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Responses about localization of center of mass

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  • Responses about localization of center of mass

    Might also want to take a look at Shimba (J.of Biomechanics, 1984)
    too. We had pretty good luck with it in the past -- even relatively
    large segmental body movements. It's not appropriate for all
    applications (read paper /error analysis CAREFULLY).

    You should make sure it works OK by doing a weighted pendulum test,
    with the base on the force platform.

    What's kinda nice about it is you only need a well calibrated force
    platform (we had to calibrate ours to about +/-.3 lbs accuracy) to
    calculate Horizontal xy COG displacements. Vertical displacements
    still have the same problem of dealing with low-freq drift artifact

    just my $0.02 cents.


    Luis Mochizuki writes:
    > Dear All,
    > I would like to thank all the responses to question concerning the
    > use of one mark position to the determination of body center of mass.
    > Especially to
    > (jb262)
    > Univesity of Maryland.
    > Jose Haroldo
    > for the answers in portuguese !
    > Luis Mochizuki
    > Lab. Biomechanics
    > School of Physical Education and Sport - University of Sao Paulo
    > __________________________________________________ ____
    > Your wish has been expressed before by many of us who are interested in
    > studying posture, but it is not very realistic. The closest one may
    > come
    > is to have the subject stand "perfectly" still, at which point we may
    > assume the center of mass (COM) is directly above the center of pressure
    > (COP). From that point on, any motion of the COM will be represented by
    > the double integration of the GRF, with appropriate scaling by the body
    > mass. To check the accuracy of the calculations, it is best to have the
    > session end with another period of static equilibrium. The difficulties
    > inherent in this approach include:
    > 1) a standing person is never "static",
    > 2) this method tells little about vertical position (for those to whom
    > this
    > is significant one may use estimates of standing height from
    > anthropometric
    > tables),
    > 3) errors in measuring forces are incorporated and carried forward in
    > time.
    > Most people find that accuracy is enhanced by including kinematic
    > measures
    > of the motion of body segments.
    > Good luck.
    > Larry Abraham, EdD
    > Kinesiology & Health Education
    > The University of Texas at Austin
    > Austin, TX 78712 USA
    > (512)471-1273 FAX (512)471-8914
    > ______________________________________________
    > Hello Luis,
    > I read your question regarding the center of mass and balance. Using
    > current segment parameter tables found in the literature, you will not
    > get a precise measurement of center of mass for the whole body better
    > than within + or - 5 cm, (my guess). this may be accurate enough for a
    > simple inverted pendulum model of balance though. If the subjects are
    > not swaying extensively, you may wish to measure each subject's CofM
    > position directly while supine on a reaction board, then mark this
    > location. this will give you a fair estimate of the CofM position;
    > however, if the configuration of body segments change substantially, the
    > true CofM location will move.
    > I recall a paper by Yang JF, Winter DA and Wells RP. (1990) Postural
    > Dynamics inthe standing human. Biological Cybernetics 62:309-320. which
    > may give you a direction.
    > Best of luck,
    > David Pearsall, PhD
    > Assistant Professor,
    > Dept of Physical Education
    > McGill University
    > Montreal, Quebec, Canada
    > ____________
    > Luis,
    > The center of mass in activities such as sit-to-stand, picking up an
    > object, or landing from a jump can not be modeled using a single point,
    > because of all the independent motion of the extremities. However, if
    > you
    > are looking at stabiligrams, you might be studying only quiet standing.
    > If
    > so, it is a reasonable assumption that the CM is 55% of body height from
    > the floor. Any marker centered on the torso at this level will give you
    > a
    > reasonable estimate of CM location.
    > _________________________
    > ()
    > /\
    > Jim Patton_/)
    > /\
    > / \
    > _\ _\
    > Doctoral Candidate,
    > Biomedical Engineering
    > Northwestern U.
    > EMAIL:
    > _____________________________
    > The best way I have seen has been in Dudley Childres's lab here at
    > Northwestern. Although I'm not thoroughly familiar with his work, I do
    > know that because of the limitations of their motion analysis system
    > (CODA), it is really dificult to use more than 3 markers. Hence, a lot
    > of
    > their work deals with center of mass. A recent article dealing with CM
    > location in gait is :
    > Gard, S.A, and Childress, D.S., "Investigation of Vertical Motion of the
    > Human Body During Normal Walking. Gait & Posture 5 (2) p. 161.
    > Abstracts
    > of the Second Annual Meeting of the North American Society of Gaitand
    > Clinical Movement Analysis, Chicago, IL April 9-12, 1997.
    > Their website address is:
    > Personally, I would use the center of the posterior edge iliac spine and
    > the left and right anterior superior iliac spine to form a pelvic
    > coordinate system, then use this to estimate the CM. I've never tried
    > this, though.
    > Also, you may want to see the article I did with Clive Pai in the April
    > issue of the Journal of Biomechanics this year. It deals with the
    > dynamics
    > of the center of mass in terminating movements.
    > Best of luck, and let me know how things turn out.
    > JIM
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