force measurement.
Based on this work I have little problem with 'targeting' the
platform as long as the targeting protocol ensures the natural gait pattern
is not noticeably distorted; e.g. a very short or very long last step.
It is obvious that wide variations in walking speed - stride
length, stride rate could affect pressure and I presume force measures. The
problem is that we often wish to ensure we are measuring a 'natural' gait
pattern and we worry that targeting may distort this. I would argue that
provided the targeting protocol is reasonably representative of the natural
stride rate and stride length, there is nothing to worry about. The easiest
way we found to maintain a natural gait pattern and allow targetting
(and hence fewer lost trials) was to use a stride length control (SLC).
The exact protocols for this is described in the paper.

Dr Neil Fowler
Dept Exercise and Sport Science
Manchester Metropolitan University
Hassall Road

Tel 0161 247 5466
Fax 0161 247 6375

Dear all,

I am about to install a 6-camera 3D Peak Performance system here, and am
teaming up with a civil engineer who is interested in earthquakes. We
are building a moving platform which will shaken by a large hydraulic
actuator. I am hoping this will provide a nice facility for studying the
effect of perturbations on gait.

My question relates to the best placement for force platforms under such
conditions. I know we have discussed this before. However, one
arrangement that we didn't mention, which I am thinking might be worth
considering is to have two long (e.g. 1.5 m or so) force platforms
longways side by side, with the subject walking down the center-line
between them. I am thinking that, providing the subject can walk
reasonably straight and has a step width greater than an inch or so (I
suspect it may be considerably greater than this when I start shaking
the platform around!), this would allow every step to be recorded.

What are your thoughts on such an arrangement? Has anyone ever seen it
tried anywhere?

Dr. Chris Kirtley MD PhD
Associate Professor
HomeCare Technologies for the 21st Century (Whitaker Foundation)
NIDRR Rehabilitation Engineering Research Center on TeleRehabilitation
Dept. of Biomedical Engineering, Pangborn 105B
Catholic University of America

Dear Chris,

I think your proposal is most interesting and multiple measurements are
very useful. We use it particularly for amputee and CP gait studies.
Although we have not tried to make a moveable platform, you and other
subscribers may be interested in our (in house designed and built) 3.3
metre twin force platforms at CBEC. More details are on our web site.
Chris Stevens

Chris Stevens
Biomedical Engineering Group,
University of Surrey

Tel: +44 (0)1483 259683
Fax: +44 (0) 1483 879395

Dear Chris,
Here are some ball park figures from David Ewins, my research supervisor
who can be contacted on

Supply, installation and calibration of 3.3 m. dual force platform walkway
45,000 pounds, excluding shipping and taxes.
David has also developed  video vector software for use with the platform,
cost depends on amount and type of hardware required.

I have also copied both your emails to David, who is probably a better
person to answer your question on Qualisys model.

Chris S


The motor control lab at the University of Oregon, under the direction
of Marjorie Woollacott, has a pair of moveable platforms embedded in
a walkway.  The walking surface of the platforms was about 30cm x 54cm,
and each plate had a maximum excursion of about 30cm.  The plates
were housed in independent carriages that could be translated on tracks
allowing the plates to be staggered by any amount, or to be side by side.
The two plates were offset to the inside of their individual carriage,
only a small distance between plates.  This allowed us to do walking studies
(plates staggered, capturing sequential foot contacts) and standing
(plates side by side).

As part of my doctoral work, I spent a fair amount of time working with
this system and ironing out many kinks (from the sound of the hydraulic pump
to post-collection force plate data processing necessary to separate out the
effects of accelerating the plate and its strain gauges).  If you have any
questions I'd be happy to share some of our experiences out there, or I can
put you in touch with people at Oregon who may have something to offer you
in the planning stages.


Krisanne E. Bothner, Ph.D.
Motion Analysis Center
Mary Free Bed Hospital & Rehabilitation Center
2020 Raybrook SE Suite 101
Grand Rapids, MI   49546
voice: 616.954.2318    fax: 616.954.2475

I think that the basic problem with this arrangement is that the subjects have
to walk down the center line and that any deviation from the center line will
place two strikes on the same plate.  It might be worth testing the
arrangement by marking off the force plate sizes and positions on a suitable
area and making a few videos of typical subjects walking across...

Edmund Cramp,
Motion Lab Systems, Inc.
4326 Pine Park Drive, Baton Rouge, LA  70809  USA
+1 225 928-4248 (voice, 2 lines), +1 225 928-0261 (fax)
email <>
web site <>

Hello! This has been tried and it can be concluded that it is not a very
good idea if you want to record unrestricted, natural walking behavior!
Best regards - Håkan

Håkan Lanshammar, Professor, Head of Department
Dept. of Information Technology, Systems and Control, Uppsala Univ.
P.O. Box 27, S-751 03 Uppsala, SWEDEN
E-mail:, Tel: +46-18-471 30 33, Fax: +46-18-50 36 11

Using the concept you describe we have in our laboratory 2 AMTI
platforms 1.2 meters in length placed next to each other with a gap of
1.2 cm in between. We find this to be adequate for most of our patients
(neurological and amputees). At times if the patient or more commonly a
normal subject walks with a long stride he/she may not be able to get a
right and a left step on the plates unless we position them on the
walkway. In the worst of scenarios we use foot switches in addition to
the force plates to mark the other foot event. Rare we have problems
with the width of base unless they have severe hip adduction and in
cases like that we use foot switches to mark foot contact events.

Alberto Esquenazi, MD
Director Gait & Motion Analysis Laboratory and
Regional Amputee Center
1200 West Tabor Rd.
Philadelphia, PA 19141 USA
Voice: 215 456 9470  Fax: 215 456 9631
A Member of the Jefferson Health Network

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