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Geoffrey.walsh
07-02-1999, 02:54 PM
I am posting in some further replies together with a few comments of my
own. There has been a little editing and here and there some shortening.

Geoffrey Walsh
.................

From: "Andrew G Jameson"

Geoff

I have just compelted my PhD coursework here at the University of
Mississippi, and our lab has recently done alot of work on muscle
spindle responses. I have read most of your replies with interest but
did not see anyone making any reference to eccentric loading of a muscle
before contraction or muscle spindle response.

My understanding of this phenomenen can be simplified and understood
best in the following example.

When you ask someone to jump as high as they can in a vertical jump they
first bend at the knees and lower their center of gravity before
concentrically contracting the quads, hip extenders and ankle plantar
flexors.

If we ask them to jump without a pre-set bend of jump then they can not
jump as high!

If we let them lower their body to a dipped position with the knees bent
and wait there for a second or two then they can jump higher than with
no , pre-set dip but not as high as they could if they had no wait
period at the
bottom!

So what does this indicate?

Maybe the muscle spindles (two types, velocity dependent and absolute
length dependent) are coming into play. We know that when a muscle
spindle is excited in the muscle it causes a motor response and
contraction of the muscle.

An example of the length dependent type is the tightening of the
hamstrings when you extend them!

An example of the velocity dependednt can be seen with the Knne Reflex
hammer experiment used by medical doctors.

I suggest that the bouncing movement of the players with the knees
flexed is placing a eccentric load on the quad muscles and illiciting
some "velocity dependent" muscle spllindle activity. This coupled with
an active
contraction when the player decides to move would mean that that the
player is able to recruit the motor untits and muscle fibers quicker and
gain a more rapid and explosive leg extension, leading to a quicker
movement to the ball.

In summary, the player has a response time to the ball and then another
which represents summation of action potentials to activate the muscle
fibers. If the muscle fiber recruitment is already at a low level then
maybe they can get to the required excitation level quicker when called
upon.

Obviously, this would also happen in the ankle plantar flexors and hip
extensors.

Also, a bosd at rest has more "Inertia" and is consequently harder to
move.

Hope that this is of some interst to you and I welcome your perspective
of
this phenomenen.

Cheers Andrew

Andrew George Jameson
Applied Biomechanics Lab
University of Mississippi
University, MS 38677. USA
662 232 5570
662 234 5574
---------

Comment by GW

There is an enormous literature on muscle spindles. The people in this
field I have known usually do intriguing ingenious and well documented
work but to a man (or woman) they are pretty shifty and evasive when
asked what the spindles do in the control systems. So this contribution
is very useful. (The tendon jerk is an epiphenomenon, in itself of no
significance.)

Of course the inertia of a body is exactly the same when it is at rest
as when it is moving, (momentum and kinetic energy, it will not have
when at rest).

A fringe political party in the UK shows on television its devotees
jumping several yards from a squatting position . How is this done ?

If someone lies down and straightens the knee the hamstring tendons
cetainly can be felt. This surely cannot have anything to do with
spindles, the muscle will be electrically silent. What occurs is the
result of passive stretching. There is in a way a paradox in that the
muscle is relaxed and tense at the same time; relaxed as it is not
active, tense because it is being stretched.

The tennis players preparatory movements do not usually show much knee
flexion, although with the lateral movement the knees are fairly often
held slightly flexed.

GW
---------------------
"IAIN.CHARLTON"

