2006 Ideas + Reports on Ski Skating -- Roberts XCski

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Ken Roberts - - Cross Country Skiing

what's here

asymmetric leg-push for V1 : Dec 2006
jump-skate : Dec 2006

earlier in 2006:

[ to be added ]

asymmetric leg-push for V1

posted to xcskiforum.com 06dec25:

subject: asymmetric leg-push better in V1

There's some old coaching lore about making the skating leg-pushes symmetric on both sides. I tried to obey it for several years -- and did lots of No-poles skating partly in order to force myself to learn it. But I'm losing faith in it.

A few days ago I again looked closely at the 2006 Olympics Individual Sprint races, and some other video clips of elite racers -- and noticed that sometimes their leg-pushes did not look symmetric. Most obviously Bjorn Lind's "jump Open Field Skate" has very different roles for each leg -- one jumps and the other doesn't. More frequently in (non-jump) V1 I saw some of the elite racers set down their poling-side foot closer in underneath, but their recovery-side foot more out to the side.

Just today I consciously tried asymmetric legs on my own V1 out on asphalt -- setting down my recovery-side foot a little more out to the side with a little less knee-flex -- and that seemed more effective.

I thought about the physics of V1 skate (a.k.a "offset", "paddle-dance"), and it seems like the Forces thru the leg on each different side are very different:

(a) the poling-side leg is assisted by poling, so there's less Force required;

(b) the reactive force of accelerating the upper body upward must be added to the recovery-side leg muscles;

(c) a higher proportion of that vertical reactive force will be directly transmitted into propulsive force if angle from ski to hip is leaning more away from vertical.

Seems to me this difference in the physics implies that:

The poling-side leg can and should take advantage of its Poling assistance to push through a longer Distance range-of-motion (which requires setting down further inside underneath), to deliver more Work; and push faster to deliver a higher rate of Power.
To transmit higher force magnitudes, the recovery-side leg needs to start a little straighter (more "bone" transmission) -- which fits with setting down a little more outside, with less focus on the initial "side-sweep" phase and more on the final "extension" phase.

And the recovery-side leg-push will be a little slower. My observation from counting frames in elite racer video clips is that the time the recovery-side leg-push is just a little longer than the time for the poling-side push.

So my (new) reasoning is that trying to impose that old "symmetry" rule on V1 skate implies that the recovery-side leg is not able to apply Force magnitude which is high enough for maximum sustainable power; or the poling-side leg is not able to achieve range-of-motion and quickness high enough for maximum sustainable Power -- or both. Go faster or longer with V1 by each leg pushing in its own specific way.

Ken

jump-skate

posted to rec.skiing.nordic 06dec20:

subject: jump-skate

I just started trying to do some sort of jump skate. Does someone have some stories about learning it or using it?

Is there link to a video clip of somebody doing it? Or a web page that expains it?

So far I've only found reference to a "jump V1". Do racers sometimes also use a "jump V2"?

Are there some tips about how to do it better?

Is it supposed to work basically like taking the feet airborne in classic double-poling? Or is there some clever trick with how it interacts with the skating leg-pushes?

Thanks for any help,

Ken

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posted to rec.skiing.nordic 06dec21:

subject: jump-skate

Colin wrote

The technique involved is to exaggerate the leg push on both sides to hop into the air.

Thanks a lot for the analysis, Colin -- it's not what I was expecting "jump skate" to be. Very helpful suggestion to look at the 2006 Olympic Women's Sprint races.

Here's my observations from the Semi-finals and Finals of the 2006 Olympic Sprint races, both Women's and Men's.

a) the women used jump-skate more than the men.

b) Didn't see anybody doing a jump in their V2 skate (a.k.a. 1-skate, double-dance).

c) lotsa skaters got both feet air-borne from both sides in their initial acceleration out of the start gate. I couldn't tell if the timing of the poling was V1 (a.k.a. offset, paddle-dance) or Open Field Skate (a.k.a. 2-skate, single-dance, V2A).

d) Some of the Men looked like they using some jump skate in their final sprint to the finish. If I had to guess I'd say the timing was closer to V1 than to Open Field Skate. The Men who used only non-jump V2 in their finish sprint beat the men who used some jump-skate V1.

