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unknown user
12-11-1995, 09:04 PM
I apologize for the delay, but I am very interested in the discussion
about surface marker use in 3D analysis of human movement. I would also
thank Paolo who has mentioned our paper on Clin Biomech 1995. It seems to
me that it may be relevant in the discussion context. Just few thoughts
coming from the paper and from my experience, related to the specific
question.
1) All of us are interested in obtainig movement data associated with the
anatomy and function of human body. Anatomical landmark trajectories,
center and axes of rotation, inertia parameters (as suggested by Paolo),
joint angles etc. All the efforts to define and to standardize such
important variables ought to be addressed to the clinical relevance,
anatomical literature and 'normal' joint functions
2) In order to obtain quantitative data about bone anatomy, position, and
orientation during human activities, we must use stereophotogrammetric
techniques and 'points' on the skin surface (markers or other detectable
references) or visually estimated joint centre. As mentioned by other
people this latter technique may be very inaccurate, and its assumption
of the joint as 2 DoF mechanism may be not sufficient to describe its
function in clinical and also in sport context. Stereophotogrammetric
techniques introduce experimental sorces of error and difficulties in
collecting marker trajectories, particularly for markers positioned in
awkward locations. This is particularly true in activities that require
wide movements and in sport performances. The data collection would
suggest to pay attention to resolve the 'technical' problems related to
the specific trials under analysis
3) Due to the particularly easy procedure for transforming location and
orientation of a system of reference to another, all the previuos
considerations have suggested to separate the 2 totally different
problems and to solve them in their specific field. The trasformation
(utilized by many people, even when not clearly stated) needs to know the
local position of relevant anatomical landmarks in the frame obtained
from surface markers. This may be obtained with an easy procedure
suggested in the paper, or using equations (as suggested in many other
papers), or from other considerations. The definition of the 'technical'
frame, may therefore be related just to marker visibility problems,
minimizing the areas of marker missing
4) The use of surface markers would not introduce FURTHER sources of
error. Their placement irrespectively to anatomical location allows for
the study of their optimal design, geometry and position, according only
to optimization criteria and according to possible future findings about
areas on the skin that show minor skin movement artefacts. The use of a
redundant number on them, may also be used to minimize stereophotogrammetric
and eperimental sources of error. Finally, their
placement without reference to bony landmark, may minimize the number of
cameras that have to be used, and therefore the cost of the equipment.

Hoping that would be useful
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Alberto Leardini
Oxford Orthopaedic Engineering Centre
University of Oxford
Headington, Oxford OX3 7LD ENGLAND
tel: ++ (0)1865 227688
fax: ++ (0)1865 742348 email: alberto.leardini@ooec.ox.ac.uk
"But the man that is will shadow
The man that pretends to be" T.S. Eliot
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