Support Moment: What people said...

Dear CGA enthusiasts

It's my feeling that the support moment is an appealing concept but
it has no basis in Newtonian (or any other) mechanics.  While it may
seem intuitively reasonable if only the sagittal plane moments are
considered, the same cannot be said when true 3D moments are
examined.  For instance, the abduction-adduction moments at the hip
and knee are of the same order of magnitude as the extension moments.
How would they contribute to the support moment?

With all due respect to Dave Winter and his significant contribution
to our understanding of gait biomechanics, I believe that the
support moment concept has not caught on because it's an
inappropriate mechanical parameter.

Kit Vaughan
University of Cape Town <kvaughan@anat.uct.ac.za> 


In discussing Winter's support moment concept, I think it's important to
understand a little of what led him to propose it.  Like many other
researchers, Dr Winter had been employing an inverse dynamics link segment
model.  However, examining individual joints, especially the knee joint,
sometimes revealed some paradoxical results:  it was not unusual to find that
the moment of force at the knee during single support was flexor, not
extensor.  Yet, these subjects did not collapse.  When one includes the hip
and ankle joint moments of force, ie by adding the extensor moments, an
overall pattern of extensor during stance and slightly flexor during swing
emerged.

I am not entirely sure I agree with Dr Vaughan's statement that the support
moment is not based on Newtonian mechanics.  As vectors, the moments can be
added.  The real issue, of course, is this:  is the addition of these vectors
simply a 'mathematical parlour trick' or does it, as Dr Winter felt, reflect
some underlying 'efficiency' of the nervous system, ie, efficiency in that an
overall goal of support is set and the lower limb joints then have some
flexibility in achieving this goal? In the sagittal plane, the consistency of
results might lead one to believe that the latter is true.  I do not have
enough experience with 3D mechanics to comment on how moments in other planes
contribute to Winter's 'theory'.  I do know that the concept has never been
rigorously examined in light of the emergence of 3D motion analysis systems.

Andrew W. (Drew) Smith, PhD
211 Lee Avenue
Toronto ON Canada M4E 2P4
Voice:  +1 (416) 699-9506
Email Address: awsmith@globalserve.net
Home Page: http://www.globalserve.net/~awsmith
ICQ:  6164882  Nickname: Daddio
EmailExpress: 6164882@pager.mirabilis.com


It's Monday, so I'll throw some fuel on the fire... Although I have no evidence
to dispute Dr Winter's assertion that the pattern revealed by summing the lower
limb joint moments represents some underlying neurological characteristic -- and
it is an attractive idea -- my problem in resolving the utility of this method
is simply this: Why stop at the hip? What if you summed moments all the way up
to the neck? After all, one must presume that the pelvis and trunk also play a
role in the control of the CoM during movement, and if it's legal to sum moments
between three body segments, why not several body segments? What does the
'support moment' mean in this context? During single support of gait, the back
and neck moments are actually quite small, so the resulting 'support moment'
would be very similar to that obtained by summing the ankle+knee+hip moments. To
me, this does not sound like a plausible idea, but I'm interested in your comments...
Ronny

------------------------------
Chris A. McGibbon, PhD (Ronny)
MGH Biomotion Lab, Boston MA <ronny@gait52.mgh.harvard.edu> 


I have a comment on Dr. Kirtley's view of the so-called support moment.

>Are you saying, then, that there is, in fact, no a priori reason to
>expect that the sum of the hip, knee and ankle moments would resemble the
>vertical force trace - that it's just co-incidence?

All the kinematic & kinetic variables in locomotion are somehow related to
the ground reaction force because it is one of the major external forces
acting on the subject's body. So we expect some sort of relationships
between the GRF and the moments or forces, etc. I expect this relationships
to become stronger as we move our focus from any particular segment or joint
to the whole leg or the whole body due to the existence of relative motions
among the segments. Isn't this a natural phenomenon?

What about the relationship btween the acceleration of the foot and the GRF
vs. the relationship betwee. acceleration of the leg or perhaps the whole body and the
GRF? Or what if you add the net joint forces of the leg joints vs. dealing
with a particular net joint force?

One other thing. The support moment is basically the leg stretching
(extension) moment. In movements like squat it makes a perfect sense. But
depending on the orientation of the leg, knee flexion moment and the hip
flexion moment may also play a role of preventing the body from collapsing.
For example, if the thigh is in a hyper-extended position, the hip flexion
moment is more favorable to support the body than the extension moment, and
so on. So, I believe that the fact which joint moments support the body
should be assessed in relation to the orientation of the leg segments.

