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View Full Version : Re: Standardization of c.m. positions in high jumping



Joe Faust
10-22-1999, 06:33 AM
To: All and Dr. Hatze,
Standardization at 56% and 70.5% for the C=0.56T and C'=0.705T with T
being barefoot tallness of a jumper, is founded on data excavated by Jesus
Dapena from studies noted below.* That "1/2%" comes from Dapena's
measurements over jumpers he has studied with careful photometrics. **

We anticipate altering the chosen figures when new studies indicate
better "rules of thumb" figures. The error off ideals stemming from not
respecting individualized body morphology, and the repugnance of "ballpark"
figures derived from questionably applicable data is shared by me; and I
welcomed your comment.

A first preference is to have each jumper and coach determine a local
individual center of mass position C using the reaction board. Such
individually measured C will be flagged as individualized so as to
distinguish it from a standardized C. However, for generalized current
reporting, the decision to adopt some standard figure was made in order to
give some momentum toward the meaning and use of the ratio J. (J=(H-C)/C)
with H being the height of the bar cleared.

In our presentation of the standardization of C=0.56T, we note that one
would end with a J that would also be "standardized" and so carry with it
error away from an ideal. Summarily, it is hoped, the pain of such error
on average would be less than the emotional pain suffered by jumpers who
compare jumping ability simply through the H achieved.

When local accurate C measuring seems to be done well nearly
universally, then a "standardized" C should be dropped. We have announced
that we will publish on each jumper's web page at www.HighJumping.com both
series of numbers --- the ones derived from a standarized C and the ones
derived from a locally measured individualized C. Similar comments ride for
C' and J' which regard the take-off separation instant.

Michael Lattany hardly receives the jumping fame that he deserves. His
standardized J=1.33. [[ Michael Lattany ( H=7'5"(226cm) T= 5'8"(173cm)
C=97cm J=1.33 W=? [21" (53cm)over his head; near world's best-ever jumper])
2 or 3 time Big Ten Champ in the 70's.
My coach Michael Lattany (PR 7'5") graduated from University of Michigan, he
now resides in Troy, MI and works at an engineering company in the Detroit
Metro Area. " ]] Dwight Stones' standardized J=1.16 for and H=2.30m.
My best H=2.25+ results in J=1.19 standardized. These J numbers seem to
reflect the jumping ability far closer than absolute H comparisons that
leave really great shorter jumpers often unnoticed.
....
....
* Studies referenced by Dr. Jesus Dapena to High Jump World in support
of the 56% figure chosen:
...
"According to Croskey, M.A.; P.M. Dawson; A.C. Luessen; I.E. Marohn and H.E.
Wright, "The height and the center of gravity in man",
American Journal of Physiology 61:171-185, 1922 (wow, old stuff!), samples
of 50 men and 50 women gave the following results:
c.m location
(% of standing height)

Men Women
54 0 9
55 9 18
56 24 17
57 16 4
58 1 2

Men: 56.18 +- 0.234; women: 55.44 +- 1.09. Notice that they only
measured each individual to the mearest whole percent, so this probably
tended to increase the standard deviation a little.
Notice that the standard deviation was larger for the women. I would
suspect that high jumpers are more homogeneous than the overall population,
and that therefore their standard deviations
would be smaller than the ones reported above.

Based on this, I'd say that it is probably not too inaccurate to use
population averages (location as a percent of standing height) for
individuals. Jesus Dapena "
.....
" Another paper. According to Cotton, F.S., "Studies in centre of gravity
changes", Australian J. Exper. Biol. Med. Sci. 8:53-67, 1931, samples of 45
men and 38 women gave the following results:

Men Women
Mean 56.74 56.07
Standard deviation 0.65 0.67
Minimum 55.2 54.7
Maximum 57.9 57.3

Notice that this guy made the measurements to the nearest 0.1 percent
of standing height.
To judge variability, you need to look at results WITHIN each study, i.e.,
don't pool together the results of the two studies. This is because there
could be slight methodology differences between the two studies which could
affect the means of the two studies, and therefore pooling the
results could show a larger variability than there really was.

Cotton used a reaction board method like the one later described by Hay,
whereas Croskey et al. used a more primitive method which would have tended
to produce more random error, thus magnifying the variability found. So I
would trust the standard deviations
found by Cotton (0.65 for men; 0.67 for women) more than those found by
Croskey et al.
Jesus Dapena "
.......
** P.S. We are keeping a record of the evolution from standardized
figures to eventual locally individualized measured and reported centroids
on the file:
http://www.highjumping.com/c-number.html
where even now resides other pertinent comments which give body to the
present note. Thank you.
Joe Faust.
........
----Original Message Follows----
From: herbert.hatze@UNIVIE.AC.AT
Reply-To: herbert.hatze@UNIVIE.AC.AT
To: BIOMCH-L@NIC.SURFNET.NL
Subject: Standardization of c.m. positions in high jumping
Date: Thu, 21 Oct 1999 16:04:38 +0000

I must confess to being profoundly astonished by a recent list
posting (Joe Faust) regarding standardization of center of mass
locations in high jumping. While the reaction board method does
indeed yield a good estimate for the vertical mass centroid location
of a specific subject lying on the board in a well defined position,
a general standardization value of 56 % of the barefoot tallness for
ALL types of high jumpers must be regarded highly questionable. The
standardization of a value of 70.5 % of T (specified to an accuracy
of HALF A PERCENT!) for the vertical mass centroid location at the
moment of lift-off is really difficult to understand considering the
comparatively large variability in the body configurations at
take-off, in addition to the variabilities introduced by the
different body morphologies of the athletes.

H. Hatze, Ph.D.
Professor of Biomechanics
University of Vienna

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