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Joe Faust
10-18-1999, 04:25 AM
To All,
On 23 Sep 1999 I asked Biomech Listserv for help on methods to find the
vertical distance of the center of mass of a standing human subject. Members
of the list were generous; and find below a list of their names and
contributions. Thank you. By far, the reaction board method was the
dominant suggestion. Segmentation analysis and force plates were suggested.
A list of references are included below.

In two ways the world of high jumping will be impacted by the
contributions. We have now a ready instruction for our readers about the
reaction board method along with illustrations and a dynamic web page for
continued investigations in this precise area. The synergistic
communications around this topic has resulted in an acceptance of a
standardization of the vertical standing c.m. position we call C# or C at
56% of the barefoot tallness T of a high jumper from the ground. Data will
be collected to sharpen this standard. Coaches and high jumpers will now
more frequently check their local personal c.m. positions and report how it
compares with the standard figure.

Further, a standard has been recently accepted at High Jump World for
the position of a high jumper's c.m at the point of takeoff when the contact
with the ground during takeoff is at the instant of separation; that
standard is called Cprime or C' and is 70.5% of T. These standards were
proposed by biomechanics professor Jesus Dapena of Indiana University and
accepted by High Jump World.

Both C and C' will be used to produce "jumper" numbers J and Jprime or J'
respectively. These dervived numbers permit an equalization among tall and
short high jumpers so that more people might stay in the game and enjoying
jumping (relavitely) high.

MEMBERS OF THE LISTSERV AND THEIR CONTRIBUTIONS ARE HERE SUMMARIZED:
>Adam Thrasher recommended: David Winter's book _Biomechanics of human
>movement_ for a reaction board reference.

> An "sei" recommended: Özkaya, Nihat, 1956: Fundamentals of Biomechanics.
>for a reaction board reference.

>Peter F. Vint recommended: "Using a reaction board would be the easiest,
>low cost method of finding the center of mass of a person in a standing
>position
(albeit they must lie down on the reaction board). This method can be
found described in most beginning biomechanics text
books. You can also find a laboratory exercise that explains this at:
http://www.uncg.edu/~pfvint/Lab_CM.html. Another
application related to jumping can be found at:
http://www.uncg.edu/~pfvint/Vertical_jump.html. "

>Jim Dowling recommended: " A simple and accurate method of measuring C is
>to use a board and a scale. You need to know how much the jumper
weighs in his/her jumping gear which is established by standing on the
scale. Next you place the board such that it is
lying horizontally flat with one end on the scale and you record the
scale reading. Next, you have the jumper lie down on
the board in the "attention" position with his/her feet at the end and
you record the scale reading and the length of the
board. It is now a very simple calculation to find C.

C = (S x L)/m
where:
S is the difference in the scale readings of the board with and
without the jumper lying on it.
L is the length of the board
m is the weight of the jumper.

Let me give you an example:
I use a board that is 7 feet long (L=7) and I have my jumper stand on
a scale. His weight is 160 lbs (m=160). I place
the board horizontally on the scale and the scale reads 20 lbs. I ask
the jumper to lie down with his feet at the end (7
feet from the scale) and I record the scale reading of 90 lbs. The
change in the scale is 70 lbs (90-20) so S= 70. Now I
can calculate C = (70 x 7) / 160 = 3.06 feet

The scale units can be kilograms and the board length can be in meters
if you prefer. If the board length is in meters then
the units for C will also be meters. I hope this helps.

Jim Dowling, Ph.D.
Associate Professor of Biomechanics
Department of Kinesiology
McMaster University
Hamilton, Ontario, Canada "

>Jack P. Callaghan recalled a summary of 5 years ago that related to my
>query from the use of "force plate" methods. There was in such data
>cautions regarding quiet stance as perhaps inappropriate during the use of
>force plates.

Thank you, ALL.
Anyone wishing to follow our use of the C, C', J, and J' numbers during the
coming years may visit the ever changing pages here listed:
http://www.highjumping.com/j-numberform.html
http://www.highjumping.com/c-number.html

We look forward to a camera with a built-in expert program that could
take as input a moving high jumper and output computed c.m. positions
relative to environment landmarks. In playback the c.m. would be watermarked
on each frame digitally stored.

We appreciate the work biomechanic scientists are doing.
Thank you,
Joe Faust
High Jump World, editor.



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