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  • Standardization

    A couple of days ago I received an E-mail message from Biomch-L
    with a draft of the report on "Recommendations for Standardization in
    the Reporting of Kinematic Data", from the ISB Committee for
    Standardization and Terminology. Almost at the same time, I received
    through regular mail ("snail mail") a hard copy of the same report
    (Draft version 4.0 of March 23, 1992, ISB Newsletter 45:5-9, 1992.)

    I welcome the drive to standardize terminology and conventions,
    although I think it is going to be difficult to come up with reference
    frames and conventions that will apply in a satisfactory way to a joint
    at all points of its range of motion.

    The plan is to develop a standard set of conventions for the
    presentation of data in the refereed literature in Biomechanics.
    Therefore, the final result of this project will affect ALL OF US, and
    I am glad that Peter Cavanagh, Chair of the Committee for
    Standardization and Terminology is asking for input. I think Biomch-L
    would be a good place to establish a public dialogue on this topic,
    because the "criss-crossing" of comments may stimulate the thinking of
    readers that might otherwise remain passive. Since Peter is a Biomch-L
    subscriber, the comments will reach him too. My hope is that the
    process will be constructive.

    Being the lazy bum that I am, I would much prefer to watch "from
    the sidelines" as this whole question unfolds. However, I feel that
    this issue is much too important for that, and I have no option but to
    contribute my own "two-cents' worth". So here it goes.

    (My comments will refer to the hard-copy draft mentioned above,
    rather than to the electronic copy distributed through Biomch-L,
    because the former showed drawings which made it a better copy for me
    to read, but the text sems to be identical in both.)


    In "Part 1: Definition of a global reference frame", the proposal
    recommends that a global XYZ reference frame be adopted, with the X
    axis horizontal and pointing forward (in the general direction of the
    motion), the Y axis vertical, and the Z axis horizontal and pointing
    toward the subject's right. The rationale is that this will permit
    two-dimensional (2D) studies to use an XY reference frame and
    three-dimensional (3D) studies to use an XYZ reference frame, so the X
    and Y axes will be common in the 2D and 3D studies. I don't think this
    is a good idea.

    I would propose that in 3D studies the X axis should be horizontal
    and pointing toward the right, the Y axis horizontal and pointing
    forward, and the Z axis vertical. In 2D studies, a YZ reference frame
    should be used, with the Y axis pointing forward and the Z axis

    Rationale: The vertical axis is a special axis from a mechanical
    standpoint, because this is the only direction in which gravity acts.
    Therefore, we could classify the axes of the global reference frame
    into two categories: the horizontal axes in one group, and the vertical
    axis (the "special" axis) forming the other group (by itself). When we
    choose names for these axes, it would seem cleanest to give consecutive
    letters to the axes of each group, so X and Y for horizontals and Z for
    verticals would make sense.

    Why is the committee proposing instead X and Z for horizontals,
    and Y for verticals? I think it is because in 2D analysis X has
    traditionally been used for the horizontal and Y for the vertical, and
    the committee is (very correctly) trying to produce consistency in 2D
    and 3D terminology. But why have people generally used X and Y to
    label the reference axes in 2D analyses? Why not A and B? Because, to
    minimize confusion, it is better to use letters that are less common,
    such as the last letters of the alphabet. Why then do the people that
    do predominantly 2D analysis use X and Y instead of Y and Z? Because
    although they are studying a 2D problem, they realize that there does
    exist a third dimension, so they keep the last letter (Z) "reserved"
    for the remaining dimension, even though their problem is strictly 2D
    and no use at all will be made of the third dimension. Occasionally, a
    person that generally uses 2D analysis will need to make a 3D analysis.
    Since this person is used to seeing X and Y on the plane of the writing
    paper, it is only natural for that person to keep them that way, and to
    define Z as an axis normal to the plane of the paper, i.e., pointing
    horizontally if the drawing shows a side view of the subject.

    Present trends indicate that in the future, research work will be
    more and more in 3D rather than in 2D. And motion in reality is, of
    course, 3D and not 2D. So, why should we let the inertia of 2D customs
    dictate what our terminology will be in 3D analysis? Why let the tail
    wag the dog? If we want to have consistency between 2D and 3D
    analyses, I believe that it is the conventions of 2D analysis that
    should be modified to use Y for the horizontal and Z for the vertical.



