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exploiting the head loop in V1 skate



 
 
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  #1  
Old December 22nd 03, 10:14 PM
Ken Roberts
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Default exploiting the head loop in V1 skate

Everybody who's looked at a video of an elite World Cup racer doing V1 skate
(a.k.a. offset, paddle-dance) up a hill has noticed that in a front view the
racer's head makes an obvious loop.

Today I think I figured out a detailed understanding of how this loop
enables them to go up hills easier and faster. And how I had gotten the
timing of my head loop all wrong in my own V1 video on Sunday.

The explanation is long and tricky . . . There's four parts to the loop, and
they're not what I thought they would be. But getting this loop right is
like "free gasoline" for my skating up hills. (Even if you don't believe
the loop has any benefit, there's an valuable surprise in the fourth phase
below.)

Here's how I now understand the head loop:
V1 skate has a "hang" side (where I push down and back on the poles) and an
"off" side (where I recover my hips and poles up and forward). On each side
I need to further break down the skate leg-push motion into two phases: the
first phase where ski lands in the snow and glides mainly in line (with also
a sideways push by the hip abductor muscle), and the second phase where it
pushes mainly out to the side (and to the back some too), and also glides
in-line.

So the leg-pushes of the full V1 stroke cycle have four phases: Hang1,
Hang2, Off1, Off2. Here's what I see happening in each one:

Hang1 -- focus on pole-push down: Head and shoulders are quickly come to a
stop, transferring their previous downward momentum to the arms and poles.
Arms move down to the skier's waist -- both transmit the "inertial" force
from the dropping head and shoulders to the poles, and add direct force of
their own. Hips and knees are fairly quiet in this phase. Some push on the
ski out to the side by the hip abductor muscles, which gets the ski more on
edge.

Hang2 -- focus on leg-push out to side (and back some too): Head and
shoulders move strongly _sideways_ away from the direction of the leg-push,
and already _upward_ lifted by the back muscles. The _reactive_ forces from
this combined move go down and sideways in the direction of the leg-push, so
the upper body _adds_ to the force of the leg-push (by Newton's Third Law).
Knee joint extends and pushes the ski out to the side with the quadriceps
and rear-gluteus muscles. Arms finish pushing the poles backward.

Off1 -- focus on lifting the upper body: Head and shoulders move further
upward, and rotate back the other way to face the skiers overall line of
forward travel. Land the ski with on well-bent knee. Knee straightens to
lift the hips and the weight of the entire upper body. The back muscles
continue the extension they started in Hang2 and further lift the weight of
the shoulders and head (this lifting adds to forward-motion power by
building potential energy which is later released in the pole-push).

Off2 -- forward fall and leg side-push: Head and shoulders move strongly
sideways and then crunch strongly down onto the hang-side, _before_ the
hang-side ski lands. Hips stay mostly level. Reactive force from the
sideways shoulder move gets added to the skate-push out to the side by the
"off" leg. Reactive force from recovering the arms forward adds some force
to the backward component of the skate-push. But reactive force from the
downward fall of the shoulders _subtracts_ force from the skate-push, so
it's better to delay that move to the last part of this phase and then do it
quickly.

Conclusion: This loop of the shoulders and head enables the upper body to
deliver an amazing combination of sideways reactive force, downward reactive
force, inertial force, and gravitational potential energy -- to add to the
obvious direct pushing forces thru the skis and poles (and minimizes extra
load on the primary skating and poling muscles).

Comments:

(1) My big flaws in my video clips that Sharon took of me skating V1 on
Sunday are that I make all my side-to-side shoulder moves one phase too
_early_, and start all my up-down shoulder moves half a phase too _late_.

(2) The timing of the downward "crunch" by the abdominal and chest muscles
came as a big surprise to me. Instead of directly helping push the pole
tips into the snow, the crunch force is used to generate momentum in
_advance_ in the Off2 phase, which is then applied thru the pole tips during
the next Hang1 phase as an _inertial_ force.

(3) It's best make the sideways moves of the head and shoulders during the
phases when the skis already edged (Hang2 and Off2). Because if the ski is
gliding nearly flat (like in much of Hang1 and Off1), then it doesn't to a
good job of transmitting the sideways reactive forces to the snow.

Note: In this respect skis on snow are completely different from rollerskis
or inline skates on dry pavement, which easily transmit reactive side-forces
when rolling flat -- or even when rolling on the "wrong" outside edge.
That's how I had gotten my timing of my side-to-side shoulder moves all
wrong: I figured them out practicing on pavement.

(4) Some coaches see the Off1 phase as a waste to be minimized, because it
looks like a loss of _momentum_. But actually it's a very valuable phase
for delivering forward-motion _power_, but in a clever indirect way. And
perhaps it is physically the most _efficient_ phase, since simple direct
lifting is an excellent way to apply the big power of the big power to the
main problem of hill-climbing: namely moving the weight of the body uphill
against the force of gravity.

So many new moves to play with -- I can hardly wait to get back on snow.
How could the forecast for Wednesday be for rain?

Ken


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  #2  
Old December 29th 03, 01:59 PM
Ken Roberts
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Default exploiting the head loop in V1 skate

Another move in the upper body sequence of the V1 skate of World Cup winners
is the raising of the hang-side shoulder at the start of the pole-push.

Climbing the hills at Mt Van Ho last weekend, I found this to be my new key
move for making it _easier_ to survive the ridiculous hills of Porter
Mountain. Of course it's another plain violation of the New Skate "quiet
upper body" religion. Seems like every time I find a new way to violate the
New Skate in my V1 technique, the easier it gets for me to ski.

My great memory from Saturday was _gliding_ up through the steepest part of
East Mountain. Of course I was pushing too -- but with the extra power from
that new raise + dip of my shoulders, it felt like miraculous floating
upward -- at least compared with my previous expectations. (East Mountain
is the hill that had to be modified for the 1980 Winter Olympics after the
Soviet coach claimed that it was too hard).

The biomechanical advantage of this raising of my hand-side shoulder
(together with dipping of my off-side shoulder) is obvious and direct, not
some tricky "reactive force" thing. When I "unwind" my pole-push from that
tilted-shoulder start position, I engage new muscles in my chest and abdomen
that are aimed more directly into my primary hang-side pole-push, and also
support my off-side pole-push. I find this extra aid allows me to drive my
hang-side pole _down_ in a whole new way, and my legs really appreciate the
help.

To make skating up a hill _easier_, I need to go beyond "core stability" to
core _action_ -- use new core muscles for extra _propulsion_ power.

To make skating up a hill easier, I need to go beyond just "naturally"
turning from side to side -- find the muscles to _power_ the turning,
enhance the timing and coordination to optimally _exploit_ the turning for
additional power -- to take some load off my primary muscles.

Ken



 




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