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summary: FOB & pelvic motion

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  • summary: FOB & pelvic motion

    thanks to the folks who replied to the post appended below:

    >we're currently validating the use of FOB to monitor pelvic
    movement as people with SCI reach while seated.
    validation is by using radiographs, but our question
    arose while doing repeatability
    we've attached birds in 2 manners to measure pelvic tilt (pitch), both
    with pros and cons:
    1) birds affixed to the ASIS and PSIS
    pro: sensors are on ilium, the target structure
    con: many folks have substantial tissue over these prominences (location &
    movement error)
    2) single bird affixed to sacrum via sacral belt
    pro: much, much easier to don/doff and much less tissue over bone
    con: movement of SI joint potentially adds error

    data shows excellent repeatability of both approaches, but we'd be
    curious to hear
    if anyone has tackled a similar issue or has opinions on SI mobility in the
    seated posture of persons
    with SCI.

    We have used Ascension's FOB in our product, The MotionMonitor, for the
    6 years. It has been our experience that placement on the sacrum is
    superior to placement on other areas that involve more soft tissue
    With a tight application to the sacrum, we then digitize joint centers and
    other landmarks that move rigidly with the sacrum and pelvis. While we
    not validated these with radiographs, we have used Leardini and Cappozzo's
    femur rotation method to locate hip centers and find that the results
    correspond nicely with his data which, if I recall correctly, did include
    validation by radiographs.

    Let me know if I can be of further assistance.
    Lee E. Johnson
    tel: 773-528-1935
    fax: 773-528-2149
    Innovative Sports Training, Inc.
    ....The Total Solution in Motion Capture.

    Go with the sacrum (use two sensors however). The overwhelming evidence
    the lack SI joint motion supports this choice among other reasons.
    The limited SI motion that is possible, only occurs with extreme
    end-of-range stress ( as in doing the splits), and SI motion in general, is
    only clinically significant in pregnant women, young women ,and hypermobile
    pathological conditions and trauma.

    The limited SCI pelvic motion in a chair is highly unlikely to elicit any
    significant stress on the SI .

    If you really want to be sure, then do both and compare on a group of
    subjects. - Digitize three points on each illium in several positions
    recording from the sacrum sensor simultaneously. Reconstruct the euler
    angles and compare the pitch.

    Kevin J. McQuade PT,PHD
    Rehabilitation Research Fellow
    University of Maryland School of Medicine
    Balitmore, Maryland 21201

    if you just go with the sacrum. using two is just a way of increasing the
    accuracy because the different placements will have different skin slippage
    and with two you can take the centroid or use some other algorithm to
    average out the error, or at least know what it is.

    define your sensor orientation such that the largest motion you want is
    azimuth - that is best for euler angles and will avoid any gimbal lock
    associated with euler rotations.


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