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    Dear BIOMCH-L readers: a couple of months ago, I asked some questions regarding
    motion analysis equipment for the MAC. I received several replies, which I have
    finally got around to compiling. Thanks very much to all who replied.

    __________________________________________________ ___________________________

    My original posting was:

    Dear BIOMCH-L readers:

    We are starting a new orthopaedic biomechanics research laboratory at San
    Francisco General Hospital. One equipment item we are considering to purchase is

    a cinegraphic motion analysis system for either 2-D or 3-D studies. I wanted to
    ask BIOMCH readers two questions related to this:

    1) do you know of any motion analysis systems which can be hooked up to a
    Macintosh computer (with a PowerPC chip and National Instruments A/D board)
    for data acquisition and control? The only one I have heard of is from a
    company called MacReflex. Do any BIOMCH readers have experience with this

    2) how many cameras are required for acquisition of 3-D kinematics? I have
    heard responses which vary from four cameras to more than six.

    I will post all responses. Thanks.

    Steve Robinovitch, Ph.D.
    Asst. Professor, Dept. of Ortho. Surgery (UCSF)
    Director, Biomechanics Laboratory
    San Francisco General Hospital
    tel: (415)206-6864
    FAX: (415)206-3288

    __________________________________________________ _________
    Hi Steve:

    About two years ago I looked into purchasing a motion analysis system for
    our lab (UCSF/UCB Ergonomics lab). At that time, and I don't think much
    has changed since then, the only one is the MacReflex system. Lawrance
    Livermore Labs have one that they purchased about two years ago. We
    really wanted to go with a mac based system but the MacReflex wasn't the
    best deal. We went with a Selspot system working off a 486 machine. We
    recently went to the new intel pc chip (pant...). Ron Tal, of our lab,
    investigated connecting the selspot system to the Mac and realized it was
    a very large scale effort, but doable. We stayed with the pc system,
    though. You can purchase a A/D board to go with the selspot, but it cost more
    than an NI board.

    To do 3-d analysis, you need a minimum of two cameras. The requirement
    for more camera usually is dependent on the range of motion and the
    marker placement you require. For example Bob Mann's lab at MIT only
    used 2 camera system to analyze gait, however, people at SF shiner's
    hospital use 6 for their gait experiments.

    To get more information about both MacReflex and Selspot systems you can
    contact Manfred and Victoria Berger of Innovision systems in Michigan
    (they've probably already sent you e-mail - they subscribe to
    biomech-l). We have worked with them over the past two years and find
    them very helpful. Their e-mail address are

    I don't have the name and andress of the person at LNL with me. I'll
    e-mail you tomorrow with it.

    If you are interested in coming to our lab to talk about the selspot
    system we have please let me know. We're about a 30 minute drive from
    SFGH. We are using a six camera system to measure wrist position.
    We use a 2 camera system to measure finger motion during tapping.

    Good luck.
    510 231-9405.
    Jack Tigh Dennerlein ~0-0~ \- /

    __________________________________________________ ________________________

    The number of cameras that you need will depend on the pathologies
    you are investigating as well as the type of movement.

    In our case we wanted to do gait analysis of Spinal cord injured
    patients using FES-assisted walking. In turned out that for a
    bilateral analysis we needed at least 6 cameras because many markers
    were blocked from the different views as a result of all of the
    rotations of the segments.

    Good luck.
    Michel Ladouceur, PhD(c)
    McGill University
    School of P.& O.T.
    3630 Drummond, Room 105
    Montreal (Quebec)
    H3G 1Y5

    Tel: 514.398.4519
    Fax: 514.398.8193
    e-mail: is true that my tactic is to make sweeping categorical
    statements. Whether or not this is a fault ... is debatable. My own
    feeling is that it leads more quickly to the solution of scientific
    problems than a cautious sitting on the fence.

    E. Mayr, _The Growth of Biological Thought_, Harvard University
    Press, 1982, p. 9

    __________________________________________________ ________________________

    Hi, Dr. Robinovitch,
    I don't really know the answer to your first question. About your second
    question, theoretically speaking, you need at least two cameras to acquisit 3d
    kinematics. The purpose to use more than two cameras in 3D kinematics
    acquisition is to improve the quality of the kinematic data (minimize the
    need for interpolation and extrapolation and the constraints to the data
    aquisition procedure). The exact number of cameras needed really depends on
    the motion to be analyzed. Generally speaking, six cameras should be
    sufficient for most of the human body motions analyzed in an orthopedic
    biomechanics laboratory.

    Bing Yu, Ph.D.
    Orthopedic Bimechanics Laboratory
    Mayo Clinic
    Rochester, MN 55905

    __________________________________________________ ________________________

    Steve Re: the above query.

    We have a Lab set up which has Peak for low speed, and Motion Analysis Inc
    for high speed and auto analysis of video. About two years ago we wanted a
    qualitative anaysis system similar to the Peak Video Illustrator and decided
    to develop a Mac based system. This uses the Radius VidoeVision capture card
    and software in the Hypercard environment (The PowerMac 8100 series is the
    recommended Mac configuration). A version for qualitative analysis (VQAS),
    with limited quantitative capability, has been marketed for about the last 6
    months in NZ.

    A quantitative version of VQAS is under development to do "full" kinematic
    and inverse dynamic analyses. In the first instance this is most likely to
    be for 2D planar motion!

