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#41
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Skate technique USST two cents
Mark's point that the dissipative-force analysis can be simplified seens
right for motions which take place all in the 2-dimensional vertical / forward-backward plane -- like many poling motions. Or in situations where there's no simple machine available to convert sideways move into useful forward work -- like weight-shift in Classic striding. Mark Drela wrote I think you're making this more complicated than it has to be. But I think the analysis _must_ get more complicated in skating. Because in skating, some sideways "dissipative" forces can be converted into useful forward-motion work -- by the magic of the angled ski operating as an "inclined plane" mechanism. Therefore we cannot simple count torques or forces typically labeled as "dissipative" as all wasted. Instead we must determine which _component_ of the force can be converted into forward-motion work (and with what efficiency). The big example is sideways torso swing, which generates useful forward-motion components in _both_ the "eccentric" and the "concentric" contractions. The reason is the _de-celeration_ of the mass of a body part also generates reactive forces by Newton's Third Law -- it's not just acceleration that does it. And in a complicated fully-three-dimensional motion sequence, there can arise clever ways to exploit those reactive forces from de-celeration. That's just the successful World Cup ski racers do. And they all use the same basic methods and moves to exploit those otherwise-typically-"dissipative" forces. Aspiring racers may choose not to be bothered with learning and refining those moves. They might succeed to the National level. But they're not going to make it on the World Cup circuit. You can't compete there if you start by giving up 2% on simple Newton's-Third-Law physics. Ken |
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#42
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Skate technique USST two cents
In article , "Ken Roberts" writes:
Mark Drela wrote I think you're making this more complicated than it has to be. But I think the analysis _must_ get more complicated in skating. Because in skating, some sideways "dissipative" forces can be converted into useful forward-motion work -- by the magic of the angled ski operating as an "inclined plane" mechanism. Therefore we cannot simple count torques or forces typically labeled as "dissipative" as all wasted. Not true. Muscle-force torque in the same direction as joint rotation produces power, and torque opposite to the direction of joint rotation dissipates power. This principle is the same for limb motion in any direction. Joint torques get converted to forces via the bone lever arms between joints, so talking about forces muddies the picture a bit. But for illustration or coaching purposes we can talk about leg extension and "leg force". The outward skating leg forces do positive propulsive work if the leg is being extended while you're pushing outward -- that's the usual skating stroke. Dissipation occurs when you pull inward while the leg is still moving out by inertia. Here are two possible ways to make the leg reverse direction and go back inward at the end of the stroke: A. Push it back by doing a last push by pointing the foot. B. Pull it back with the upper leg muscles Method A uses the leg's kinetic energy towards propulsion. Method B dissipates the leg's kinetic energy. In actual skating both are surely present, but good technique maximizes A minimizes B. Similar options are present in DP. At the end of the pole stroke you can: A. Do a sharp final push with the arms to stop the torso's downward movement. B. Relax the arms and use the back muscles to stop the torso's movement C. Let the torso slam into the thighs. Method A does useful propulsive work with the torso's energy, while B and C dissipate the torso's energy. Good technique maximizes A and minimizes B and C. |
#43
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Skate technique USST two cents
Mark's simplified dissipative force principle applies to many moves in
skating, but not to all of them. Mark Drela wrote Similar options are present in DP. I already agreed in a previous post that the Double-Pole push supports the simplified principle. Here are two possible ways to make the leg reverse direction and go back inward at the end of the stroke And I now gladly add the recovery of the leg as a supporting example for the simplified principle. But I'm still claiming that sideways torso rotation is an important skating move that does _not_ follow Mark's simplified "dissipative force" principle. (Not the only one, but the biggest.) So how about some consideration of that move? I'll make a very specific claim: During the second phase of his leg-push on the poling-side ("hang-side") of his V1 skate ("offset", "paddle-dance"), Carl Swenson starts a move to rotate or "swing" his whole torso toward his non-poling "off" side. While his torso is in the midst of rotating, he transfers his body weight to the other ski on his non-poling side, and starts edging that ski. Then he uses his abdominal muscles to slow and then stop the torso rotation move. My claim is this _deceleration_ action generates a reactive force (by "F = ma") -- and that this reactive force has a partial directional component vector which is perpendicular to the edge of Carl Swenson's non-poling ski. I am claiming that this component of the reactive force therefore _adds_ to the skate-push force thru the non-poling-side ski, and therefore accomplishes useful forward-motion work, and that helps Carl climb up the hill faster and/or easier. Does that make any sense? What are some improvements or corrections to that analysis? Ken |
