Skate technique USST two cents

Mark's point that the dissipative-force analysis can be simplified seens
right for motions which take place all in the 2-dimensional vertical /
forward-backward plane -- like many poling motions. Or in situations where
there's no simple machine available to convert sideways move into useful
forward work -- like weight-shift in Classic striding.

Mark Drela wrote
I think you're making this more complicated than it has to be.

But I think the analysis _must_ get more complicated in skating. Because in
skating, some sideways "dissipative" forces can be converted into useful
forward-motion work -- by the magic of the angled ski operating as an
"inclined plane" mechanism.

Therefore we cannot simple count torques or forces typically labeled as
"dissipative" as all wasted. Instead we must determine which _component_ of
the force can be converted into forward-motion work (and with what
efficiency). The big example is sideways torso swing, which generates
useful forward-motion components in _both_ the "eccentric" and the
"concentric" contractions.

The reason is the _de-celeration_ of the mass of a body part also generates
reactive forces by Newton's Third Law -- it's not just acceleration that
does it. And in a complicated fully-three-dimensional motion sequence,
there can arise clever ways to exploit those reactive forces from
de-celeration.

That's just the successful World Cup ski racers do. And they all use the
same basic methods and moves to exploit those
otherwise-typically-"dissipative" forces.

Aspiring racers may choose not to be bothered with learning and refining
those moves. They might succeed to the National level. But they're not
going to make it on the World Cup circuit. You can't compete there if you
start by giving up 2% on simple Newton's-Third-Law physics.

Ken

In article , "Ken Roberts" writes:

Mark Drela wrote
I think you're making this more complicated than it has to be.

But I think the analysis _must_ get more complicated in skating. Because in
skating, some sideways "dissipative" forces can be converted into useful
forward-motion work -- by the magic of the angled ski operating as an
"inclined plane" mechanism. Therefore we cannot simple count torques
or forces typically labeled as "dissipative" as all wasted.


Not true. Muscle-force torque in the same direction as joint
rotation produces power, and torque opposite to the direction
of joint rotation dissipates power. This principle is the same
for limb motion in any direction.

Joint torques get converted to forces via the bone lever arms
between joints, so talking about forces muddies the picture a bit.
But for illustration or coaching purposes we can talk about
leg extension and "leg force".

The outward skating leg forces do positive propulsive work
if the leg is being extended while you're pushing outward --
that's the usual skating stroke. Dissipation occurs when you
pull inward while the leg is still moving out by inertia.

Here are two possible ways to make the leg reverse direction
and go back inward at the end of the stroke:


A. Push it back by doing a last push by pointing the foot.
B. Pull it back with the upper leg muscles

Method A uses the leg's kinetic energy towards propulsion.
Method B dissipates the leg's kinetic energy.

In actual skating both are surely present, but good technique
maximizes A minimizes B.


Similar options are present in DP. At the end of the pole stroke
you can:

A. Do a sharp final push with the arms to stop the torso's downward movement.

B. Relax the arms and use the back muscles to stop the torso's movement

C. Let the torso slam into the thighs.


Method A does useful propulsive work with the torso's energy,
while B and C dissipate the torso's energy. Good technique
maximizes A and minimizes B and C.

Mark's simplified dissipative force principle applies to many moves in
skating, but not to all of them.

Mark Drela wrote
Similar options are present in DP.

I already agreed in a previous post that the Double-Pole push supports the
simplified principle.

Here are two possible ways to make the leg
reverse direction and go back inward at the
end of the stroke

And I now gladly add the recovery of the leg as a supporting example for the
simplified principle.

But I'm still claiming that sideways torso rotation is an important skating
move that does _not_ follow Mark's simplified "dissipative force" principle.
(Not the only one, but the biggest.)

So how about some consideration of that move?

I'll make a very specific claim: During the second phase of his leg-push on
the poling-side ("hang-side") of his V1 skate ("offset", "paddle-dance"),
Carl Swenson starts a move to rotate or "swing" his whole torso toward his
non-poling "off" side. While his torso is in the midst of rotating, he
transfers his body weight to the other ski on his non-poling side, and
starts edging that ski. Then he uses his abdominal muscles to slow and then
stop the torso rotation move.

