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Questions
see also Leg motion Questions
Seems like V2 is the most powerful technique for elite racers on gentle
terrain. Isn't power just what is needed to go up steep
hills? So why should V2 not be best for steep hills also?
What matters for getting up a steep hill is delivering enough power
to maintain enough forward speed to keep on skating. Seems like
the more times I get more help from more muscles, the more power I
should get. So two pole-pushes ought to be more powerful than
one. Or at least, if two pole pushes are more powerful on the
flat, they also ought to be more powerful up a hill.
The obvious counter to this is that the "dead spot" of
passive-glide time needed to prepare for each pole-push on each
side slows the skier down more on a hill -- that it's the dead spot that
makes the skier work harder to get speed back up again, puts more stress
on arm muscles. The slow-down during the dead spot up a hill is a
larger percentage of the skier's average speed than on the flat.
And when failure comes, it comes at the dead spot.
But in the physics the dead spot is not so important:
- The speed to switch from V2 to V1 offset is much higher than the
failure point, so avoiding the failure point is not the real
problem.
- Each dead spot is less than 0.3 seconds, so for an athletic skier
the loss of speed is not much more than 10%.
- The skier can carry extra kinetic energy from the other phases to
get through the dead spot.
The more important reasons in the physics are:
- Muscle-configuration "gearing": The poling motion
simply delivers less power at lower speeds. Each motion has a
Speed-Power curve. In Classic skiing, poling is thought of as
a "high-gear" motion, best for higher speeds.
The force must go through the shoulder joint, which is a long way
from the surface of the snow -- so its leverage is good for
converting shorter muscle movements into much longer snow-surface
movements. But this same leverage ratio makes it difficult to
deliver high forces at slow speeds. So their lower power
output drags down the skier's average power at lower speeds. [
see
more on this ]
- Pole-angle "gearing": I can fight this gearing
problem to some extent by focusing my pole-push on the
"low-gear" sub-range of its motion -- the first part where
the pole is angles back more. But eliminating the first part
of the pole-push cuts the range of motion, so I get less benefit
from the cost of setting up for each pole-push. Unfortunately
the speed is slowest just at the start of the pole-push, and the
start-position is the worst angle for delivering high force at low
speed. [ see
more on this ]
- Inefficiency of using leg muscles to drive poling up hills:
In pure double-poling in Classic skiing, using the leg muscles is a
major source of the power of poling. This is done by dropping
the hips and upper body down and back to drive the poles, then using
the big leg muscles to lift the body-mass above the hips up high in
recovery to build "potential" energy to be released in the
next pole-push.
One the flats, using the capacity of these "hip-lifting"
muscles like this makes sense -- applying vertical work to drive
forward motion -- the hallmark of "Big V2".
But when climbing up a substantial hill, over 80% of the skier's power
is applied to fighting vertically against gravity -- not just
moving forward. The skier lifts the
hips and large body mass above them, only to drop it down again
to drive the poles, then lift it all that again to really finally
move that mass up the hill. Lifting it once is required, lifting
twice it through the same vertical range has the smell of inefficiency and wasted
energy. Moving one part of the body down to move another part
forward and up sounds strange and like wasted energy.
Better just to lift the body mass above my hips once, and
"lock in" that gain.
Yet another reason why each poling stroke is not delivering as much
benefit as on the flats.
- There's no isometric bent-knee problem for leg muscle lift to
solve up a hill.
On the flats the use of leg muscles to raise the upper body also
solves a different problem: the isometric strain of gliding on a
flexed knee joint. At high speeds on the flats, it is difficult
to start pushing immediately with the leg on the new ski -- difficult
to set the ski down on its inside edge with reliable control and
balance.
So I need to passively glide a little ways on the new ski.
But even though that bent-knee position is optimal the next
skate-push, it's very strenuous to hold that position statically until
the skate-push actually starts. So it is necessary for me to
un-bend that knee some to get out of that position. But I can
also derive benefit by using that move to raise my hips and large body
mass above them, then using that raised mass to drive the
pole-push. This same problem occurs on both sides, so making a
pole-push on each side has another reason.
But climbing up a hill, my speed is slower, so it's easier to set
my new ski down on its edge and start pushing on it virtually
immediately. Another reason for making a pole-push on each
side goes away.
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Seems like the more times I get more help from more muscles, the faster
I should go. So two pole-pushes ought to be faster than one (or
none).
Two reasons why this obvious argument breaks down:
- "Gearing" problem: The velocity of the pole-push
is limited by the fact that the pole can only apply force while its
tip is stopped on the surface of the snow. So the relative
velocity of the tip of the pole versus the skier's body must be the
same as the skier's forward speed. So the faster the speed,
the higher the skier's muscle-speed must be. At some point the
muscle-speed gets less than optimal part of that muscle's
Speed-Power curve. Eventually it the speed gets so high that
the muscle cannot deliver hardly any effective contribution to
forward motion at all. The skate-push is not subject to this
limitation. [ see
more on this ]
- Pole-angle "gearing": I can fight this gearing
problem to some extent by focusing my pole-push on the
"high-gear" sub-range of its motion -- the first part
where the pole is more nearly vertical. But eliminating the
second part of the pole-push cuts the range of motion, so I get less
benefit from the cost of setting up for each pole-push. [
see
more on this ]
- Single focus on poling speed: Perhaps by focusing all my
pole-push muscles on only one pole-push, with more time for best
set-up, and more recovery time in between, I can get less reduction
in range-of-motion than with two pushes, and more time in between
allows my poling muscles to recover better from the stress of higher
muscle-speeds. So the poling in Open
Field Skate could continue to be worthwhile at some higher
speeds, when V2 is not.
- Aerodynamics: The cost of air resistance drag goes up
roughly as the square of the skier's speed. The skier has more
air resistance when raise upper body to start the pole-push.
So the air resistance cost, and the time wasted in preparing for the
push, starts to outweigh the benefit.
Then at some higher speed, even the single pole-push per cycle of Open
Field Skate can no deliver enough muscle-speed and range-of-motion to
outweigh the cost of higher air resistance.
Then the better aerodynamics of the consistently low position of
Skate-No-Poles takes over.
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