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RE: Body segment axes

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  • RE: Body segment axes

    Dear BIOMCH-L readers,

    In last week's posting, Herman Woltring raised
    the question of how to define body segment axes and joint rotations.
    This is indeed a fundamental problem in basic as well as applied
    biomechanics, which can only be solved by some sort of standardization.
    My own experience on this subject comes from 2-dimensional multibody
    modelling, and (to a lesser extent) from clinically oriented 3D
    kinematic analysis on horses.

    1. Multibody models are commonly used for (inverse) dynamic analysis of
    movement, using measured kinematics as input. Usually, these models
    assume hinge or ball joints between the body segments. I think that
    this is the reason why in these models the 'joint centers' are often
    used to define the long axis (being one of the coordinate axes) of the
    segment. This definition also slightly simplifies the equations of
    motion. In 3D models a second axis must be defined somehow, usually the
    lateromedial axis.
    Kinematic data, obtained from landmarks placed at arbitrary points on
    the segments, can be used for dynamic analysis if the positions of these
    points with respect to the segmental axes are known. For my 2D work, I
    obtained this information from radiographs showing the 'joint centers',
    as well as the kinematical landmarks (marked by steel rings).
    I think this method has a few serious drawbacks:

    - It relies heavily on the assumption that joints have a fixed center of
    rotation, and that these points can somehow be identified.
    - For 3D applications, the definitions are not sufficiently strict, and
    transformation of marker data to rigid body kinematic variables
    depends on marker coordinates with respect to the segmental reference
    frame. It is difficult to obtain this information.

    Possibly it would be wiser to define standard segment axes with help of
    three well-defined points on the outside of each bone. If these axes do
    not coincide with the traditional anatomical axes, this will have to be

    2. For clinical applications, it is practically inevitable to use
    segment axes based on external markers. For this reason, I tend to
    prefer methods of kinematic analysis that give results that are (within
    certain limits) independent of marker placement. This is the best way
    to ensure that data obtained from different recording sessions or
    different individuals can be compared safely. In fact, this seems to be
    the way most software supplied with kinematic analysis systems works.
    A typical analysis method is to put two markers (proximal and distal)
    on each body segment, and make sure that during the recording the
    walking direction is along one of the coordinate axes (e.g. the Y-axis)
    of the laboratory reference frame. Joint angles are obtained from
    projections of the 'stick diagrams' on the sagittal (YZ) and frontal
    (XZ) planes. Incorrect marker placement will only produce a constant
    error in the angles, and it is easy to extract parameters from the
    signals that are insensitive to this.
    Of course, the example given above does not provide data for all six
    degrees of freedom (DOF) of the body segments. In my opinion, this loss
    of information is not too serious for clinical applications because the
    set of kinematic variables describing human movement is by no means
    independent. A more practical reason for this 2x2D approach is, that a
    complete 3D analysis of, for example, the femur requires a simultaneous
    view of markers on the medial and lateral condyles. The system we are
    currently using (CODA-3) does not allow this, you would need a video-
    based system with many cameras.


    - For basic research (requiring full 3D kinematic data, 6 DOF per
    body segment) standard segmental axes should be adopted, based on
    the same palpable bone landmarks where the markers are attached.

    - In clinical biomechanics, we should only measure those variables that
    are reliable and reproducible. If a 3D analysis produces less reliable
    results or requires too much effort, stick to a 2D (or 2x2D) approach.

    These are my own personal opinions, and I would welcome further
    discussion on BIOMCH-L about this subject.

    Ton van den Bogert
    Dept. of Veterinary Anatomy
    University of Utrecht, The Netherlands.