My claim is this _deceleration_ action generates a reactive force (by "F =
ma") -- and that this reactive force has a partial directional component
vector which is perpendicular to the edge of Carl Swenson's non-poling ski.
I am claiming that this component of the reactive force therefore _adds_ to
the skate-push force thru the non-poling-side ski, and therefore
accomplishes useful forward-motion work, and that helps Carl climb up the
hill faster and/or easier.

Does that make any sense? What are some improvements or corrections to that
analysis?

Ken

Hi Ken,

I was holding this back for a rainy day, but alas, I can no longer
contain myself. The MIT Center for Sports Innovation has done some work
in the area of skate motion analysis that may prove instructive to this
thread
(http://web.mit.edu/afs/athena.mit.ed...i/skating.html).


The idea is that today's athletes expect to use tools to optimise their
performance. That these tools exist in many sports, such as cycling, but
that sophisticated tools are not available to the in-line skater. As a
first step, MIT tried to figure out the qualitative and quantitative
aspects of in-line skating performance.

My suggestion to you, 1) Call MIT. 2) Come up with a research plan &
budget to develop better tools to understand the qualitative and
quantitative aspects of skating ski performance. 3) Build something at
least as cool as an SRM (http://www.srm-usa.com). 4) Sell it on the
Internet to everyone on RSN. 5) Get really rich, retire and ski every day.

See you at the Loppet Jimmy ...

Todd



Ken Roberts wrote:
Mark's simplified dissipative force principle applies to many moves in
skating, but not to all of them.

Mark Drela wrote

Similar options are present in DP.


I already agreed in a previous post that the Double-Pole push supports the
simplified principle.


Here are two possible ways to make the leg
reverse direction and go back inward at the
end of the stroke


And I now gladly add the recovery of the leg as a supporting example for the
simplified principle.

But I'm still claiming that sideways torso rotation is an important skating
move that does _not_ follow Mark's simplified "dissipative force" principle.
(Not the only one, but the biggest.)

So how about some consideration of that move?

I'll make a very specific claim: During the second phase of his leg-push on
the poling-side ("hang-side") of his V1 skate ("offset", "paddle-dance"),
Carl Swenson starts a move to rotate or "swing" his whole torso toward his
non-poling "off" side. While his torso is in the midst of rotating, he
transfers his body weight to the other ski on his non-poling side, and
starts edging that ski. Then he uses his abdominal muscles to slow and then
stop the torso rotation move.

My claim is this _deceleration_ action generates a reactive force (by "F =
ma") -- and that this reactive force has a partial directional component
vector which is perpendicular to the edge of Carl Swenson's non-poling ski.
I am claiming that this component of the reactive force therefore _adds_ to
the skate-push force thru the non-poling-side ski, and therefore
accomplishes useful forward-motion work, and that helps Carl climb up the
hill faster and/or easier.

Does that make any sense? What are some improvements or corrections to that
analysis?

Ken

(Pete Vordenberg) wrote in message . com...
Hello all,

snip...

Good work Pete. Some interesting comments with this thread. I
find it somewhat ironic that Pete prefaces his technique ideas with
the USST philosophy on getting the word out and challenging ideas,
rather than challenging individuals, and this is followed by a lot of
sniping back and forth and less and less about the ideas he presented.

We are very lucky to have the USST in its current state, not only
working with the USST athletes, but opening itself and its resources
to elite skiers, and presenting educational information to citizen
racers, nearly all of whom are not even members of the USSA
organization which pays Pete's (woefully inadequate) salary. These
guys are doing this because they love the sport and they are doing
what is best for the sport. If you nit-pick through pages of detailed
descriptions made by anyone, you are going to find seeming
contradictions. Look at the big picture and you should see that he is
describing top-level technique at the elite international level to us.
Focus on minor imprecision in his language, and you can write pages
on technical contradictions in what he says.

A note on the "Physics" analysis of skiing. Certainly, we must
address physics and biomechanics in our technique discussions, and the
physicists out there can offer a tremendous amount in these dialogs.
However, physicists and biomechanicists alike, please be aware that it
is foolish to break skiing technique down into 100% physics or 100%
biomechanics.

