see also
[ under construction ]
intro
generate muscular Work
sweep of leg to push on ground
Why V2 fails for climbing up steep hills:
To exploit the double-pole-push requires substantial
up-down motion of the weight of the upper body. But that takes time for
recovery during which the main propulsive force can only be one of
either the Sweep muscles or the Extension muscles -- because both cannot
at the same time be in a good configuration angle for a substantial
percentage of direct contribution to forward propulsion. But if the foot
is set down close to underneath, the Sweep muscles working alone without
aid from Extension or from poling is not enough to reliably sustain
forward speed until the next double-pole-push.
But if the foot is set down further outside in order to
start using Extension push immediately after set-down, then (a) the pole
on that side must be planted further outside from the hip, so it tends
to be aimed more inward and tends to absorb sideways kinetic energy of
the upper body which otherwise could be converted into propulsive work
by being transmitted into the next skating leg-push; (b) the pole on the
other side must be aimed more sideways (so one pole is pushing more
sideways in the direction which the other pole is tending to block); and
(c) there is more downward impact force on the snow at set-down and at
larger edging angle, so there is higher friction; (d) since more of the
Extension leg-push is going into the skating push and less into the
vertical potential energy for the pole-push, the double-pole-push is
less effective; (e) if the foot is set down further outside, then each
skating push is shorter in distance, also shorter in time, so there
isn't enough time for much up-down motion of the upper body.
So once the foot is set down significantly outside from
underneath its hip, there just isn't time to make an effective
enough double-pole push to be worth it. Making only one double-pole push
per stroke-cycle as in V1 skate solves the problems of V2 by:
(a) allowing enough time to generate
enough up-down motion of upper body weight for an effective pole-push;
(b) on the P-side the foot can be set down
close underneath it's hip because the Sweep muscles have immediate
assistance from the pole-push, so . . .
(c) the P-side pole can be planted closer to
its hip and shoulder, so it does not block or absorb the sideways energy
of the upper body and does not oppose the sideways aim of the R-side
pole-push.
Which leaves the problem of how to get through the
R-side leg-push which has no assistance from the pole-push. Two possible
ways:
(d) by dropping the shoulders later during the
R-side push, a reactive upward force is applied to the R-side leg and
foot, which makes it easier for the ski to keep gliding up the hill
without stopping, and easier for the R-side Extension muscles to keep
pushing to full extension.
(This dropping implies that there must have
been in a rising of the shoulders earlier in the R-side push, which
applied an added downward force on the R-side leg and foot -- but that
extra load comes before the leg-push reaches its Extension phase, to the
Extension muscles are in static mode ("isometric"), so they are better
able to sustain the extra force load).
(e) If the R-side push goes on too long a time
to avoid stalling and stopping, can shorten out by setting the foot down
further outside and having a shorter leg-push which focuses mainly on
the Extension push. This creates no problem with planting the pole tip
further outside because no pole plant is needed until after the R-side
leg-push has finished. This shorter R-side stroke does result in less
up-down motion of upper-body weight, and so a somewhat less effective
double-pole push (but not so ineffective as the V2 version of this
situation).
At some point with planing the R-side foot
even further outside, there isn't enough time remaining in the R-side
stroke for enough up-down motion of the upper body weight for this one
double-pole push in the stroke-cycle to be more effective than two
single-pole pushes -- since the only advantage of one double-pole push
over two single-pole pushes is the contribution of upper body muscles
and vertical motion other than the direct pushing by the shoulders and
arms. That's when we give up on V1 skate and switch to "Single-poling
Skate" (a.k.a. "herringbone skate" or "coaches skate".
extension of leg to push on ground
up-down motion of body mass
side-to-side motion of body mass
arms + shoulders push on poles
hip-abdomen-chest push on poles
? which kinds of move take priority ?
With six different kinds of move, they cannot all be
performed with each one at its optimal configuration and length and
timing. So which ones get priority for optimal exploitation and which
ones are more compromised to fit in?
"side-to-side motion of upper body mass" usually gets
compromised in V1 skate nowadays. It would tend to deliver higher Power
at lower turnover frequency, and with the Extension push at a higher
leg-tilt angle (aimed more sideways and less upward). Ten years ago
there was more concern to optimize side-to-side motion, but now the pro
racers focus more on high turnover and on the contribution of poling.
Nowadays the pro racers are glad to accept the power contribution of
whatever side-to-side motion of upper body fits easily with their other
moves, but they're not much concerned to do anything special to increase
it beyond that.
