View Full Version : Tethered water skiing

Dr M. C. Siff
10-10-1995, 10:58 PM

Brett Lee 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.

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

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ONE OF THE REPLIES has implications for other issues in biomechanics
and bears further comment.

Craig Nevin 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

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My comment:

It appears that Craig Nevin's definitions of inversion and eversion of the foot
are different from those traditionally used in functional anatomy
and physiotherapy. Possibly, it is just his wording of his reply,
but if not, it is important to resolve this fundamental point.

He stated that "... 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".

Inversion (and eversion) of the foot is the complex end-product of
several concurrent actions of the joints of the lower extremity. Very
often inversion is confused with supination (as some vague sort of
outward or lateral flexion of the ankle). Inversion cannot
accurately be viewed as the result of motion at the ankle, since it
is defined as "the action of the foot produced by simultaneous
medial rotation of the knee (and varus of the lower extremity),
plantarflexion of the foot and supination of the sole of the foot".
Similarly, eversion is the inverse action to inversion.

Moreover, it has to be noted that rotation of the knee is impossible
unless the knee is flexed (e.g. see Kapandji 'Physiology of the
Joints' for this information, as well as for definitions and
mechanisms of joint actions in general. The degree of
inversion/inversion is dependent on the degree of knee flexion, as
well as on the degree of hip extension/flexion, so it is somewhat
simplistic to concentrate on fairly gross actions such as 'inversion'
(supination?) of the foot.

Craig also stated that "The range of motion of the foot is much
greater with inversion". The range of motion of the foot not only
is determined by the degree of hip and knee flexion/extension, but
also by the degree of hip abduction and rotation of the pelvis about
the vertical axis. Since ballet dancers (who chronically train to
increase the range of lateral rotation of the proximal head of the
femur) display an exceptional ability to /laterally rotate
the lower extremity (with an unflexed knee, as required by the
aesthetics of their discipline), I would be interested in reading any
references which compares the relative degrees of 'pure' inversion and
eversion (or degrees of inversion vs eversion for different relative
angles of action of the all the joints of the lower extremity).

Brett Lee's question raises this and several other interesting issues
in the biomechanics of water skiing, skating, surfing and other sports
which involve lateral displacement, rotation and balance on various
moving devices. For instance, you will notice that the responses by
Craig Nevin and others make to mention of the role played by other
parts of the body in initiating and controlling the motion.

Considerable research reveals that motion of the head alone
has a profound effect on motion of the entire body, as does
activation of the various reflexes in the muscles of the neck. The
Positive Supportive Reaction and other reflexes also play a vital role
in overall motor control.

Moreover, sideways motion of the body is not necessarily initiated by
actions of the lower body, but by unilateral contraction of the
muscles of the trunk and neck. Craig Nevin also refers to the
interaction of the skier with the tow rope, but probably deliberately
simplifies the analysis by ignoring the importance of the actions of
the highly sensitive control mechanisms of the hands.

What a lot of this boils down to is the fact that one cannot ignore
the fact that balance and imbalance is determined by the brain, the
central command computer which manages dominance, specificity, the
storage of training input etc. Brett will find it an interesting
exercise to deliberately try to change his foot placing as a very
educational exercise.

He might also find it relevant that the kicking leg in football or
soccer, though it may be dominant, does not necessarily exhibit
greater strength than the support leg. One also has to examine the
difference between dominance in stabilisation vs dominance in

Thanks, Brett for a question which really stimulates biomechanics
students to think - I will certainly use it as an example for my
students, if you don't mind!

Dr Mel Siff

Dr Mel C Siff
School of Mechanical Eng
University of the Witwatersrand
South Africa