I am not saying that natural laws do not apply, but I am saying
that skiing is a horrifically complicated motion, and unless you have
1000 supercomputers running algorithms day and night, figuring out all
of the variations due to terrain, body type, snow conditions, ski
flex, and the myriad other factors that make up the equation of a
skier going down the track, you are not going to be able to understand
skiing, nor will you be able to explain it. There are too many
parameters involved to make a strictly scientific framework.

This fall at a lecture at the CU Engineering department, I had
two experts on physical models of skiing tell me that I push off of my
ski perpendicular to the direction of the ski's travel. I argued that
I was actually pushing more from the side and that one of the most
fundamental errors I see people make in skating is to push "backwards"
or directly perpendicular to the ski. They were so caught up in this
model that they had created, that they could not see that something
could possibly be different than what they had predicted. Even though
they were 100% wrong due to assumptions and simplifications they had
made in order to make their model reasonably simple, they were certain
that I could not possibly be correct because I was describing what I
observed, not what fit into their framework.

That is not to say that science should not be a part of the
discussion. Having physicists evaluating our discussion keeps us
honest and gives us insight. But scientists need to realize that
scientific analysis is only part of the equation, and detailed
scientific analysis may not be required or even desired for every
point in a technique discussion. Sometimes, we need to look at the
bigger picture and talk about imprecise things like ski feel and
"over-rotating".

-Nathan
http://nsavage.com

"Nathan Schultz" wrote:
This fall at a lecture at the CU Engineering department, I had
two experts on physical models of skiing tell me that I push off of my
ski perpendicular to the direction of the ski's travel. I argued that
I was actually pushing more from the side and that one of the most
fundamental errors I see people make in skating is to push "backwards"
or directly perpendicular to the ski. They were so caught up in this
model that they had created, that they could not see that something
could possibly be different than what they had predicted. Even though
they were 100% wrong due to assumptions and simplifications they had
made in order to make their model reasonably simple, they were certain
that I could not possibly be correct because I was describing what I
observed, not what fit into their framework.

I think this might be complicated by the difference between what it feels
like we are doing and what we are actually doing. It also depends on our
frame of reference. For instance, if your upper body (and head) are
inclined at a forward angle, from the frame of reference of your body,
kicking directly to the side will have a rearward component when viewed from
a frame of reference outside of your body. I admit that I go for the kick
to the side feeling most of the time and that it's a good teaching tool, but
when I look down, the starting position in the kick of my feet is farther
forward than the ending position relative to my center-of-mass and the
direction of travel. I see this in all the videos too. Some good side
views at Janne's site are PerElof6, Zorsi and Valbusa1. (Valbusa1 shows
both V2 and V1 on the same slope). They look like they are pushing to the
side relative from the frame of reference of their angled bodies, but with a
rearward component from the outside view (look at the sweep of their feet).
If you can point to a clip where you can see the sweep of the feet going
perpendicular to the direction of travel, it would be nice to observe.

I don't think those engineers have it right that they see the kick as
perpendicular to the ski though (from the outside view). It might be at
times, but I think the actual angle varies depending on the technique and
terrain if you look at the hills too. I found this article by Borowski the
other week where he advocates kicking perpendicular to the ski. But I'm not
sure how old the article is or if it's his current thinking.
http://www.sierranordic.com/Tech_tip_six.html .

I was hoping not to get pulled into this thread. Except to say that I like
Pete's ideas and have felt many of his cues learning on my own. Reading his
tips reinforces that I'm on the right track and gives me some more ideas to
use. The progression sequence of "short quick"... to "now your skiing" is
great (somewhat like what I've read of Galanes and other coaches too). But
I was hoping to clarify some of the discussion. As I said, I do like the
"kicking to the side" as a technique tip, but I wanted to point out that
there is a rearward component to it when viewed from the outside (not quite
90 degrees to the side, but not always perpendicular to the ski).

Andrew Lee

In article , (Nathan Schultz) writes:

This fall at a lecture at the CU Engineering department, I had
two experts on physical models of skiing tell me that I push off of my
ski perpendicular to the direction of the ski's travel. I argued that
I was actually pushing more from the side and that one of the most
fundamental errors I see people make in skating is to push "backwards"
or directly perpendicular to the ski.

I know I'm going to catch flak for this,
but these "experts" were essentially correct.