#44
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Skate technique USST two cents
Hi Ken,
I was holding this back for a rainy day, but alas, I can no longer contain myself. The MIT Center for Sports Innovation has done some work in the area of skate motion analysis that may prove instructive to this thread (http://web.mit.edu/afs/athena.mit.ed...i/skating.html). The idea is that today's athletes expect to use tools to optimise their performance. That these tools exist in many sports, such as cycling, but that sophisticated tools are not available to the in-line skater. As a first step, MIT tried to figure out the qualitative and quantitative aspects of in-line skating performance. My suggestion to you, 1) Call MIT. 2) Come up with a research plan & budget to develop better tools to understand the qualitative and quantitative aspects of skating ski performance. 3) Build something at least as cool as an SRM (http://www.srm-usa.com). 4) Sell it on the Internet to everyone on RSN. 5) Get really rich, retire and ski every day. See you at the Loppet Jimmy ... Todd Ken Roberts wrote: Mark's simplified dissipative force principle applies to many moves in skating, but not to all of them. Mark Drela wrote Similar options are present in DP. I already agreed in a previous post that the Double-Pole push supports the simplified principle. Here are two possible ways to make the leg reverse direction and go back inward at the end of the stroke And I now gladly add the recovery of the leg as a supporting example for the simplified principle. But I'm still claiming that sideways torso rotation is an important skating move that does _not_ follow Mark's simplified "dissipative force" principle. (Not the only one, but the biggest.) So how about some consideration of that move? I'll make a very specific claim: During the second phase of his leg-push on the poling-side ("hang-side") of his V1 skate ("offset", "paddle-dance"), Carl Swenson starts a move to rotate or "swing" his whole torso toward his non-poling "off" side. While his torso is in the midst of rotating, he transfers his body weight to the other ski on his non-poling side, and starts edging that ski. Then he uses his abdominal muscles to slow and then stop the torso rotation move. My claim is this _deceleration_ action generates a reactive force (by "F = ma") -- and that this reactive force has a partial directional component vector which is perpendicular to the edge of Carl Swenson's non-poling ski. I am claiming that this component of the reactive force therefore _adds_ to the skate-push force thru the non-poling-side ski, and therefore accomplishes useful forward-motion work, and that helps Carl climb up the hill faster and/or easier. Does that make any sense? What are some improvements or corrections to that analysis? Ken |
#46
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Skate technique USST two cents
"Nathan Schultz" wrote: This fall at a lecture at the CU Engineering department, I had two experts on physical models of skiing tell me that I push off of my ski perpendicular to the direction of the ski's travel. I argued that I was actually pushing more from the side and that one of the most fundamental errors I see people make in skating is to push "backwards" or directly perpendicular to the ski. They were so caught up in this model that they had created, that they could not see that something could possibly be different than what they had predicted. Even though they were 100% wrong due to assumptions and simplifications they had made in order to make their model reasonably simple, they were certain that I could not possibly be correct because I was describing what I observed, not what fit into their framework. I think this might be complicated by the difference between what it feels like we are doing and what we are actually doing. It also depends on our frame of reference. For instance, if your upper body (and head) are inclined at a forward angle, from the frame of reference of your body, kicking directly to the side will have a rearward component when viewed from a frame of reference outside of your body. I admit that I go for the kick to the side feeling most of the time and that it's a good teaching tool, but when I look down, the starting position in the kick of my feet is farther forward than the ending position relative to my center-of-mass and the direction of travel. I see this in all the videos too. Some good side views at Janne's site are PerElof6, Zorsi and Valbusa1. (Valbusa1 shows both V2 and V1 on the same slope). They look like they are pushing to the side relative from the frame of reference of their angled bodies, but with a rearward component from the outside view (look at the sweep of their feet). If you can point to a clip where you can see the sweep of the feet going perpendicular to the direction of travel, it would be nice to observe. I don't think those engineers have it right that they see the kick as perpendicular to the ski though (from the outside view). It might be at times, but I think the actual angle varies depending on the technique and terrain if you look