Dr Gabor Barton (MD)
Senior Lecturer in Biomechanics
The Research Institute for Sport & Exercise Sciences,
Liverpool John Moores University
Room 2.51 Henry Cotton Campus, 15-21 Webster Street, Liverpool, L3 2ET
Tel: +44 (0)151 231 4333/4321 Fax: +44 (0)151 231 4353
Thanks to Dr. Daryl Phillips, I was informed that a "teach-in" has just appeared on Dr. Chris Kirtley's Clinical Gait Analysis webpage on the CoM'nalysis module used in the F-Scan /teach-in/efficiency/
Maybe some of you more mathematical types can help answer some of Dr. Kirtley's questions at the end of the webpage better than I can. Norman and Bart, go ahead, give it a shot for the rest of us!
Norman, I know that you must have something to say about all of this!
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
107 Scripps Drive, Suite 200
Sacramento, CA 95825 USA
Voice: (916) 925-8111 Fax: (916) 925-8136
I am traveling, and will get back on this. This will not be a one e-mail, but several. I need to reflect on presenting the material (most clearly) since I do not have a lecture room situation (white board, Power-Point, with questions & answers). Please bear with me in doing so, since I prefer several short e-mails than a few lengthy ones. There are fundamental and theoretical concepts and principles I need to present in help to clarify and provide an understanding. I do wish the readership to be become more knowledgeable on gait, and with the methods for its analysis.
I could answer yes to the questions, but this would not do justice to the readership.
You wrote in your reply:
Kevin Kirby invited us to...
Are you really sure... hummm, seems that Kevin was out on the Pacific ocean, fishing salmon, and that he was thinking of you and I when placing bait on his hook...
I have received several e-mails indicating that the following
link in the Bibliography on Dr. Kirtley's web site does not work:
<http://www.tekscan.com/pdfs/CoM'nalysis.pdf> Murphy N (2002) Behind COM'nalysis. Technical and marketing flyer, TekScan Inc., South Boston MA.
Specifically, the URL for this reference is: http://www.tekscan.com/pdfs/CoM'nalysis.pdf
Note that there is an apostrophe ' in the word CoM'nalysis and some operating systems do not recognize this symbol.
For those who which to access, but are unable, please use:
http://www.tekscan.com/pdfs/CoMnalysis.pdf (without the apostrophe)
Of interest to some, posted on Dr. Kirtley's web site is an analysis before treatment (not wearing prescribed orthotics). If you do access the site http://www.tekscan.com/pdfs/CoMnalysis.pdf, scroll down the pages, and you will also see the results with treatment (wearing the prescribed orthotics) for the subject.
Norman, I hope you do not feel offended by me dissipating
notes about this CoM module. After all, the mailbase is a forum for
discussion and I am quite interested to know your views on the usefulness
of the CoM analyis. Despite the podiatric community in Canada and the US
knowing you as the official promoter of the F-scan system, in this way
tying your hands and head to a commercial company, I always appreciated
your honest and knowledgeable comments about plantar foot pressure
measurements. And your last contribution on the mailbase about the use and
non-use of pressure plates is a good illustration of this.
But let us start with some few comments of mine (which
are based on the
information presented on Cris Kirtley's web site)
First, I am always wary to categorize complex phenomena
like human gait by
one figure, here the Gait Perfection Index. There is always the danger or
temptation for many clinicians to base their judgment on this one figure
without taking the effort to look further to other much more informative
indicators as the individual pressure curves etcŠ(Norman, in your email
about pressure plates, you yourself stressed the need to take account of
many other gait variables you deemed useful for a thorough gait
evaluation). From my own experience, I know that the COP path on a colorful
maximal pressure picture of the foot, is sometimes the sole information
retrieved by many using pressure mats for gait evaluation, because it is
presented on the first flashy screen displayed by most pressure systems I
Secondly, I am not sure and, by my knowledge, it is not proven
yet that the
vertical Com trajectory for normal gait should be a pure 2Hz sinus curve.