    In "Part 1: Definition of a global reference frame", the axes of
    the global reference frame, fixed to the ground, and with origin at
    point O, are called XG, YG and ZG. (Sorry, I can't do subscripts in
    E-Mail!) I realize that the "G's" stand for "global", but the letter
    "G" is often used to designate the center of mass (or center of
    gravity), and therefore this terminology could be confusing. I would
    propose instead XO, YO and ZO.



    In "Part 2: Definition of segmental local center of mass
    reference frames", the local X axis is assigned to the
    anterior/posterior direction; the local Y axis to the proximo/distal
    direction; and the local Z axis to the medio/lateral direction. I
    don't think this is the best choice.

    As with the axes of the global reference frame (see "COMMENT A"
    above), we could classify the local axes of most segments into two
    categories: the anterior/posterior and medio/lateral axes in one group,
    and the proximo/distal axis (by itself) forming the other group. This
    is because the moments of inertia of the segments are generally large
    and similar to each other with respect to the anterior/posterior and
    medio/lateral axes, and much smaller with respect to the proximo-distal
    (longitudinal) axis. Therefore, when we choose names for these axes,
    it would seem cleanest to give consecutive letters to the axes of each
    group, so X for medio/lateral; Y for anterior/posterior; Z for
    proximo-distal would make sense. This terminology would also fit well
    with the terminology proposed in "COMMENT A" above for the global
    reference frame.



    In "Part 2: Definition of segmental local center of mass
    reference frames", the committee puts forward two alternative opinions
    with respect to the definition of the segmental local axes:

    The "majority opinion" in the committee proposes that, for all
    segments, the medio/lateral axis should point toward the right, the
    anterior/posterior axis forward, and the proximo/distal axis in the
    proximal direction;

    The "minority opinion" (Prof. John Paul) proposes that left-handed
    reference frames be used for the segments on the left side of the body.

    I feel that this is an important point, and that more thought
    needs to be given to it. I have not been able to decide which is the
    best way to go in this, but here are some of my thoughts on this
    question. The problem with the proposal of the MAJORITY OPINION is
    that a positive torque (or angle change) in the internal/external
    rotation direction will mean "internal rotation" in one leg, but
    "external rotation" in the other leg. The same problem will occur with
    abduction/adduction, while flexion/extension will be OK. With respect
    to LINEAR kinetics, positive forces in the medio/lateral direction will
    mean medial forces in one leg, but lateral forces in the other; forces
    in the other two directions will be consistent in both legs.

    What would happen if, instead, we sought the goal of Prof. John
    Paul's MINORITY OPINION, in which adduction (and presumably also
    internal rotation, although he does not mention this explicitly), as
    well as flexion, would have the same sign in both legs? For this (and
    using X for medio/lateral; Y for anterior/posterior; and Z for
    proximo-distal as proposed in COMMENT C above), the right thigh would
    have an X axis pointing toward the right, a Y axis pointing anteriorly,
    and a Z axis pointing from the right knee toward the right hip, while
    the LEFT thigh would have an X axis pointing (AGAIN!) toward the right,
    a Y axis pointing posteriorly, and a Z axis pointing from the left hip
    toward the left knee.

    The surprise is that the reference frame just defined for the left
    leg would still be RIGHT HANDED! So left-handedness does not seem to
    show up as a problem at all!!

    However, while I can't see a problem with respect to angular
    kine(ma)tics, I do see one with respect to LINEAR kine(ma)tics. For
    instance, a force in the positive Z direction would be pointing
    proximally in the right thigh, but distally in the left leg; similar
    inconsistencies would also occur in the X and Y directions.

    I have not found a perfect solution, so I think we have a hard
    choice. But I think I would rather use the modified reference frame
    last described, because with that reference frame the three torques
    will be consistent in both legs, and in my opinion, joint torques are
    more important parameters than joint forces. If there is a sacrifice
    in consistency, I would rather make it in the joint forces than in the
    joint torques. Also, to make the forces be consistent in the right and
    left side segments, all that would be needed would be to reverse the
    signs of all three components (X, Y and Z) in the left side segments.
    That may actually be cleaner than reversing only the sign of the forces
    in the medio/lateral (X) direction, which is what would be needed if we
    used the original reference frame (see 4th paragraph before this one).



