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Geoffrey.walsh
06-29-1999, 04:30 AM
I am posting in further replies about the preparatory movements of a
player about to return a serve and further down a few further
observations of my own.
-------------->
>From M.D.Lakie@bham.ac.uk
Organization: The University of Birmingham

Hi Geoff,

Glad to hear that you have leisure to watch television! You see the
same thing in a cat just before it leaps on its prey, but I think most
of the movement is confined to the rear end, so not much paralax
advantage. Also, is there much head movement in the tennis players? If
not, then perhaps the movements either eliminate thixotropy or act as a
callibration of the movement system, getting it ready for action. In
cricket good batsmen keep their heads VERY still which suggests that
they do not rely on enhanced parallax, but many of them have some
preparatory waggles of the bat as the bowler approaches.

Here is a not very related phenomenon for you to consider. It is
probably related to the scudding moon effect, but I haven't seen it
described. As the train approaches London from the west, (Reading) it
goes close to Heathrow. jets can often be seen taking off or landing on
a parallel track to the train. The train travels fast, as do the
jets. Interestingly, when the direction of travel is opposite and the
effective velocity highest, the planes seem to be stationary in the
air. It is a curious and compelling effect - the first time I saw it I
was sure that the plane would stall. It does NOT seem to work when the
motion is in the same direction, which might make more sense. Any
thoughts?
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From: Samuel Phillips

Geoff,
I am responding to a commment from Takashi Yanagaw about players not
being able to percieve distance quickly enough. I do not believe this
to be true. I do not know much about tennis, but in order to hit a
baseball it is very important to be able to percieve the spin on the
ball. Some professional baseball players will have batting practice
with letters printed on the ball to practice focusing. If an exeptional
baseball player can do this, I would think an exeptional tennis player
would also be able to judge many things about a tenis ball quickly
enough to react.
Sam

Sam Phillips
samph@eden.rutgers.edu
Biomedical Engineering (732)878 2881
Rutgers University
---- Subject: Tennis
--------------------
From: kathy.price@reebok.com

Geoffrey,

As a longtime recreational player and a tennis team player in school, I
can tell you that a player sways side to side on their toes in
preparation for receipt of the serve. By keeping one's body in motion,
one can better have the muscles "ready for action" (as you stated), at
least that is what our coach always told us. I've never heard or ever
read about such a motion providing an "optical parallax thus enhancing
distance perception" as you suggested. It is a very interesting and
creative idea though. I'd be curious if any one respondsto you
regarding that possibility.

Sincerely,
Kathy Price
----------
My further obervations are -

The player has the feet much further apart than the distance between the
hips. In swaying laterally the lower part of the body is thus in the
shape of a trapezium, the legs pelvis and ground constituting a 4 bar
linkage. The movement takes place at the hips and ankles. As the body
sways the pelvis must tilt. The head normally shows some translatory
movement, but rotary movement is minimal. The small tilt of the pelvis
must, I take it, be compensated by movements elsewhere, probably at the
neck. A small amount of translation will not substantially impair vision
and long as there is no rotation,

Sometimes the knees bend a little, fairly often there are small rapid
hops.

A philosopher long ago worte a book on ‘the Meaning of Meaning’. I think
I looked at it but did not get very far. I raise this because of the
different levels of explanation that the various people replying have
considered. To say that the movements are preparatory may well be right
but is descriptive rather than analytical.

The movements may as Martin Lakie suggests be shaking out thixotropic
stiffening and he raises the question of a cat the head of which may be
still, the rear end oscillating, when about to pounce. I have seen cats
stalking birds, at times only the end of the tail can be seen to be
moving. The tail will play a significant role in the mechanics of a leap
The moment of inertia etc is so easily adjustable..

Vision is a slow sense , to intercept or hit an object coming towards
you means it is necessary to estimate its flight and time of arrival
from data obtained about, say, 0.5 sec before.

Objects are perceived as approaching if their images progressively
enlarge, as receeding if the images diminish. It is very remarkable if
someone can by looking at a tennis ball detect its spin, (rather than
estimating it by its curved flight). This should be worth following up
with lab experiemnts, not with real balls but perhaps computer generated
images - a Ph.D project for someone perhaps !. A perfectly plane sphere
would give no cue, everything will depend on surface texture. I see the
observation is about a baseball so I suppose the size difference may
help.

(In my youth as a student in Boston I made a few attempts to play
baseball, my contemporaries were very charitable - I cannot now remember
much about it. What is the diameter of a baseball ? Few Europeans will
be able to make much of a guess)

As to the planes, those heading West from Heathrow will be going up,
although travelling towards the train their image size may thus be about
constant and they are perceived as stationary.

This is contrast to the hedges, cows etc on each side of the track.
which will be whizzing past.

Planes heading towards Heathrow will be descending, their images will
get larger, so they are preceived as moving.

Another aspect is the idea that such preparatory movements are serving
to calibrate the system - sensory or motor and muscular or both ?

Someone may think of an ingenious way of testing this under controllable
conditions. It could be very important. Sensory endings adapt often
rapidly and muscle properties depend on past history.

The movements may be updating the control apparatus until the very
last moment. Any suggestions for relevant lab experiments to test the
hypothesis ?

Geoffrey Walsh




--
Email— Geoffrey.Walsh@ed.ac.uk
http://www.ed.ac.uk/~gwalsh
Phone (0)131.664.3046

64, Liberton Drive,
Edinburgh
EH16 6NW
UK

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