What is the inter- and intra- subject variability of this curve ? I can
agree with the fact that the CoM trajectory in all probability will
RESEMBLE a 2Hz sinus, but I would like to see the results of a study
investigating a large amount of subjects without apparent gait abnormality
or medical history of the lower extremities. As long we do not have this
information, it is my conviction that the use of Purity Index, and for the
same reason also the Symmetry Index, is quite premature for clinical gait
Thirdly, the logic behind the Conservation Index, being the inverse
sum of all twelve harmonics, escapes my gray cells as an indicator of gait
And even in the case the three indices would be found valuable parameters,
why combining them in one single general index, here the Gait Perfection
Index. I would rather present the three indices separately as they are, as
each tells its own story about the gait analyzed.
And finally, a last comment, at least for tonight (it is about
2 o'clock by
now). It is well known and Cris Kirtley iterated this on the teach-in,
that energy efficiency depends heavily on the phase difference between the
potential energy and kinetic energy curves. Perfect out-of-phase would
infer maximal energy recovery, and thus maximal efficiency. Which CoM
module index does take the latter into account ?
Well, I quit now and as my gray cells are gradually fading out,
I'll go for
a nice sleep. Norman it is your turn now to grasp your pen and present your
I am looking forward to it.
Regards to all,
Bart Van Gheluwe
Laboratory of Biomechanics
Vrije Universiteit Brussel -Fac. LK
Pleinlaan 2, 1050 Brussel, Belgium
Tel.: 02/629.27.33 (31)
I've been using the COM program linked to my fscan for the past
year or so.
What I like about it is that it provides a sort of reference for pre and
post op 'sort of quantification' for orthotic treatment. It may be argued
that as a true value of efficiency there may be some problems but I think it
is a good way to measure an effect comparing before and after. I have done
a study recently scanning 27 patients with a multitude of conditions. In
all but 1 if the COM graph improved so did the symptoms. The case that did
not was a patient with a large intermetatarsal neuroma who was using a pain
avoidance strategy and when I 'improved the efficiency' she got more pain.
She subsequently had surgery and is doing fine.
On a similar note I have just evaluated 1150 orthotic prescriptions
subjectively, all were written based on sagittal plane theory. These are
all musculoskeletal patients some complex trauma and neurology cases where
total resolution may be unrealistic. Orthotics are both foot and limb
devices and none of the foot ones were casted, all just treated using
sagittal plane theory with very small amounts of correction. The results
show 92% of patients had significant to total resolution of symptoms. This
compares to previous studies I have done showing over 95% improvement. This
latest study was with more involved cases so this may explain the drop in
The fscan study is ongoing and I do rely more on the fscan (using
evaluation process to Howard Dananberg and Bruce Williams). I am now
starting to link video clips to the COM and they seem fairly reliable in
what you see visually and what you see on the graphs. Also the com graphs
seem sensitive to 2mm of material thickness which enables very fine tuning.
I think Howard has mentioned this before and it still amazes me how little
'posting' you need to get a good clinical result.
I would add that in many of the non fscan studies (see above)
treatment based on proprioceptive awareness of body weight, this seems
excellent for post hip and knee replacement patients and post surgical ankle
fusions and bunion ops to help fine tune sagittal function. This is great
for frontal plane FF conditions too and for assessing medially deviated
I sent the COM info to Chris Kirtley to get a more traditional
overview so I
will be interested to see what comments it produces globally. I guess I was
just looking for an accurate reference which was sensitive enough to see
changes in orthotic function and then cross referencing this with subjective
Robert Fleck, Robert.Fleck@NUTH.NORTHY.NHS.UK
While I do not dispute your findings Bob, I have my reservations re: the indices produced by CoMnalysis. I compared my "normal" walk with a much exaggerated Quasimodo type gait: much limping- dragging one foot behind me, much vertical displacement of CoM- CoMnalysis told me that this was more efficient and more symmetrical. Never mind.