e) Chandra Crawford's key breakaway sequence in both the Semis and Finals was to start an earlier stronger No-poling skate just before the last hill, then V2 up the hill. (jump-skate had nothing to do with it)

f) Chandra Crawford did some strokes of jump V1 on the hill before the last one, but looked like only getting near the top. Another skier used more strokes of jump V1 to pass her. One interpretation is that Crawford only used as much jump V1 as she needed to stay in range of the front.

g) the faster Men did not use jump V1 on the critical last hill -- only V2. (Perhaps the Men might have used more jump V1 if the critical hills on the course were bigger and steeper).

h) On the hill before that, virtually all the Men used some V1, but I only one or two skiers including some jump, and for only one or two strides. I think this is because most of the men did not want to lead at that point, and save their strength for the last 400 meters.

My tentative conclusions:

The main benefit to elite racers of using jump-skate is get more intense force in each leg-push, not to apply more force to the pole-push.

because . . .

If the benefit were about the pole-push, they would not jump on both sides in V1.
I didn't see anybody landing their jump first with full weight on their poles before getting their foot down to the ground.
In videos I've seen of the short-distance inline-skate races, the winners get both feet air-borne lots of times in their accelerations, and they're not even carrying poles.

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posted to rec.skiing.nordic 06dec21:

subject: jump-skate

Adam wrote

if you jump, your skis come of the snow for a split second, which results in no friction against the skis=less resistance to traveling speeds.

I agree that's a benefit from jumping, but I doubt in most groomed snow conditions that it's large enough to justify the energy cost and time cost of jumping. In softer snow the benefit is larger while both skis are in the air -- but then when the next ski lands it sinks deeper than if there had been no jump. (anyway the starting zone of a World Cup race is about the least likely place to find soft snow)

it increases power when you land and come down on > your poles, transferring that extra power from the jump into velocity.

Yes -- to whatever extent you've gotten more body mass higher off the ground, you might as well derive more benefit from that by transferring some of it to the poles. Actually this was going to be my guess for the main benefit which justifies jump-skate.

But now I find it unconvincing as the main justification for jump-skate, because if it were true then: (a) "jump V2" would be even better than "jump V1"; and (b) elite racers would jump more and higher off their recovery-side foot than off their poling-side foot. (c) there would be no use for elite racers to use "jump" in No-poling skate accelerations.

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posted to rec.skiing.nordic 06dec21:

subject: jump-skate

Whoops. I just looked again at the Men's final of 2006 Olympics individual sprint. Bjorn Lind, after his initial jump V1 acceleration out of the starting gate, then switches to V2, and it sure looks like a "jump" V2 to me. And it sure looks like he's doing it to land with more force thru his poles.

There's one stride where he might even be doing a "double-push" with his leg: hopping up off his Left leg and then landing on his same Left leg, but with his ski aimed differently. I'd heard rumors that the Swedish team was working on some sort of jump double-push for sprinting, and maybe it's there on video -- but there's so much complicated action that it's hard for me to feel sure about it.

Anyway, very cool to watch in slow motion the giant upward extension Bjorn Lind is getting later in the start zone in the 2006 Finals.

I wanna know when they're having some Sprint races around where I live so I have an excuse "get wild' like that.

Funny I was trying "jump V2" with landing on my poles yesterday on rollerskis, and the acceleration felt so powerful -- but then I looked at the Olympic sprint videos and was thinking that those elite racers must have found a way to do non-jump V2 that was somehow even more powerful. Maybe Bjorn Lind thought different.

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posted to rec.skiing.nordic 06dec21:

subject: jump-skate

Whoops again. Actually I think Bjorn Lind later in the starting zone is doing "jump Open Field Skate" (a.k.a. V2A, single-dance, 2-skate) -- not V2. Anyway the key point is the same: It's a big jump "for jumping's sake". It's not the quick-quick jump V1 he was doing in his first few strides out of the gate.

What's really surprising is that he's not jumping from his Right leg to his Left leg (like in jump V1). Rather he's jumping from his Left leg to his Left leg.

What I see him doing is:

(1) skate-push with Right leg.

(2) near the end of that leg-push, sets down Left leg.

(3) jumps off Left leg. He gets his left foot up into the air -- and _not_ by retracting it or pulling it up toward his hip. Just the opposite, he pushes so hard with his left leg out to full ankle-extension (a.k.a. plantar-flexion), like he's trying to push his toe down as close to the ground as possible -- but his whole body weight is going up so high it's just impossible for his ski and toe to stay in contact with the ground no matter how far he could stretch his leg.

