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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
Jose_A_BARELA@umail.umd.edu (jb262)
Univesity of Maryland.
Jose Haroldo
labmov@lab01.sarah.br
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
l.abraham@mail.utexas.edu
http://www.edb.utexas.edu/abraham97/lda.html
______________________________________________
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: j-patton@nwu.edu
http://sulu.smpp.nwu.edu/~jim
_____________________________
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:
http://pele.repoc.nwu.edu/
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
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
Jose_A_BARELA@umail.umd.edu (jb262)
Univesity of Maryland.
Jose Haroldo
labmov@lab01.sarah.br
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
l.abraham@mail.utexas.edu
http://www.edb.utexas.edu/abraham97/lda.html
______________________________________________
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: j-patton@nwu.edu
http://sulu.smpp.nwu.edu/~jim
_____________________________
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:
http://pele.repoc.nwu.edu/
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