I think you must have been doing something wrong. I tried
this myself and
the com was much much worse. Also when I see each patient I repeat the test
by magnifying the problem if I can to see if the efficiency is worse. For
example if someone has a leg length discrepancy and I am happy with the set
up I then put the raise in the other shoe and run a test to see if the com
is worse. I'm not sure what you are doing but the only thing that affects
the com in my experience is if you start deleting or averaging cells in the
fscan program or the calibration is not correct.
I'm sure there is lots more work to be done here but all I was
looking to do
was to try and add some form of quantification to my outcomes. This seems
to be the best and most practical for me so far.
If you like mail me the fscan files and I will take a look
Robert Fleck, Robert.Fleck@NUTH.NORTHY.NHS.UK
I think the inclusion of internal work, elastic energetics and physiological
work rather upsets the TekScan model.
Adam Shortland PhD, MIPEM, SRCS
One Small Step Gait Laboratory,
Chris alluded to Cavagna's equation and both Gabor and Adam picked up
on it. The equation actually separates out "vertical work" from "horizontal
work". I don't really like this because energy is a scalar quantity (and therefore so is work) and I can't see the justification for separately
calculating "vertical work" and "horizontal work". I think the equations based on this under-estimate the efficiency of walking by not taking
into account the possibilities of energy transfer between potential energy and those changes in kinetic energy that arise as a consequence of changes
in the horizontal component of the velocity of the centre of mass.
Richard Baker PhD CEng
Gait Analysis Service Manager and Director of Research
Hugh Williamson Gait Laboratory, Royal Children's Hospital, Victoria
Tel: +61(0)3 9345 5354, Fax: +61(0)3 9345 5447
Adjunct Associate Professor, La Trobe University
Honorary Senior Fellow, University of Melbourne
When I was Dave Winter's student at Waterloo those many years ago, we spent
(not surprisingly) a fair bit of time discussing
work-energy-power in gait. Tekscan's innovation has its roots in some early research on gait energetics which used the COM
as the single measure representing the body. That early research was flawed because it ignored the interactions within and
between segments (so we were told); so I guess Tekscan may be going down a 'deadend'. In theory, there are an infinite
number of ways that I could 'improve' my gait purity (and that doesn't even touch on abstinence from alcohol ) by
manipulating how my limb move - assuming I had a built-in gait purity meter, of course.
I guess my concern is the oversimplification of the body in these systems.
I mean, let's get over the fact that the human body
is complicated and stop trying to make analysing human movement a 'simple' thing. That can only result in a dilution of
information and understanding. Analysing human movement can be complicated and I'd rather see more effort go into
producing better 'movement analysts' (for lack of a better word) than trying to 'dumb down' measurement tools to appease
Department of Sport & Exercise Science
University of Auckland
Private Bag 92019
Auckland, New Zealand
Yesterday at the ASB meeting in Toledo I happened to talk to Norman Murphy
of Tekscan who explained some of this to me.
Unfortunately, I had to run to a session before he got to the part about efficiency.
The use of the fourier analysis to look at gait symmetry makes perfect
sense to me. If left and right leg have the same effect
on COM, the COM movement will only have even harmonics, and therefore the first harmonic is an indicator of asymmetry. A
similar analysis to quantify asymmetry in horses (but using head motion, not COM motion) was described by Christian Peham
in Vienna (J Biomech 1996).
But maybe Tekscan is putting the cart before the horse (no pun intended).
Horses use asymmetrical vertical motions of the
head to reduce the vertical ground reaction force on one painful forelimb, and increase the force on the other limb. Experts in
horse lameness have understood this, hundreds, maybe thousands, of years ago and they always look for asymmetry in head
movement to (1) find out which limb is painful (the horse can't tell you!) and (2) quantify lameness. These people have also
very well understood the 180 degree phase shift that comes from differentiation of a sine wave to get inertial force from
So vertical motion is used as an indicator of vertical ground reaction
force, and that may be why it is so useful. But.... if you
already know bilateral ground reaction force (as in Tekscan's system) why bother? Could you not directly quantify the
asymmetry from the ground reaction force, instead of going through this whole process?