One cannot significantly influence the _direction_
of the force that the snow applies to the ski,
only the magnitude. This direction is mostly
perpendicular to the ski base, with a small
rearward tilt due to snow friction. There is
no other physical mechanism to change this.

In turn, the leg force applied to the ski nearly
equal and opposite to the snow force, so the leg
force direction is fixed. Yes, the leg force
is slightly different because the ski and lower leg
are accelerating back and forth, but this difference
is very small. So the bottom line is: The only way
the skier can significantly change the direction
of the leg force is to change the angle of the ski
itself.

The "push sideways" versus "push back"
is a difference in perception, not reality.
I agree that the perception distinction
may be useful in coaching to modify technique.
But also being aware of reality is not
necessarily bad.

I agreed with like 99% of what I read in Vordenberg's post. But I don't
learn much from reading long _long_ articles that I already agree with.

I did find a drill that looked helpful ("Hot Feet"), and said I would
practice it. And so I did, and it helped (more accurately, it was the
"Nathan Schultz" variation in that post that helped me).

For all the generic reverence expressed for Vordenbergs's post, who else
except Jeff Potter and me actually said they used anything _specific_ from
it? (And who of those generic reverers has actually paid for a copy of
Pete's "Momentum" book and read it through, like I have?)

Nathan Schultz wrote
Pete prefaces his technique ideas with the USST philosophy
on getting the word out and challenging ideas . . .

Which were those "challenging" ideas? Most sounded pretty mainstream to me.

When is the USST going to be "getting the word out" on how Carl Swenson
keeps getting such great results while violating a major principle of
Vordenberg's New Skate articles? (How's that for "challenging"?)

Focus on minor imprecision in his language, and you
can write pages on technical contradictions in what he says.

No I cannot.

I worked through every sentence of the Skating sections of Vordenberg's
post, and the only thing I could find to question was the _interpretation_
of one drill. (Such an overall assessment could be taken as high praise.)

The "old Pete" was more fun.

Ken

Well, Ken, I can claim to be another who used something specific from
Pete's article (which I appreciated very much without having chimed
in earlier, though I should be doubly grateful, since none of my
tax $CAN go to pay his salary!). My lack of symmetry in V1 leg action,
"bogging down on the non-poling side" as he puts it, has been pretty obvious to
me, so I've been taking his advice about skating off with the knee
pushed down and forward, and consciously trying to avoid standing up.
This Saturday's skate race is o the home course of Al Pilcher, a guy
who had some very good results in the Pierre Harvey era for the canadian
team. It's just never-ending ups and downs, mostly too steep for more
than one or two strokes of V2 (sorry, 1-skate, after all we're in
Orangeville, Canada!), so lots of offset. But you guys at Lake Placid
have to ski 5 times farther, a different, but not necessarily worse,
kind of pain. We get to do a classic race Sunday on the same course.
So I'll take his advice about not over-striding, though I'm not
sure that has been one of my classic sins.

Best, Peter

I find the physics discussion of skate skiing very interesting, while
at the same time not considering it likely that absorbing it will
make me a faster skier. So I ask the following just as a matter of
intellectual interest. (End of disclaimer for those who get impatient
with this kind of thing.)

The question relates to Mark Drela's post concerning the push by the
leg, saying that it can't in reality be sideways if the ski is angled
quite a bit, but must be close in direction to perpendicular to the
length of the ski. Probably it comes down to the definition of the
word "push", some kind of operational definition, rather than mathematical
definition. Imagine an astronaut floating in space on a nice long tether,
whose space suit ends on each leg in a Pilot boot attached to his
indifferently waxed RCS's. So there's no snow to push against, and no
track direction, but his center of mass and body give a perfectly good
reference frame for the following to be meaningful. The skis are
at angle 45deg to the direction he is facing. First he swings his leg
straight out to the side. Next time he swings his leg to the side and
backwards so that it's perpendicular to the ski. I've been assuming
that what the coaches are saying is that he should be spending his
time doing the first, not the second, if it's a bit of training for
his next ski race in the polar regions of Mars that he's training for.

Everything I just said sounds very tediously obvious to me, but maybe
nedds correction. The real queston is to explain why the first rather
than the second makes you have better race results.

Best, Peter