"arms + shoulders push on poles" usually gets
compromised in V1 skate nowadays. Mainly in that P-side shoulder turns
away from the P-side and starts rising up before the pole-push is
finished, instead of continuing to help drive the P-side pole to its
finish. This is to fit with higher turnover frequency. Also when
climbing up a very steep hill where there isn't enough distance and time
in one stroke cycle to allow for a full-length pole-push, it's the "arm
and shoulders" active push phase at the finish which gets shortened (not
the transmission of up-down motion and upper body weight in the early
part of the push).
"extension of leg to push on ground" is an important
source of Power, but in recent pro racer technique it gets shortened and
taken at a less effective angle -- because the same muscles can also
make an effective contribution to propulsive Power by raising the mass
of the upper body to build gravitational potential energy.
but the overall use of the leg extension move (for
"push on ground" and for "up-down motion of body mass" combined) is not
compromised, in the sense that in V1 skate by pro racers, each leg is
always pushed to nearly full extension. The only compromise is in
the starting configuration -- where the bend of the knee and hip joints
is less than it used to be (higher hips nowadays than back around 2000
"sweep of leg to push on ground" is normally not
compromised because: (a) this move in its good configuration transmits a
very high percentage of its work immediately directly into forward
propulsion; (b) the distance of its push is limited anyway, so it
doesn't gain much from low frequency or lose much at higher frequency;
(c) tends not to interfere much with other drivers of Power and speed;
(d) tends to have lower friction than the Extension push; (e) there's no
other way for these muscles to contribute positive propulsive work.
So normally it only gets shortened (by landing the foot further outside)
on very steep hills, in favor of allowing more focus on the Extension
push (because that's better able to sustain the higher forces required
for surviving to the top of a very steep hill).
The other key moves above are compromised normally only
in order to achieve higher turnover frequency -- not to improve the
effectiveness of some other move.
transmit muscular Work
typically a muscle move must simultaneously transmit
work from other muscles and actively generate work of its own.
if one muscle move does not do a good job of
transmitting, either the stroke-cycle must be delayed or else other
muscles must have their effectiveness reduced by transmitting that work.
Two cases of special interest:
transmit up-down motion of body to poles
transmit side-to-side body motion to foot-ground contact
Power = rate of Work
turnover frequency
Speed + Power + resistance
reducing expenditure of Work and Power
Two strategies to go faster: Work harder or Make the
skiing easier.
The second way to go faster is to modify the skier's
moves or positions to reduce the amount of resistance against motion at
the current speed. Then the skier can go at a higher speed (which
results in higher resistance, but not as high as it would have
been without modifying the moves).
The nice thing about the strategy of reducing
resistance is that it tends to have fewer "side effects" than the
strategy of recruiting more muscle mass to add more work. The two
problems of recruiting more muscle mass are: (a) the added muscles
consume critical shared resources -- especially oxygen transported in
the blood; and (b) the work from the added muscles usually must be
transmitted thru already-used muscles whose independent contribution is
then reduced.
Despite the side effects, usually it's still worth it
to learn to recruit more muscles because: (a) the body adapts to
increase the amount and effectiveness of available shared resources
(e.g. enlarges the heart and major blood vessels); and (b) transmitting
force from other muscles moves the already-used muscles to a better
point on their force-speed-power curve, so there's a net gain in power.
friction of gliding on the snow
determiners of the amount of friction in the snow
not affected by the skier's motions techniques:
-
softness of snow
-
history of the snow (e.g. melt +
refreeze)
-
temperature
-
moisture
-
design and shape of the ski
-
design and materials of base of the
ski (relative the weight and skiing style of the skier)
-
structure in the base of the ski
-
wax or other substances on and in the
base of the ski
determiners of the amount of friction which are
affected by the skier's motion techniques:
downward
force: reducing downward force has a significant influence
on choosing technique moves for V1 skate. One of the changes to top pro
racing technique around 2000-2007 was to use a little less downward
force on the ski.
A certain minimum amount of downward force is required
in order to enable the edge of the ski to "cut" down into snow to be
able to "hold" its grip to transmit propulsive force components aimed
out backwards and sideways into the snow.
(gravity climbing uphill)
V1 skate is most often used when climbing up a hill, so
handling gravitational resistance is important for V1 technique. Really
there's no way to reduce the total gravitational resistance in
climbing up a specific hill. If your rate of overcoming gravity is
lower, your rate of getting up the hill is slower. It's the law.
You can try some tricks with gravitational resistance
to help manage it better in its distribution over the stoke-cycle --
which might offer some subtle indirect way to increase
sustainable speed. Here's a couple of ideas . . .