(4) lands on his Left leg and his Poles (not obvious which lands first or how the weight and landing impact force are initially split between the leg and the poles)

(5) skate-push with Left leg and double pole-push pretty much simultaneous.

(6) steps onto Right foot as pole-push finishes.

(1) skate-push with Right leg. . . .

Can't wait to try this "jump OFS" on rollerskis. (but wish it could be snow)

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posted to rec.skiing.nordic 06dec21:

subject: jump-skate

Colin wrote

... exaggerate the leg push on both sides to hop into the air. This is something that is forced rather than a byproduct of another motion.

A very interesting and tricky point. Let me try an opposing perspective . . .

Yes the "jump Open Field Skate" stroke (a.k.a V2A, 2-skate) (like Bjorn Lind does sometimes) really is "forced" -- it really is jumping up "for jumping's sake"; but the hop in "jump V1" is mostly just a byproduct of explosive leg-pushes.

Here's my argument about "jump V1":

The skating leg-push has two key phases: First "sweeping" the foot directly sideways out from underneath (using muscles mainly on the side of the leg, like medial hip rotation and hip abduction and perhaps ankle pronation); Then later pushing more in downward "extension" (using muscles mainly mucles on the front and back of the leg, like knee-extension, hip-extension). The early phase has little vertical component of force, so it's not important for jump-skate. The later phase has a major vertical component, which is the proportional to something like the cosine of the angle the pusing leg is leaning away from vertical.

So a greater amount of vertical work is done in the later phase if: (a) you start the extension phase earlier, when the vertical angle is smaller so a larger proportion of the force goes vertical; and/or (b) you push harder during the extension phase. A first approximation is that if you do enough of (a) and (b), then it's pretty easy for your full body to go airborne, and if you do little enough of (a) and (b), then it's pretty hard to go air-borne.

There's a special term for racers who choose to do less of both (a) and (b) while climbing a very steep hill at a critical point in the race: "Getting Dropped".

The normally expected "byproduct" of a World Cup racer pushing with her or his legs in the main "extension" phase early enough and hard enough to be competitive with a pack group on a critical hill (or out of the starting gate) is to go airborne.

The only way I see to get (most of) the benefit of an earlier and stronger leg "extension" push for accelerating on a steep hill without both feet leaving the ground would be to make a deliberately "forced" move of the next foot down into early contact with the snow.

But I think this "forced" downward move of the next foot also somewhat reduces some of the force and work from the current leg-extension phase: Allowing the foot to start to move down produces an "inertial" or "reactive" force which absorbs some of the leg-push force. And after the next foot contacts the ground, some of the gravitational force thru current leg-extension push is lost. One advantage of making the "jump" is that these two events which would be negative for propulsive leg-force are assured to occur after the extension push is complete, at a moment when there is no extension force available to be weakened. So holding the next leg up off the ground turns out to be a clever timing move -- or I guess really its a "non-move".

This strong push is combined with an extra-quick tempo.

I can believe that "jump V1" has a faster turnover frequency than non-jump V2. But I would think that jump V1 would tend to have a slower turnover rate than non-jump V1. Because you can't start the next leg-push until your foot lands on the ground from the previous one. Seems almost a tautology that your next foot is on the ground ready to push earlier if it gets set down before the previous foot lifts off -- or simultaneous with the previous foot coming off the ground. Of course that's with other things being equal, which they never are. But I'd like to hear which of those "unequal other things" it is that makes jump V1 have a higher "extra-quick" turnover rate than non-jump V1.

I recently heard an argument against hop-skate and the whole idea of stepping up the hill while doing offset.

"Stepping up the hill" is a whole other (interesting) topic -- which I think is completely independent from jump-skate. I think you can do a jump way up in the air without the next foot landing way further up the hill. . (Of course if switching to jump V1 results in the skier going overall faster than non-jump V1 at the same turnover frequency, then every body part of the skier is going to tend to be a little further up the hill after each stride -- that's just just by definition of the arithmetic -- has nothing to do with any special "stepping up" move).

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what's here

asymmetric leg-push for V1

jump-skate

My tentative conclusions:

more . . .

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