Young-Hoo Kwon

 Young-Hoo Kwon, Ph.D.
 Biomechanics Lab, PL 202
 Ball State University

 Phone: +1 (765) 285-5126
 E-mail: ykwon@cs.bsu.edu
 Homepage: http://www.cs.bsu.edu/~ykwon


I disagree that the frontal moments at the knee are negligible. If
you doubt this, take a look at our publication in the Journal of
Biomechanics (29:383-388, 1996).  We show that for stair climbing,
the abduction moments are between 25 and 45 Nm (compared with the
extension moments of 60 to 85 Nm). Furthermore, I challenge the
participants to say whether the abduction or adduction moment
should count towards the support moment (i.e. which would be positive
and which negative?).

In response to Drew Smith:

I am not entirely sure I agree with Dr Vaughan's statement that thesupport
moment is not based on Newtonian mechanics. As vectors, themoments can be added.
 The real issue, of course, is this:  is the addition of these vectors simply a 'mathematical parlour trick' or
does it, as Dr Winter felt, reflect some underlying 'efficiency' of the nervous system?

Newtonian mechanics tells me that the sum of external forces acting
on a system is equal to the rate of change of the system's linear
momentum (which is the mass of that system multiplied by the
acceleration of the system's centre of gravity).  This is the linear
form.  The angular analogue is that the sum of external moments
acting on a system is equal to the rate of change of the angular
momentum of the system.  You will notice that I have emphasised the
word external.  When we refer to "system", what do we mean?  The
traditional inverse dynamics approach in human gait normally starts
at the foot which is the first system.  Having solved for the
resultant forces and moments at the ankle joint, the next system is
the calf, and so on (cf. Vaughan, Davis, O'Connor, Dynamics of Human
Gait, Human Kinetics Publishers, Champaign, IL, 1992 for a more
detailed explanation).  When we consider the whole of the lower limb
(in the support moment paradigm), then the force and moment at the
ankle and knee joints are internal.  That is what I meant by saying
that the concept has no basis in Newtonian mechanics.

It's not for me to say whether the support moment is a "mathematical
parlour trick", although I am on record (Human Movement Science, Are
joint torques the Holy Grail of human gait analysis? 15:423-443,
1996) as saying that joint moments are a "mechanical ruse"!

Kit Vaughan
University of Cape Town


I'm not too sure what the answer is here.  Using Dr Winter's logic from the 2D
case, I suppose any joint moment that prevents collapse should be added.  So,
if we assume that the hip abductor moment is maintaining the support of the
pelvis, then maybe the equation might include it.  As for the knee and ankle,
I'm not too sure how they might fit the model.

My experience is that the support moment is appealing to some of my colleagues
with a clinical background because it seems to address the issue of function.
By that I mean, as biomechanists we can measure almost infinitesimal changes in
a wide range of variables associated with movement.  We can collect sufficient
data to show statistical significance in these changes.  The question my
clinical colleagues often ask, though, is, can you demonstrate that the person
can perform the task better/faster/more efficiently, etc.than before?

Dr Vaughan's distinction (below) between internal and external forces and
what the system is, while being quite valid, may be somewhat irrelevant to
some of the readers of this listserver, only in that, I believe, they are
looking for answers to improving function rather than developing Newtonian (or
otherwise) models.  Our job is to explore topics such as the support moment in
an objective manner and present our findings for peer review.  What anyone
believes after that is up to them.

Drew Smith, PhD
--
Andrew W. (Drew) Smith, PhD
211 Lee Avenue
Toronto ON Canada M4E 2P4
Voice:  +1 (416) 699-9506
Email Address: awsmith@globalserve.net


I would like to add some comments on the support moment discussion.
Many of what will follow is based on discussions with Bert Otten,
also from our lab.

In general I agree with Kit Vaughan : '"....the support moment (Ms) is an
inappropriate mechanical parameter..." Its first characteristic, to
begin with, that it resembles the vertical ground reaction force
(Fy), is only very superficial: the resemblance is no more than that
it has also two peaks. The 'trough' in the middle is much lower in Ms
than in Fy, because in midstance the leg is fairly straight and knee
and hip moments are both low.

The second  characteristic, the low CV, has some mechanical
explanation, however. It is hard to send pictures by e-mail, so 
please draw one for yourself, of a leg with ankle, knee and hip, all
somewhat flexed, and with the F originating from the centre of
pressure, somewhat in front of the ankle, its line of action behind the
knee and in front of the ankle.