at the hills too. I found this article by Borowski the other week where he advocates kicking perpendicular to the ski. But I'm not sure how old the article is or if it's his current thinking. http://www.sierranordic.com/Tech_tip_six.html . I was hoping not to get pulled into this thread. Except to say that I like Pete's ideas and have felt many of his cues learning on my own. Reading his tips reinforces that I'm on the right track and gives me some more ideas to use. The progression sequence of "short quick"... to "now your skiing" is great (somewhat like what I've read of Galanes and other coaches too). But I was hoping to clarify some of the discussion. As I said, I do like the "kicking to the side" as a technique tip, but I wanted to point out that there is a rearward component to it when viewed from the outside (not quite 90 degrees to the side, but not always perpendicular to the ski). Andrew Lee |
#47
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Skate technique USST two cents
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#48
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Skate technique USST two cents
I agreed with like 99% of what I read in Vordenberg's post. But I don't
learn much from reading long _long_ articles that I already agree with. I did find a drill that looked helpful ("Hot Feet"), and said I would practice it. And so I did, and it helped (more accurately, it was the "Nathan Schultz" variation in that post that helped me). For all the generic reverence expressed for Vordenbergs's post, who else except Jeff Potter and me actually said they used anything _specific_ from it? (And who of those generic reverers has actually paid for a copy of Pete's "Momentum" book and read it through, like I have?) Nathan Schultz wrote Pete prefaces his technique ideas with the USST philosophy on getting the word out and challenging ideas . . . Which were those "challenging" ideas? Most sounded pretty mainstream to me. When is the USST going to be "getting the word out" on how Carl Swenson keeps getting such great results while violating a major principle of Vordenberg's New Skate articles? (How's that for "challenging"?) Focus on minor imprecision in his language, and you can write pages on technical contradictions in what he says. No I cannot. I worked through every sentence of the Skating sections of Vordenberg's post, and the only thing I could find to question was the _interpretation_ of one drill. (Such an overall assessment could be taken as high praise.) The "old Pete" was more fun. Ken |
#49
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Skate technique USST two cents
Well, Ken, I can claim to be another who used something specific from
Pete's article (which I appreciated very much without having chimed in earlier, though I should be doubly grateful, since none of my tax $CAN go to pay his salary!). My lack of symmetry in V1 leg action, "bogging down on the non-poling side" as he puts it, has been pretty obvious to me, so I've been taking his advice about skating off with the knee pushed down and forward, and consciously trying to avoid standing up. This Saturday's skate race is o the home course of Al Pilcher, a guy who had some very good results in the Pierre Harvey era for the canadian team. It's just never-ending ups and downs, mostly too steep for more than one or two strokes of V2 (sorry, 1-skate, after all we're in Orangeville, Canada!), so lots of offset. But you guys at Lake Placid have to ski 5 times farther, a different, but not necessarily worse, kind of pain. We get to do a classic race Sunday on the same course. So I'll take his advice about not over-striding, though I'm not sure that has been one of my classic sins. Best, Peter |
#50
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Skate technique USST two cents
I find the physics discussion of skate skiing very interesting, while
at the same time not considering it likely that absorbing it will make me a faster skier. So I ask the following just as a matter of intellectual interest. (End of disclaimer for those who get impatient with this kind of thing.) The question relates to Mark Drela's post concerning the push by the leg, saying that it can't in reality be sideways if the ski is angled quite a bit, but must be close in direction to perpendicular to the length of the ski. Probably it comes down to the definition of the word "push", some kind of operational definition, rather than mathematical definition. Imagine an astronaut floating in space on a nice long tether, whose space suit ends on each leg in a Pilot boot attached to his indifferently waxed RCS's. So there's no snow to push against, and no track direction, but his center of mass and body give a perfectly good reference frame for the following to be meaningful. The skis are at angle 45deg to the direction he is facing. First he swings his leg straight out to the side. Next time he swings his leg to the side and backwards so that it's perpendicular to the ski. I've been assuming that what the coaches are saying is that he should be spending his time doing the first, not the second, if it's a bit of training for his next ski race in the polar regions of Mars that he's training for. Everything I just said sounds very tediously obvious to me, but maybe nedds correction. The real queston is to explain why the first rather than the second makes you have better race results. Best, Peter |
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