    In "Part 1: Definition of a global reference frame", the proposal
    recommends that the horizontal axes of the global reference frame
    should point respectively to the right and forward. It states that
    "All directions are given for the subject facing the direction of
    working or travel that is of most interest to the particular activity".
    Such a definition makes sense for a gait analysis situation or a
    javelin throw, for example. However, it will not always be appropriate
    for other activities, such as a Fosbury-flop high jump, where the
    athlete at the end of the run-up will be moving in a diagonal direction
    with respect to the bar. In the high jump, a reference frame oriented
    to fit with the final run-up direction can sometimes be useful, but
    often a reference frame aligned with the high jump BAR is still more
    useful. Therefore, I think the requirements for the directions of the
    horizontal axes should be redefined to admit occasionally directions
    that are orthogonal but not aligned with the direction of working or
    travel of the subject.



    In "Part 1: Definition of a global reference frame", the
    committee proposes that the origin of the global reference frame be
    located at an important point for the activity being analyzed (for
    instance at the center of one of the force plates being used). I agree
    with this, but in our laboratory we have added an extra "twist" that we
    find very useful.

    In our laboratory, we typically add 10.00 meters to all
    horizontal coordinates. In effect, this puts the origin of the global
    reference frame at a point located at a nice, round, number of meters
    away from the "important point". Using a reference frame where Z is
    the vertical, this makes the "important point" have coordinates (10.00,
    10.00, 0.00). This eliminates negative coordinates, and we feel that
    it facilitates the interpretation of the data. For instance, instead
    of saying that the center of mass moved from position X1 = -0.12 m to
    position X2 = +0.08 m, and finally to position X3 = +0.28 m, our data
    would say that the center of mass moved from position X1 = 9.88 m to X2
    = 10.08 m, and finally to X3 = 10.28 m. Our way of expressing the data
    still allows us to see clearly that the c.m. passed over the X
    coordinate value of the "important point" (it is just as easy to see
    that the c.m. goes through the X = 10 meters position as it would be to
    see that it goes through the X = 0 meters position), but our way of
    expressing the data also allows us to see very easily in this example
    that the changes in position in each of the two intervals are constant
    delta X values of 0.20 meters (9.88 ... 10.08 ... 10.28), while using
    the other method, with the important point at (0.00, 0.00, 0.00), this
    would be harder to see (-0.12 ... 0.08 ... 0.28). So we feel that the
    change in sign associated with the important point being at position
    (0, 0, 0) produces some confusion which can be avoided by making the
    important point be at (10, 10, 0) and thus keeping all coordinates



    In "Part 5: Relative attitudes", I believe that the labels of
    vectors X2 and Y2 in Figure 3 (ISB Newsletter) were mistakenly



    I believe that the biggest bone of contention is going to be in
    "Part 5: Relative attitudes", in the formulation of the joint rotation
    conventions for each joint of the body. That is going to be a tough
    nut to crack. We'll cross that bridge when we get to it ...



    In "Part 6: Joint Moments", I believe that the word "moment"
    should be replaced by "torque". The word "moment" must be the most
    overused one in mechanics. What would you rather see, "The force
    exerted a large moment at that moment because of its large moment
    arm.", or "The force exerted a large torque at that instant because of
    its large moment arm." ? I would hate to see the word "moment"
    enshrined as the official term for "torque" in biomechanics.



    In "Part 6: Minority report", "With reference to part 1:", it is
    proposed that the letters XYZ be avoided. The rationale is that many
    equipment manufacturers have already formatted data according to their
    own XYZ system, and awkward transpositions, such as Yisb = Zkistler
    could be avoided by using other letters in the proposed ISB
    standardization, for instance the letters RST. I don't agree. I don't
    think ISB should relinquish to the manufacturers the "exclusive" right
    to use XYZ!!

    Jesus Dapena
    Department of Kinesiology
    Indiana University
    Bloomington, IN 47405