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ISB Standardisation Document

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  • ISB Standardisation Document

    The following message is the outcome of email discussions on the topic of
    standardisation by Fred Yeadon, Ton van den Bogert and Giovanni Legnani.
    Since the standardisation proposal is "on the table" interested parties
    should (a) accept it as it stands (b) propose changes. We think that changes
    are needed and as an initial step have made some comments on the proposal.

    Comments on the document:

    Standardization and Terminology in Biomechanics.
    Volume 1. Section A.
    Recommendations for standardization in the reporting of kinematic data.

    It should be borne in mind that the purpose of standardisation is to have a
    common convention for REPORTING data so that the results of different
    researchers can be interpreted and compared more easily. Any discussion on
    this document should bear this in mind and avoid issues of what conventions
    are best used for data analysis. It is the reporting of results that is the
    present focus of the standardisation document.

    In the "Recommendations" document:-

    Part 1. X, Y, Z axes for displacements

    It should be recognised that there IS general agreement on the directions of
    the three Cartesian axes in that they are mutually perpendicular, one axis is
    vertical, and the directions of the remaining two horizontal axes are not
    usually contentious. There is some disagreement as to the NAMING (XYZ) of the
    three axes. There is also the issue of the senses of the axes in that there is
    general agreement that "up" is positive whereas "left" may be positive or
    negative. The system proposed in the standardisation document seems to be
    supported by people who have initially used 2D analysis and have added the a
    horizontal Z axis for 3D in order to preserve consistency between 2D and 3D.

    Unfortunately the majority of researchers in biomechanics seem to favour a
    system in which the Z axis is vertical. This does not augur well for the
    acceptance of the standardisation proposal in its present form.

    However, since there is general agreement on the directions of the axes and the
    focus is on the reporting of data, standardisation could take the form of naming
    these axes with words rather than with letters. For example, it might be
    possible to use "forward", "lateral", and "vertical" to name these directions.
    The point is not that particular names be cast in stone but that it become
    common practise to use appropriate words rather than XYZ.

    Another issue is that the XYZ system used should fit well with the system used
    to describe rotations. This will be addressed after the comments on Part 4 and
    Part 5.

    Part 2. Definition of segmental reference frames

    The system proposed in the standardisation document is consistent with the
    system proposed for the absolute reference frame in Part 1. However, as noted
    above, most researchers are not in agreement with the adoption of the proposed
    Part 1. An alternative will be proposed after considering relative orientation
    in Part 5.

    Part 4. Absolute Orientation

    In the standardisation document it is proposed that the absolute orientation of
    a body segment be defined by successive rotations about lateral, longitudinal
    and frontal axes. Since the system for defining the orientation of the whole
    body has not yet been published the present proposal prejudges the issue.
    Recommendations have been made to the Standardization Committee on whole body
    orientation as detailed in:

    Yeadon, M.R. 1990. The simulation of aerial movement. Part I: The determination
    of orientation angles from film data. Journal of Biomechanics 23, 59-66.

    In this system successive rotations are made about the lateral, frontal and
    longitudinal axes. This order of rotations has the advantage that the Cardan
    angles of somersault, tilt and twist agree with the nomenclature used by
    gymnasts and coaches. Since the focus is on the reporting of data such
    considerations of the established use of terminology are of great relevance.

    Part 5. Relative Orientation

    The description of the system proposed here appears to be somewhat ambiguous but
    our interpretation is that successive rotations about lateral, frontal and
    longitudinal axes bring one segment from initial alignment with a more proximal
    segment to its final relative orientation. That is we interpret the proposal
    as stating that the system described in the cited publication (Grood and Sunday,
    1983) is to be adopted. If this is so then we are in agreement with this part
    of the proposal and expect that most researchers will also find it acceptable.
    On the other hand this definition is in conflict with that proposed in Part 4,
    suggesting again that Part 4 be changed.

    Part 1 and Part 2.

    It would be attractive to think that articles could use Tables in which axes
    were labelled with words rather than X, Y and Z but since this is unlikely to
    occur there is a case for having a standard system. One rationale would be to
    let the definition of orientation dictate the axes nomenclature by requiring
    sequential rotations about the X, Y and Z axes. This leads to the adoption of
    a system with Z vertical (longitudinal).

    One final note of caution on the subject of standardisation:

    There is a danger if recommendations were to be adopted formally by an academic
    journal. Any standardised system will need to be able to adapt to various needs
    and changing circumstances. For different types of arm movements it may be
    helpful to use different systems of describing orientation. It will not be
    helpful if systems are cast in stone from on high. Researchers need the freedom
    to choose what they think is best bearing in mind the conventions prevalent at
    the time. For example even if whole body orientation is "standardised" in some
    way, it should not be imposed on all activity. What is a sensible system for
    gymnastics may not necessarily be sensible for high jumping. It should rest
    with the researchers of high jumping to arrive at a useful system bearing in
    mind what already exists.

    Fred Yeadon
    Ton van den Bogert
    Giovanni Legnani

    M.R. (Fred) Yeadon