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Brett Lee
10-10-1995, 02:17 AM
hello,

recently i asked the list a question about performance during water
skiing (slalom). i would like to thank those of you who replied, as this
brief discussion has helped me understand the sport much better.
for those interested in this topic, i have appended the replies.

in addition, an applied discussion of this topic recently appeared in
one of the skiing magazines (Water Ski, Sept/Oct 1995, pp. 40-43). in a
"very" brief summary of this article, the concept of an easier and an
"off-side" turn was explained. as the off-side turn was the
primary source of skiing errors, three suggestions were put forth: 1)
keep your hips turned 90 degrees to the boat through the wake, 2) keep
your rear knee close to (behind) your front knee, thus keeping your
hips and shoulders closed to the path of the boat, and 3) a controlled,
predictable reach (which has more to do with deceleration) into the turn.

happy aquatics,
-brett || ||
******************************* ^^^^^ ||---------------------||
Brett Michael Lee, Ph.D. ' ' || \ @@ / ||
Christopher Newport University \-/ \ -- /
blee@powhatan.cc.cnu.edu _ ||| _ ||
http://www.cnu.edu/~blee/hp/ || / ||| \ || || go
O: 804/594-7617 F:804/594-7862 ||----/---/---\---\----|| / \ for
******************************* || _\ /_ || _/ \_ it!


original posting
----------------
i ski with my right (dominant) leg behind my left leg. i can cut to the
left AND move to the left easier. my partner (dan), who ski's with his
left (non-dominant) leg behind his right leg can cut to the right AND
move to the right easier. in each case, as we move with our backs more
toward the water, the skiing is easier.

question:
^^^^^^^^
1. is there a (bio)mechanical reason (or any reason, for that matter) for
the apparent balance differences?


replies follow
--------------------
From: Craig Nevin

Brett:

I presume you are have only one ski, with your feet one behind the
other. In this case the weight is mostly on the back foot with the
front foot acting as a steering mechanism. There is also a natural
initial rotation of the hips due to the posture.

If you stand on the ground in this position you will notice that it
is far easier to invert your foot than it is to evert it. The range
of motion in the foot is much greater with inversion. You will also
notice that you can retain an upright posture far easier with extreme
amounts of inversion than with eversion. This is due to the
anatomical arrangement of the bones in the foot. You therefore have
an anatomical preference for rotations to the the lateral side of the
leading leg.

You may ask why it makes a difference which foot is in front,
since in both situations there is one foot with a lateral edge on
each side of the ski. The answer is that there are also
biomechanical reasons relating to the assumed boby posture, the line
of progression of the boat and the angle of the tow-line relative to
both the boat and skier.

First it is necessary to describe the posture of the skier as it
relates to the movements of the feet. Due to anatomical linkages
in the lower limb, with inversion, the ankle displaces laterally,
ROTATING the leading knee laterally too. On the other hand, with
eversion the ankle displaces medially and the knee ROTATES medially.
With a bit of experimentation you will see that the hips rotate in
the direction of rotation of the leading leg.

When you cut to the left (with the left leg leading) the hips rotate
towards the tow-line. When you cut to the right with the left leg
leading, the hips rotate away from the tow-line. In the first
case you have more range of movement as you must first de-rotate the
hips with respect to the tow. In the second case your hips start
partially rotated outwards, and continue rotating outwards.

As you cut to the left the tow-angle changes with respect to the
skier, tending to restore the initial angular displacement. The skier
can uses this to his advantage as it is a very stable situation.

However, when cutting right, the tow angle just makes the angle even
more unfavourable. On the right cut, once your line exceeds 45
degrees to the line progression of the boat, you cannot cut left again
to restore the angle without letting the rope become slack. This is
because if you lean to your left while getting towed to your left by
the tow-line, you will struggle not to fall as there is no resistance
from the water due to the angle of the ski. Contrast this with the
power and control you get at 45 degrees on the left side (port side)
of the boat.

If you swap your legs around, the preferred side of the boat also
changes. It has nothing to do with which is the dominant leg, except
that this might confuse your neural control mechanisms, and reduce
you response time.

Hope that explanation is adequate.

Regards

Craig Nevin
Biomedical Engineer
Department of Physiology/Sports Science
University of Cape Town, South Africa
CNEVIN@anat.uct.ac.za

----------------
From: brian garner
Hello,

I am a biomechanics graduate student at the University of Texas and was
interested in your question. Although I cannot provide any biomechanical
explanation for the phenomenon you described, I can provide an additional
data point.

I enjoy water skiing very much and have been skiing (on one ski) since I
was 12 years old. I happen to put my right leg (dominant) in FRONT of my
left leg, and, like your friend Dan, I find it easier to cut to the RIGHT.

I also find that I exert extreme force with my LEFT leg during hard cuts
in order to dig the fin of the ski into the water. When I climb back into
the boat, the quadriceps of my left leg are bulging and exhausted while my
right leg is relatively un-fatigued.

This may not add any new knowledge to your investigation, but I thought
you might appreciate the additional data point.

Sincerely,

Brian

----------------
From: Bob Colborne

Concerning your recent query to BIOMCH-L...

There are people with even slight leg length inequalities who are stronger with
their non-dominant leg. If your left leg is longer than your right by 1 cm (and
about 25% of males demonstrate this), but your right is your 'dominant' leg,
your left may still be stronger.

Check this out. Measure the circumference of your thigh and upper shank on both
limbs. Also, stand in front of a large mirror and look for evidence of a left
or right pelvic tilt.

G. Robert Colborne, Ph.D.,
Department of Anatomy & Cell Biology,
University of Saskatchewan,
Canada.