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Surface markers in 3D analysis

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  • Surface markers in 3D analysis

    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
    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
    ************************************************** ****************************
    Alberto Leardini
    Oxford Orthopaedic Engineering Centre
    University of Oxford
    Headington, Oxford OX3 7LD ENGLAND
    tel: ++ (0)1865 227688
    fax: ++ (0)1865 742348 email:
    "But the man that is will shadow
    The man that pretends to be" T.S. Eliot
    ************************************************** ****************************