    I assume from your system description (ie use of the National Instruments
    A/D board), that you also use National Instruments Labview3 as an aquisition
    and software analysis package. We started with Labview2 on the Mac and are
    now upgrading to Labview3 (.1? runs "native") on the PowerPC. Teaching labs
    have 6100 series Powermacs. If you are not using Labview I recommend taking
    a look time at its application.

    Email if you need further info.


    Barry Wilson

    __________________________________________________ ________________________

    The Mac reflex system is, like nearly all Mac software, easy to drive. Its
    results look good. However, it is a direct recording system. The image on
    screen is only the marker points. If there are obscured points, it may be
    impossible to decide where the points should be. Also, if there is a
    problem in recording, the MOTION must be performed again. This may not be

    A Video based system eg PEAK allows re-digitising of obscured points, using
    manual estimation from the video image, allowing judgemment to be made on
    probable position. Reflections from the background can be also allowed
    However, this is not a real time system and the digitising process is very
    tedious and takes a long time, especially if there are problems with the
    time coding or other video hardware.

    > 2) how many cameras are required for acquisition of 3-D kinematics? I have
    > heard responses which vary from four cameras to more than six.

    This depends on the motion and how much obscuring of marker points takes
    place. Minimum of 2 cameras BUT this assumes all points are visible to both
    cameras. Using more cameras to show points that go missing on other
    cameras is possible BUT ensure that the software will allow the collection
    of dis-continuous data. The amount of digitising time required, especially
    when lots of manual estimation of missing points occurs, can be very large.

    You may also like to read my posting of "Practical PEAK suggestions", of 24
    October 1994.
    I can send this if you can't find it.

    John Yelland

    __________________________________________________ ________________________

    I am currently using a MacReflex 2D system with one camera in my
    biomechanics/ergonomics laboratory. It seems to work quite well for my 2D
    needs. I am currently seeking funding to purchase a 3D system and am
    shooting for a total of four cameras for the system, so I share your second
    question - how many cameras are required?? I suspect part of the answer
    has to do with the type of activities people will be doing, e.g., how fast
    will they be moving, potential for well hidden markers, size of person's
    excursion area, etc. Have you talked to Bill Hand, (707) 579-2596?? I'll
    be interested to hear what you find out since I share some of your
    questions. If you have any specific questions I can answer about the 2D
    system, let me know.

    Carter Kerk

    Carter J. Kerk, PhD, PE, CSP 236E Zachry Engineering Center
    Assistant Professor (409) 862-4149 Office
    Industrial Engineering Department (409) 847-9005 Fax
    Texas A&M University
    College Station, TX 77843-3131 USA

    __________________________________________________ _______________________

    Hi Steve,

    I'm marginally familiar with the MacReflex
    system. My understanding is that it is average performance and priced right. I
    believe you will have more flexibility with a PC platform so you may wish to
    explore that option as well particularly since the cost of the computer itself
    is down in the noise when you consider cost of the whole system.



    __________________________________________________ _______________________

    Hi from Vienna-Austria

    We are working in a gaitlab using a videometry sytem with six cameras. The
    number of cameras strongly depends on your application. Theoretically it is
    sufficient to do 3-D Analysis with only two cameras. This is a very unstable
    situation as you loos markers whenever one is covered by a bodysegment
    or merging with another marker in the field of view of one of this cameras.
    To have a robust system for analysis of human movement you need three
    cameras for each side. For full body analysis this sums up to six cameras.

    For more information see also:

    Kollmitzer J., Errors and practical tests for 3-D gait analysis system
    setups with videometry and forceplates, Proc. Third International Symposium
    on 3-D Analysis of Human Movement,1994 Stockholm

    Kindest Regards

    Josef Kollmitzer
    Univ.Clinic PMR
    Waehringer Guertel 18-20
    A-1090 Vienna Austria

    __________________________________________________ ______________________

    My colleague in the dept. of Psychology here have a Mac Reflec. You can
    contact Jan Piek on

    I have just recently bought a Litek Advanced Systems V-scope motion
    monitor, which works on ultrasound. It promises to be very useful (and very
    cheap at US$5000). At present it only works with the IBM PC, but they're
    developing a Mac version, fax Joseph Eliav Tel Aviv Israel 3-497719

    You need a minimum of 2 cameras for 3D kinematics, but often need more
    because the markers are not seen by one of them when the subject moves.
    Typically, we use 3 for gait - 4 for sporting actions. It is possible to do
    limited 3D analysis with ONE camera - a paper was published a few years ago
    on this.

    Best wishes - I'm afraid movement is not the easiest thing to measure!


    __________________________________________________ __________________
    Dr. Chris Kirtley MB ChB, PhD
    Lecturer, Bio-engineering --_ / \
    / \
    School of Physiotherapy, Perth #_.---._/
    Curtin University of Technology, V
    GPO Box U1987,
    Perth 6001, Tel +61 9 351 3649
    Western Australia. Fax +61 9 351 3636
    __________________________________________________ __________________

    Steve: Regarding your posting, I don't know about question 1 (although I
    can bet it will be substantially more expensive). Regarding #2, you
    theoretically only need two cameras. This is what we use with a
    commercial Watsmart system using ired's. However, this assumes you will
    always be able to see the markers.

    Dan Gottlieb
    Dep't of Ortho
    The University of Vermont