The ankle moment Ma is then about F x orthogonal distance between F
and ankle. Idem for knee Mk and hip Mh. This is a quasi static
approximation, but not far off for stance.

Ms is defined as Ms = Ma - Mk + Mh, i.e. extension moments are
summed.

In walking the kinematics,e.g.  the joint angles as a function of
time, are very stereotypical, in contrast to the moments.
Now two things are possible (usually in combination)

I think this simple mechanical picture may explain the main features
of Ms, but it also indicates that these properties are fairly
trivial.

In the above we saw that Ma, Mk and Mh are determined by, can be
calculated by inverse dynamics from:

  1. 1) the positions of ankle, knee and hip joint
  2. 2) the position of the COP, and
  3. 3) magnitude and direction of F
On the other hand, 2) and 3)  are completely determined by the laws of
conservation of linear and angular momentum, very roughly, by the
trajectory of the body center of mass.

In this discussion it may be interesting to consider what is cause and
what is effect. In my opinion the three muscle moments are the cause and the
movement, especially the trajectory of the body CoM,
is the effect. The moments should ensure that the leg does not
collapse, that the trunk is not lowered too much,
and that some forward propulsion is generated. The ground
reaction force is just a manifestation of the three moments (the
causes) or a reaction on  the movement (the effect) , very
convenient if you can measure it with a force plate, but no ''prime
mover'' in any way.

This is not to say that in controlling the movement the GRF is
completely unimportant. For example, it should alway be ensured the
the CoP is not too far from the base of support, so that balance can
be recovered. My thesis, however, is that it is a secondary
manifestation of the events that are really important.
I am interested in your comments.

At Hof
Vakgroep Medische Fysiologie
University of Groningen
Bloemsingel 10
NL-9712 KZ GRONINGEN

Tel:   (31) 50 3632645
Fax:   (31) 50 3632751
e-mail: a.l.hof@med.rug.nl


Dear all,

Thanks for all the comments about the support moment.

I agree with both sentiments: that its biomechanical validity is
suspect, but also that we really do need some tools for assessing
inter-joint (and inter-limb) coordination and function. In this regard,
it occurs to me that the concept of limb power, in which the ankle, knee
and hip powers are algebraically summed, must also be based on the same
unstable foundations.

What I'd really like to know is whether anyone is using these measures
in routine clinical gait analysis, and if so, how? What do you look for,
and how do you interpret abnormal curves? Or has anyone come up with any
alternative/better holistic measures of limb function?

Chris
--
Dr. Chris Kirtley
Dept. Rehabilitation Sciences
The Hong Kong Polytechnic University
Hong Kong
Special Administrative Region of The People's Republic of China 


Chris wrote;
In this regard, it occurs to me that the concept of limb power, in which the ankle, knee
and hip powers are algebraically summed, must also be based on the same unstable foundations.

this is based on even poorer foundations because these are scalar
quantities and a summation has little meaning, but I don't think
that diminishes the qualitative value of looking at the relative
power contributions at different levels.  I feel that too much
significance is attached to power data anyway. The suposition that
peforming a psoas release over the brim doesn't significantly effect
hip function because hip power at toe off is not significantly
effected is a case in point.

Jeremy Linskell
Manager, Gait Analysis Laboratory
Co-Ordinator, Electronic Assistive Tehcnology Service
Dundee Limb Fitting Centre
Dundee, DD5 1AG, Scotland
tel +1382-730104, fax +1382-480194
email: j.r.linskell@dth.scot.nhs.uk
web: http://www.dundee.ac.uk/orthopaedics/dlfc/gait.htm


Dear Chris,
First of all, best wishes for the new year 2001!
In 1998 there was a lively discussion about the support moment on the
CGA list. As it goes, the problem kept on the background in my mind
for some time. In the course of a completely different project I then
hit on a calculation, that had some relevance for the support moment
problem. In short, it has resulted in a short paper:
A.L. Hof, On the interpretation of the support moment. Gait and
Posture 12:196-199 (2000).
Maybe a link to this paper can be made in the archive webpage, for
the case somebody still looks there....
 
Greetings,

At Hof
Department of Medical Physiology &
Laboratory of Human Movement Analysis AZG
University of Groningen
A. Deusinglaan 1,  room 769

PO Box 196
NL-9700 AD GRONINGEN
THE NETHERLANDS
Tel:   (31) 50 3632645
Fax:   (31) 50 3632751
e-mail: a.l.hof@med.rug.nl

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