As I said, I did not get an explanation of Tekscan's "Energy Efficiency
Index" that is calculated by their software. If it is based
on the amplitude of vertical COM motion, I agree with Drew that this is probably not related to energetics. For example, a
pendulum has lots of vertical COM motion, but moves without using any energy. In fact, there may be a negative correlation!
Tomorrow at ASB there is a presentation with the title "Flattening the Center of Mass Trajectory Increases the Cost of
Walking", by Farley and Ortega. Yesterday there was a poster with the title "Reducing vertical center of mass movement
during walking doesn't necessarily reduce metabolic cost" (by Gordon, Ferris, and Kuo). The abstracts will probably be posted
Ton van den Bogert, PhD
Department of Biomedical Engineering
Cleveland Clinic Foundation
There are several loose ends from the comments so far on gait efficiency, which I'd like to summarize.
1. Is the amount of Centre of Mass (CoM) motion in any way related to
Evidence for: Kerrigan et al (1995) found a strong correlation (r2 = 0.91) between the amplitude of the vertical displacement of the CoM and energy consumption.
Evidence against: Richard Baker reports that children with spina bifida walk with large (inefficient) motions of the body segments but little
Justus Ortega & Clare Farley (at the ASB last week) reported that walking in a way which minimised CoM motion actually HALVED efficiency.
2. Is it, perhaps, the recovery rate between potential (PE) and kinetic
energy (KE) that is related to efficiency?
If so, can we agree on a definition for this? Giovanni Cavagna calculates the KE (0.5mv2) by just looking at the rise in FORWARD velocity when the CoM falls. Is this valid? Why ignore the other (vertical and lateral) components?
3. A few people have mentioned the phase relation between PE and KE as being critical, and Cavagna's team have shown that this is highly tuned in African women who carry baskets on their heads - see: http://jeb.biologists.org/cgi/content/full/205/21/3413 - and Griffin & Kram have shown that penguins also optimize this timing: http://www.usatoday.com/weather/resources/coldscience/2000/penguins122000.htm
If this is true, and getting back to the determinants of gait, what might be the joint kinematics associated with such timing?
mailto:[n/a] as usual with your comments.
Kerrigan DC, Viramontes BE, Corcoran PJ, LaRaia PJ (1995) Measured
versus predicted vertical displacement of the sacrum during gait as a
tool to measure biomechanical gait performance. Am J Phys. Med Rehabil.
Griffin M and Kram R (2000) Biomechanics: Penguin waddling is not
wasteful. Nature 408, 929.
G. A. Cavagna, P. A. Willems, M. A. Legramandi, and N. C. Heglund
Pendular energy transduction within the step in human walking J. Exp.
Biol., January 11, 2002; 205(21): 3413 - 3422.
Heglund NC, Willems PA, Penta M and Cavagna GA (1995). Energy-saving
gait mechanics with head-supported loads. Nature 375, 52-54
Dr. Chris Kirtley MD PhD
Dept. of Biomedical Engineering
Catholic University of America
My colleague Dan Ferris forwarded this message to me. I was the organizer
the symposium, "Modern Perspectives on the Six Determinants of Gait," at the
recent ASB meeting. I'm not a member of the Clinical Gait Analysis e-mail
list, but if you don't mind I would like to respond to the points that you
summarized, which appear to be at the tail end of a thoughtful discussion.
