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- - back to main Climbing Up a Steep
Hill "secret" page
- - motion technique pages: hill
bound - herringbone
- - Learning
program page
- - Is more glide good?
- - Double cost of extra down-force
- - more "Secrets"
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Why is steep uphill so different?
- Since the skier's velocity is slower, the big leg muscles can be
used more effectively.
The big problem of classic stride is that the ski
and the skier's foot must be in static contact with the snow surface
while the skier's leg is pushing. The distance over which a
single leg-stroke can maintain ground contact and apply significant
forward-push force is limited by the geometry of the skier's leg
joints.
Therefore at high speed on gentle terrain, the time
duration of the leg-push becomes very short (around 0.1 second). This implies that the intensity of the force much be
large, in order to deliver significant power through that short
moment. Also this intensity must be delivered at high
velocity. This combination of high force intensity and velocity
is "explosive" -- like "plyometric" exercises --
and it puts more stress on the leg muscles than would a longer
or smoother leg-push.
Therefore the leg muscles are not as effective at
the high velocities attained by racers on flat terrain. Since
the distance of ground contact for the pole-push motion is much larger
than for leg-push, elite racers rely mostly on their pole-push muscles
on flat terrain. This also helps them "save" the big
leg muscles for the steep hills where they are most needed.
- It is good to increase leg-push cadence (or "turnover"),
because the more times per minute the strongest muscles push with
effective leverage in the strongest sub-range of their motion, the
more power is delivered.
- Since the velocity is lower and the cadence is higher, the glide
phase can be minimized.
See the discussion how the combination of high
velocity and low cadence makes a glide phase necessary, on the "Is
more glide good" page.
- Applying extra down-force now helps -- because when the glide
phase is minimized (and the leg-push is initiated as soon as the
new ski lands), then the double-cost of extra down-force is rewarded
with a double benefit of both "launch" down-force and
"landing" down-force.
See the discussion on the Double
Cost of Extra Down-Force page. The key difference is that in
classic stride on gentle terrain the "landing" down-force
hits in the glide phase -- but with hill bound the "landing"
down-force hits in the leg-push phase.
- The extra grip from the extra down-force is often required
-- to avoid slipping back down the steep slope.
And the extra down-force resulting extra grip is
also needed to mitigate the side-effects of some of the other changes
in motions: (a) synchronized pole timing; (b) lack of complete
side-to-side weight transfer.
Some instructors say to "straighten up",
not bend forward
One of the ideas on my main hill-climbing page is to "bend forward
strongly".
But some instructors say just the opposite: Straighten up more
as the hill gets steeper.
Actually I believe that opposite advice works better for some people
on hills -- which shows how complicated classic striding technique can
get. My guess is that what's behind this advice is an
over-simplified "quasi-static" analysis of the basic
physics. The over-simplified analysis says that the only way to
sustain the maximum down-force through the grip zone is to keep the
skier's body's center of mass centered closely over the center of the
grip zone. Therefore they advise taking smaller steps with a
straighter body.
What a more dynamic analysis of the basic physics says is that I can
temporarily allow my center-of-mass to fall behind the grip zone of the
new leg-push ski -- provided that I use other dynamic tricks as
described above. Making this dynamic physics work requires that
the skier has several things:
- knowledge of the "press the toe" trick
- enough calf-muscle power to use that trick
- enough pole-power to push the body up forward again
- dynamic balance to coordinate it all
Elite racers have all of those, and so did I back when I was figuring
out what really worked.
But skiers who do not have at least three out of four of those may
indeed find that the "straighten up" advice works better than
the "bend forward strongly from the hips" idea.
This depends on the skier and snow conditions:
- skier's velocity at different slope grades
- skier's leg-push cadence and force
- "effective static friction coefficient" for this wax on
this ski with this skier's level of technique.
Two key physics criteria for selecting hill bound over normal classic
stride are:
- the slope grade at which this skier's velocity and cadence are
such that the glide phase can be minimized
to the point where most of the "landing" down-force now
falls into the leg-push phase.
- the slope grade where the "effective static friction
coefficient" of the grip can no longer hold against slipping
back unless extra down-force is applied beyond skier's
body-weight.
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- Getting the next leg-push ski air-borne
together with my body sometimes goes naturally with applying large
extra down-force to increase my grip -- but it is not necessary
in order to get the extra benefit.
If a large enough extra down-force is applied, then
the reactive force can launch my body up into the air. I've seen
videos of elite racers (and of myself) with both skis up in the air
simultaneously. But going air-borne is not necessary for this
technique.
Although I call this steep uphill technique
"hill bound", there does not have to be a literal
"bound" into the air. What's important is not whether
I get the next kick-ski into the air. The important thing is to
get the rhythm of the non-leg-push time gap matched with my body's
upward-downward momentum, so that I get the double benefit in improved
grip -- due to both the "landing" down-force and then the
"launch" down-force fall during the time-period of the kick
(rather than the "landing" down-force falling during the
time-period of the glide, when it does no good -- see
double cost of extra down-force).
The weight of the ski is so small compared to my
total body weight, that whether the ski slides on the snow or gets
into the air a bit makes little difference to the desired
result.
because extra down-force is now available from
up-and-down motion of the upper body; and because there is no benefit
in getting into balance over a flat ski.
Since complete side-to-side weight transfer has an
energy cost, it is good to cut back on as much of it as is not
needed.
This is not the same as no weight
transfer. It just means that for going up a steep hill,
side-to-side weight transfer is only one grip-enhancement trick among
several available. These other tricks are live options here
because up a steep hill they no longer have the penalties associated
with them on gentle terrain. So I trade off the special costs
and benefits of weight transfer against those of other
grip-enhancement techniques -- and shift the "blend" as snow
and slope and bodily conditions change.
- No benefit in getting into balance over a flat
ski.
because there is no glide phase.
Because focusing down-force to the snow through the
small area of the edge of the ski deforms the snow surface, which
increases grip friction -- friction is critical for not slipping back
on a steep slope. When the snow surface is harder, it is more
difficult to deform it -- so it takes stronger edge pressure to
achieve the same improvement in grip friction.
This is one of the key physics principles behind
herringbone, but it can also be used as another little
grip-enhancement trick for hill bound (and for classic stride).
- Moving over outside the groomed set tracks for striding can give
better grip sometimes.
On some days the parallel tracks get
"glazed" from previous skiers, while the snow outside is
"fresher", so it can more easily attach to to the wax on
your skis, especially if you land your ski into it with extra force
and press into the snow. So if the groomed set tracks aren't
working so well for you, try stepping over into some snow with a
different history.
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