The resistance of gravity is not equal to full
body weight, because the skier is not trying to move directly upward
vertically. Instead the resistance force is proportional to the
mathematical "sine" function of the upward slope of the motion. The
steeper the hill, the higher the resistance. But . . .
It also matters what path you take up the hill.
The more the skier's body zig-zags from side to side, the more the
average path is aiming away from directly up the hill, and lower the
average slope of the skier's motion. Therefore lower gravitational
resistance through most of the stroke-cycle.
Note that simply aiming the ski out more to
the side does not accomplish this: The skier's total body mass must move
more side-to-side. So the pushing force must be aimed more sideways. But
that tends to work better when the ski is aimed more forward.
Another problem is that reversing the sideways
motion and then getting up to speed and traveling a large distance in
the other direction takes more time -- so the strategy of taking a more
"zig-zag" path tends to result in lower turnover frequency and thus
larger range-of-motion in the muscle moves -- not a good thing when
they're sustain the high forces needed to keep on overcoming a steep
hill.
Also increasing the proportion of sideways
motion increases the total amount of work applied to overcome friction
to get to the top of hill -- not a good idea when snow is soft so
friction is high.
So the strategy of increasing sideways motion
doesn't get used much. Instead skier's try to use a turnover frequency
and ski-aiming angle that enables the leg muscles to sustain the high
forces required, and accept whatever sideways motion results from that.
The strategy of reducing gravitational resistance can
be applied to a key move of V1 skate: using leg-extension on the
Recovery-side to raise the weight of the upper body. If the upper body
is moved more forward instead of vertically upward, then the "upward
slope" of its motion is less, so the leg-extension muscles encounter
less gravitational resistance.
The way to do this is to drop the shoulders down and
forward during the Extension phase of the Recovery-side push. Just
holding the shoulders lower and more forward does not accomplish this --
they actually have to drop forward (sometimes called "forward fall",
actually partly a sideways-forward fall). But it's easier for the
shoulders to drop if they're already leaning way forward, because
there's less support underneath to prevent them from falling.
(It could also be possible start from a higher
shoulder position and move the shoulders down and forward by using
muscular force instead of gravity, but this has undesirable side effects
which contradict the goal of reducing load on the leg-extension muscles)
The drawback of this shoulder drop move is that the
Recovery-side leg extension moves are generating a smaller amount of
propulsive work. By reducing both the resistance and the pushing
force, we have not increased the overall forward (and upward) speed. So
what's the point?
Obvious answer is that the Recovery-side leg
push is the "weak link" of V1 skate, so taking load off its
leg-extension muscles is necessary to make the overall stroke-cycle
sustainable at a higher power rate.
But couldn't we accomplish the same result
just by not pushing as hard? and not through such a large
range-of-motion?
? Possible answer?: That would help the "weak
link", but still the leg-extension muscles are operating a better point
on their force-speed-power curve by pushing against lower force ? [but I
don't know why this should be true]
Further answer: There are other important
reasons to get the shoulders low and forward: so that pole-push muscle
moves are aimed more backward and less downward, so a higher proportion
of their work goes into forward propulsion instead of just pounding down
into the snow.
But then why not just hold the shoulders more
low, and not raise them up in recovery any higher than necessary?
Possible answer: Because (a) it already takes
strain and oxygen even just to hold the shoulders in that position; (b)
the shoulders don't have anything else useful to do in the first half of
recovery, so there's no harm in lifting them higher; (c) the extra
raising builds additional "potential energy", and when the shoulders
later drop that gets converted into "kinetic energy", which gets
converted into propulsive work then they "land" onto the pole-push; (d)
raising the shoulders in the first half of recovery put more down-force
on the ski when it is flatter so there is less effect on friction, then
dropping the shoulders later takes down-force off the ski when it is
more edged so it helps reduce friction just when it's most critical.
So having written all that, it seems like
mostly the shoulder-drop move is done for good reasons of its own, and
its interplay with gravitational resistance is just a side effect.
(air resistance)
Air resistance is real, but it doesn't determine any
aspect of V1 skate motions. Some moves and positions of V1 skate
increase air resistance, but they're performed anyway. Some moves and
positions of V1 skate decrease air resistance, but they're still
performed even when speed is so low that air resistance is not
significant.
Some moves and positions of V1 make a
difference for air resistance, but that doesn't influence the choice of
the move or the manner in which it is performed.
The force of air resistance is significantly smaller
than gravitational resistance in situations (climbing up a hills) where
the V1 motion is used by strong competent skiers.
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