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Young-hoo Kwon, Ph.d.
06-30-1997, 11:19 AM
Dear colleagues on Biomch-L:

I made a posting some time ago regarding the equations in the article of
Davis et al (1991). I got some replies so far including one from the first
author of the paper. I thank to those who spent their precious time to
reply to my query.

Here is the original posting:

>Dear colleagues on Biomch-L:

>I had some problems in reading the article:

>Davis, RB III, Ounpuu, S, Tyburski, D, and Gage, JR (1991). A gait data
>collection and reduction technique. Human Movement Sciences 10, 575-587.

>On page 583, they provided equations for the hip joint. In my
observation
>based on figure 3 on the same page, equations 5 and 7 should read as
>follows:

>Xh = - ( Xdis + Rmarker ) * cos(BETA) - C * cos(THETA) * sin(BETA)
(5)

>Zh = + ( Xdis + Rmarker ) * sin(BETA) - C * cos(THETA) * cos(BETA)
(7)

>where, THETA = 28.4 deg (SD = 6.6 deg), and BETA = 18 deg (SD = 4 deg).
The
>meaning of Xdis, Rmarker are provided in the article.

>Am I misunderstading something here or is it just a matter of typo? Or
BETA
>could be negative. I wish to get some input from the readership of Biomch-
L, >especially from the authors of the paper.

In summary, figure 3 on page 583 of the above-mentioned was wrong. The
direction of pelvis tilt must be in anterior direction rather than
posterior. Here is the reply from the author:

>Thank you for your close investigation of the our article: Davis, RB
III,
>Ounpuu, S, Tyburski, D, and Gage, JR (1991). A gait data collection and
>reduction technique. Human Movement Sciences 10, 575-587. You are
correct
>in pointing out the inconsistency between figure 3 on page 583 and
>equations 5 and 7 on the same page.

>The figure incorrectly illustrates the tilt of the pelvis as seen from
the
>side (sagittal plane view) while standing. The figure should have shown
>the angle BETA as a counterclockwise rotation of the pelvis about the
>medial/lateral axis of the pelvis (about epy in the figure). This would
>then illustrate BETA more properly as a measure of the anterior tilt of
the
>pelvis. The revised figure would then be consistent with equations 5
and
>7.

>Roy Davis

So if you substitute -BETA for BETA in the equations 5 and 7 shown above,
you will get equations identical to those presented in Davis et al (1991).

Thanks!

Young-Hoo Kwon

---------------------------------------------------------
Young-Hoo Kwon, Ph.D.
Head, Biomechanics Department
Korea Sport Science Institute
223-19 Gongneung-2-Dong
Seoul, 139-242
KOREA

Phone: +82-2-9709-555
Fax: +82-2-9709-502
Internet: y-hkwon@sports.re.kr
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Effective on August 1, 1997
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Assistant Professor
Biomechanics Laboratory, PL202
School of Physical Education
Ball State University
Muncie, IN 47306
U.S.A.
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