This is perhaps too long of a reply to post to the list, but I hope you
might find it helpful. --Art Kuo (University of Michigan)
1. Is the amount of Centre of Mass (CoM) motion in any way
Possibly, but not in the way predicted by the Six Determinants of Gait.
parallel with the study of Farley & Ortega (2003), we also presented a
poster with similar evidence (Gordon KE et al. 2003). Our student, Keith
Gordon, had healthy normals walk with reduced CoM motion, and showed
markedly higher metabolic cost. This was found using two methods of reducing
CoM motion while maintaining a regular walking speed: (1) walking with very
short but fast steps, and (2) walking to reduce vertical CoM motion using
visual feedback of a sacral marker height. In normal walking, subjects do
not minimize their CoM motion, yet the preferred gait appears to minimize
metabolic cost for given speed. Our methods were slightly different from
those of Farley & Ortega (2003), but we very much agree with their results.
It is true that Kerrigan et al. (1995) found a correlation between CoM
motion and metabolic cost, but that was because CoM motion increased with
walking speed. That was a nice study, but their results shouldn't be
interpreted as making CoM a predictor, because speed is also correlated.
Keeping speed constant, it appears that metabolic cost goes up for any
increase or decrease in CoM displacement from what normal subjects prefer.
When subjects increase their vertical CoM motion, such as by taking
steps (whether or not speed increases), their metabolic cost also goes up
(Donelan et al., 2002; Gordon et al. 2003). However, we do not consider this
to be due to raising the CoM per se. We hypothesize that inverted pendulum
mechanics allow the CoM to go up and down without necessitating much
expenditure of energy. But regardless of the energy needed to act like an
inverted pendulum, there appears to be a substantial mechanical and
metabolic cost associated with the transition between inverted pendulum
phases, which we call step-to-step transition costs. The CoM velocity needs
to be redirected from one inverted pendulum arc to the next, and that
requires negative work by the leading leg. Positive work is needed to offset
this lost, and much (but not all) of this occurs when pushing off with the
trailing leg. See also Kuo (2001).
2. Is it, perhaps, the recovery rate between potential (PE)
energy (KE) that is related to efficiency?
I am skeptical that % Recovery is a good predictor of efficiency. In
different experiments where subjects walked with different step widths
(Donelan et al., 2001) and step lengths (Donelan et al., 2002), we found an
increased metabolic energy expenditure despite an increase in % Recovery. Of
course, there are other situations where % Recovery is inversely correlated
with metabolic cost, but overall I would not consider % Recovery a good
predictor. This is partially because % Recovery does not account for
simultaneous positive and negative work by the legs during double support
(Donelan et al., 2001), as well as other reasons. I am a great admirer of
Cavagna's many contributions to the understanding of walking and running,
but I find % Recovery to be of limited use. In any case, I don't believe %
Recovery has been rigorously tested as a predictor of experimentally
measured metabolic cost.
3. A few people have mentioned the phase relation between PE and
There might very well be a relationship between PE and KE phasing and
metabolic cost, but I don't know of any explanation that makes predictions
that have been tested experimentally. There have been many papers that
analyze data post hoc, and speculate on the mechanism. But as with the Six
Determinants of Gait and % Recovery, I would like to see an experimental
test before embracing any hypothesis.
Donelan, J. M., Kram, R., and Kuo, A. D. (2001) Mechanical and metabolic
determinants of the preferred step width in human walking. Proceedings of
the Royal Society of London, Series B, 268: 1985-1992.
Donelan, J. M., Kram, R., and Kuo, A. D. (2002) Mechanical and metabolic
costs of step-to-step transitions in human walking. Journal of Experimental
Biology, 205: 3717-3727.
Donelan, J. M., Kram, R., and Kuo, A. D. (2002) Simultaneous positive
negative external mechanical work in human walking, Journal of Biomechanics,
Gordon KE, Ferris DP, and Kuo AD (2003) Proc. 27th Ann. Mtg. Amer. Soc.
Biomech., Toledo, OH. #53.
Kuo, A. D. (2001) A simple model predicts the step length-speed relationship
in human walking, Journal of Biomechanical Engineering, 123: 264-269.
The poster above and paper reprints can be found at
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