CGA FAQ: Foot models

I have a practical question about the foot model. I think both the VICON
and ELITE use similar protocols, so the problem is not equipment
specific.

When placing markers on the foot, the ELITE system requires one on the
5th met. head, one on the lat. malleolus and one on the calcaneus so
that it's height from the floor is the same as that of the 5tm met.
marker. This is needed so that the neutral position of the sole of the
foot can be measured and then used to calculate ankle position during
gait with a correct offset.

The problem rises when we test the same subject with shoes on, let's say
with a heel raise on one side. The 5th. met. marker is reapplied on the
shoe, the lat. mall. remains. The question is where to stick the marker
on the heel.

There are two possibilities:
1 Sticking it on the shoe so that the marker's height is the same as
that of the 5th met., measured from the floor.
2 Sticking it on the shoe over that point of the calcaneus where the
marker was in the barefoot test.

ad 1:
-The shoes/raise becomes a part of the foot functionally, and so the
functional surface of the foot is the sole of the shoe. Let's say,
without shoes/raise the ankle would be in plantarflexion at contact.
With the raise at initial contact the ankle angle would change to
neutral reflecting the changed function of the foot, even resulting in a
first rocker. (The foot inside the shoe might still be in plantarflexion
though.)
-The foot with shoe only and the foot with shoe+raise are comparable,
because the functional surfaces of the "feet" are considered.
-The marker position can be checked accurately (the heights from the
floor should be equal).

ad 2:
-We measure the position of the foot inside the shoes (if we assume that
the heel does not move inside the shoe). We know that there is a raise
on one side and we can take that into account when looking at the
results.
-The accuracy of heel marker placement cannot be checked easily.

My question is, which of the two protocols is best?
Related to that: what are your feelings about taking markers off and
reapplying them on the shoes, and then comparing the two sets of data? A
recent paper hasn't found differences in gait with or without shoes (no
orthoses).

Gabor

-- 
Dr Gabor Barton MD                  email: G.Barton@gaitlab.demon.co.uk
Clinical Scientist
Gait Analysis Laboratory
Alder Hey Children's Hospital       tel:  +44 (0)151 252 5949
Eaton Road, Liverpool               fax1: +44 (0)870 052 1935
L12 2AP, UK                         fax2: +44 (0)151 252 5846



Gabor

I will add my 'tuppence' worth based on knowledge of VCM. The heel marker is
used to define the angle offsets, from the ankle joint centre to the 5th met
marker, for definition of the alignment of the sole of the foot. The
crudeness of the model means that p/d flexion are really defined as the
sagittal plane orientation of the sole of the foot relative to the tibial
axis system. Accepting that any pro/supination will introduce significant
errors into the results I would say that I would always try and reflect the
alignment of the sole of the foot when placing the heel marker. If you place
a heel raise under a shoe, having already performed a static test, then the
results should be valid (as long as the ankle and 5th met markers have not
been disturbed in the process). If comparing barefoot against shod, or
comparing 2 different shoes, then each heel marker placement should reflect
the alignment of the sole of the foot in that state. They will obviously be
estimations, but no better than the model can support. If you just remember
that you are modifying the 'offset' angles to reflect sole alignment, then
you shouldn't go far wrong.

Hope that helps

Jeremy Linskell <j.r.linskell@dundee.ac.uk>

There is a BodyBuilder model of the foot - downloadable from the model
depository at: /bodybuilder - that
Martin Lyster wrote before he left Oxford Metrics
<martin.lyster@the-data-company.com is his new address>. It uses a
virtual talus, though, since there is no practical way of getting
markers on that bone: /faq/foot.gif

It's a beautiful model, but is a demo only - it is pretty useless in
practice since motion capture of so many closely situated markers is
very difficult, particularly (I suspect) with Vicon (there was some
discussion of this at the Minnesota Vicon Users' Meeting, I think).
There are also problems with skin-bone motion artefact - see:

Reinschmidt, C., van den Bogert, A.J., Lundberg, A., Nigg, B.M., Murphy,
N., Stacoff, A., & Stano, A. (1997). Tibiofemoral and tibiocalcaneal
motion  during walking: external vs. skeletal markers. Gait & Posture,
6: 98-109.

Briefly, skin markers tend to overestimate linear (translation) and
angular (rotation) displacements - the patterns of motion are similar,
but amplitudes are different.

Note, too, that many people stick the toe (2nd metatarsal head) marker
too anteriorly in routine gait studies, and thereby overestimate ankle
range of motion, velocity and power.

I suspect some sort of compromise marker set would be best - for example:

• Lateral malleolus
• Os calcis
• 5th metatarsal head
• Dorsum (e.g. navicular)
• Hallux

Chris
--
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

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