IAIN.CHARLTON wrote:
>
> Geoffrey,
>
> I have just read your summary of replies, so I'm sorry if this is a bit late. After chatting with my partner (a psychiatrist), we have noticed that only the biomechanical aspects of this argument has been discussed thus far.; the mental picture is absent:
>
> My partner observes that many of the motions tennis players go through in preparation for returning serve are highly ritualistic (just watch one of the Wimbledon quarter or semi-finals and you'l1> see). This may indicate that these movements are connected to the mental preparation a player goes through. We also noticed that these movements vary greatly from player to player, and can be very representative of a player's style, e.g. Becker (from memory) actually springs forward a little when the server strikes the ball (which, mirrors his flight towards the net on serve).
>
> It also seems intuitive, as most people have pointed out, that the rapid movement of the legs and flexing of the knees serves to prepare for rapid movement in either direction. Goal keepers do the same as they are approached by an attacker to avoid being wrong footed.
>
> This seems to be a very interesting biomechanics question; my contribution is intended to hopefully maintain a diverse approach to it's discussion.
>
> IAIN.
>
> Iain Charlton
> CREST
> Stephenson Building - G26
> University of Newcastle
> Newcastle upon Tyne
> NE1 7RU
> UK
>
> tel: +44 (0)191 222 6000 ext 6193
> e-mail: I.W.Charlton@ncl.ac.uk
> http://www.ncl.ac.uk/crest/
-----------
Comment by GW

If your hypothesis is that the movements are done to relieve nervous
tension would you say the same as a lion in a zoo pacing round the cage
?
Geoff
--
--------
From: Douglas Adams

I believe it is merely a spike of heightened anticipation for return of
serve, and to get you up on your toes and prepatory motion. Obviously,
you can't maintain the highest level of readiness and anticipation for
any length of time, but you can always surge or impulsively prepare.
Throwing off your opponent's preparatory bounce would be a tremendous
asset.

-------
Comment by GW

Rifle shooters are as still as possible, this is necessary for accuracy.
In tennis the target on the other side of the net is very broad but
speed of reaction is vital.

In general where high accuracy is important but the time window is not
rigourously fixed the person should be calm, and have as little
extraneous movement as possible. But where speed is important getting
the adrenaline coursing through the veins is surely helpful.

A trade off between speed and accuracy is commonplace.

GW
-------
"IAIN.CHARLTON"

Geoffrey,

I don'tpropose that these movements are purely to relieve nervous
tension, more that they are deeply engrained in a player's mind. It is
likely that the movements originate from training, i.e. being coached
to sway, rapidly hop from foot to foot etc. to allow the player to move
quickly in either direction and I certainly agree that these movements
could definitely help to maintain neural feedback (from all sources)
and ensure that the system is ready for movement very quickly. My point
was to merely draw attention to the fact that they seem to have become
ritualistic over time. Athletes prepare for sprints, throws, jumps etc.
in a similar (i.e. ritualistic) way.

As far as the lion pacing around a cage goes, highly repetetive
movements (pacing and rocking back and forth in particular) are
certainly indicative of mental strain.

I wonder if a player would feel adequately mentally prepared to recieve
a serve if they were forced to do so without going through their usual
routine?

Regards,

IAIN.

e-mail: I.W.Charlton@ncl.ac.uk
http://www.ncl.ac.uk/crest/
------
"Bruce Etnyre, Ph.D., P.T."

Geoff,

I've been following the discussion, with interest, on your observation
that skilled tennis players move side-to-side prior to receiving a
serve. It has provoked many interesting hypotheses related to vision
and muscle preparation. I have not contributed to the discussion because
we are currently working on revision of a manuscript which has not been
published yet regarding the neurologic influences under controlled
laboratory conditions related to muscle contraction prior to performing
a skilled timing movement.

The original idea for our research came from my colleague, Takashi
Kinugasa, who has been a tennis instructor for many years and is,
himself, a skilled tennis player. He felt that his receive of serve was
better when moving side-to-side than when he was stationary and wanted
to test the fundamental basis for this empirical observation. There has
been a long history of research on neurologic and performance effects
following muscle contraction, but I will not bore you with the details.
Our manuscript includes a review for those interested.

(Abridged - Geoff)

The sprinter and tennis player examples are probably familiar to most
people. For European and other readers not familiar with baseball (North
American, Japanese, and Latin American's play it the same, to my
knowledge), before attempting to strike a pitched ball, most players
warm up by adding a weight to their bat and swing it several times prior
to stepping into the box for batting (without the weight). Some studies
have been done on the psychological effects of such a warm-up. However,
controlled studies reported there was no difference between bat swing
speed
(Abridged - Geoff)

etnyre@rice.edu


--
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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