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Re: ISB Standardisation Document

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  • Re: ISB Standardisation Document

    To the Biomch-L readership:

    I agree wholeheartedly with the Yeadon/Bogert/Legnani
    counterproposal for standardization.

    Comment #1: Cardan angles in the order X-Y-Z (i.e., successive
    rotations about the mediolateral, anteroposterior and longitudinal axes)
    should be a good and logical way to express segment orientations. I believe
    that this should be the "standard", which should be used ***almost***
    always. At the same time, we have to keep in mind that such an order (like
    any other order) will produce "gimbal lock" at certain orientations. (This
    is where some of the angles "go crazy" because there are two very different
    combinations of numerical values of the three angles that produce very
    similar physical orientations of the segment.) Gimbal lock should be
    avoided. A different Cardan angle order will achieve this. That is why I
    consider it very important also that the new proposal allows for deviations
    from the standard when necessary. There is no angle order that will prevent
    gimbal lock everywhere. Any angle order has a "ugly orientation" that
    produces gimbal lock. That is why we should not force any given order on
    every situation.

    Comment #2: I would like to recommend that the X axis generally
    should point toward the right, and the Y axis forward, instead of the
    alternative (X axis forward and Y axis toward the left). This is not really
    a very critical issue, but here is why I think that X toward the right and Y
    forward is somewhat better:

    When I am analyzing a person walking or running forward, I generally
    think of myself as an observer that is traveling with the subject. I feel
    that this helps me to gain a more intuitive understanding of what the
    subject is experiencing. When I am considering the values of forces or
    velocities in the horizontal plane, I think of myself as the subject looking
    downward at this plane. Then, if I am using the X=right/Y=forward
    convention relative to the direction of motion, when I look at the ground I
    will see the positive X direction pointing toward the right and the positive
    Y direction pointing forward, which is how I generally draw these axes when
    I make any graph on a piece of paper: I am in familiar territory. But if I
    use the alternative convention X=forward/Y=left, then when I look at the
    ground I see the X direction pointing forward and the Y pointing toward the
    left, which is more unfamiliar. I think most people would probably feel
    more comfortable with X=right/Y=forward.

    Jesus Dapena
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