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#21
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Skate technique USST two cents
I could care less about technique because I'm not a coach and I just
like to go out and ski. Yeah, I could be faster if I did everything the right way. I've been skiing and racing over twenty years though and at 46, it's no big deal if I finish a minute or two faster in a 50K. That being said, I don't think you guys need to defend Pete or rip Ken. I'd suggest staying tuned to Ken's posts in fact. I know Pete and I think he is the type of guy who enjoys discussion on technique. I think Borowski was more ****ed at Pete's "new skate" theory because it made what he said obsolete. Marty Hall's column in Ski Trax was meant to add more to the discussion. I think Pete is the type of coach who feeds off the discussion and disagreement. That's how the sport evolves. Marty wants to hear Pete's response to his criticism. About ten years ago, Steve Gaskill, Mike Gallagher and Lee Borowski all disagreed with each other on technique in the pages of the Master Skier. It makes people try new things and is ultimately good for the sport. Pete is one of the most cerebral guys in nordic skiing. Like many here, I would put my faith in what Pete says over the other guys because he is on the World Cup and he is at the center of the action. What he says about core strength is enlightening IMHO. Take a look at Elofsson, Fredriksson and Freeman for example. Their stomachs kind of jut out. Not like a guy carry a few extra pounds, but like a boxer or football player. The core strength thing makes a lot of sense to me. I might even start back with the sit-ups myself. Jay Tegeder "On the podium if the right people don't show up!" JT wrote in message ... Pete, thanks for the great post. And, congrats on the excellent results by the USST the past few years. I've been actively thinking about not "skiing big" in my classic skiing the last couple years and it is good to have that notion reinforced by you. This newsgroup, and life in general, is richer due to the diversity of people/posters. I too give anything written by Ken Roberts nothing more than a quick skin. I think he'd get more out of a 30-second instruction session with Vordenberg than he will ever get from his endless ramblings about the first-principle physics/physiology of ski technique. But, to each his own. Cheers, Brian ... who is very proud of this result (2002 Birkie): 23 Vordenberg, Pete 2:13:36 24 May, Brian 2:14:18 In article , Roger Knight wrote: Pete and others, thanks for the positive flow on this board and in real life. Don't get bogged down by those who will criticize no matter what you say....you know better than to listen to this stuff. Keep the ideas and positive results flowing, otherwise we will never improve as a country....THANK YOU! |
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#22
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Skate technique USST two cents
In article , "Griss" writes:
Being from a medical background, there's biochemistry, physiology, anatomy, biophysics, etc, and then there's the human body which defies the rules in every *individual* case. Except nobody nobody can defy the rules of physics, no matter how much authority they have behind them. I have real trouble accepting technique advice which has no apparent reason to work from a physics viewpoint, or even a physiology viewpoint. Any technique approach which aims to make you go faster must do it through in increase in NET power -- power produced minus power dissipated, or maybe through reduced air resistance. Also, aerobic power output is limited mainly by cardiac output, not by how much muscle mass can be recruited. Much technique advice that I see appears at odds with these key constraints. |
#23
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Skate technique USST two cents
Huh? Except of that one Hot Feet drill, all the talk about "quick" in Vordenberg's post was about _poling_. And even the Hot Feet drill continues on to a variation that starts like this: "To achieve a good forward driving position with the leg, slow down the tempo . . . " I will spend more time with Pete's post. Here I think he is referring to how to learn the technique, no? I am mixing the words from a few different coaches perhaps. But one concept that seems to be consistent among them is that the powerful crunch down with the poles corresponds with a forward drive of the knee which is followed by a lateral push to the side. On a V1, your description gets more an more true the steeper the hill gets. Quick tempo seems to be the norm in most top level skiers these days, isn't it? There was this guy named Thomas Alsgaard who was known for skating with a _smooth_ style, notably not "quick". But Alsgaard did not seem to suffer the deleterious results that Philip claims must follow from the "drawn out" approach. By strange coincidence, Alsgaard using this style achieved better World Cup race results than anybody on the US National Team, now or ever. Not sure what you mean by "drawn out." On the smoothness comment though, a quick application of power doesn't really look jerky on film. It actually looks really smooth because the motion is short and subtle. Dahlie, in the film studies my coach showed me as examples, doesn't look jerky at all, but seems to apply his power very quickly, looking relaxed throughout most of the stroke. Alsgaard is the same way. So I'm not feeling embarrassed about my choice of styles -- though I sometimes am surprised by how poorly I _execute_ some of them in my videos so far. We all feel the same way! |
#24
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Skate technique USST two cents
Being from a medical background, there's biochemistry, physiology, anatomy, biophysics, etc, and then there's the human body which defies the rules in every *individual* case. Except nobody nobody can defy the rules of physics, no matter how much authority they have behind them. I have real trouble accepting technique advice which has no apparent reason to work from a physics viewpoint, or even a physiology viewpoint. It seems to me that any discussion of physics without some measurement of the magnitude of the forces involved isn't too helpful. To give a hypothetical example: if I twist my torso to increase propulsive force by an average .5%, this may be offset by a 2% average loss on the d/p from being in a less stable position...or the figures might be reversed. We just don't know. None of the physics discussion I have seen has had any measurement of the relative magnitude of the forces applied by different muscles/movements etc--one sometimes hears figures on the proportion of upper and lower body, but not much more precise than that. In the absence of these kinds of measurements, isn't on-snow performance a better means to evaluate technique recommendations than theoretical deduction? |
#25
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Skate technique USST two cents
It's great to see on this newsgroup some goals other than racing -- like
feeling good (Grissy) and looking good (Jay W). Jay Wenner asked: Is the goal here to ski with technique that looks good, or to ski fast regardless of how it looks? Slow: one of my current goals is to learn to skate slow. On gentle terrain and fast snow, I'm interested in playing with what looks good sometimes. When skating up a steep hill, the only thing I care about is making it to the top without suffering lactate pain for the next hour. My problem is that I'm one of those guys for whom skating up a hill fast _feels_ good -- powerful and strong -- in that moment. I just don't like paying the price later. Therefore, I've been trying to learn how _slow_ I can skate up hills (regardless of how it looks). So I spent a lot of time yesterday afternoon at Mountain Dell looking for skaters plainly slower than me, and then following them up all kinds of hills -- because they were the proof that it was really possible to go that much slower and still be skating. And it worked. In my third hour I was finally able to climb hill after hill without stopping to rest. Seems to me there's several kinds of "looking good" and "feeling good" -- but that merits a whole other thread of its own. Ken |
#26
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Skate technique USST two cents
Maybe I'm just not understanding what is meant by "cardiac output" here, but
most of what I've read about exercise physiology says it's more complicated than that. Mark Drela wrote aerobic power output is limited mainly by cardiac output, not by how much muscle mass can be recruited. If I'm understanding Mark right, this is a key issue in choosing among techniques. Because if (A) most of us are limited by central VO2max, then there's little point in looking for more muscles to use -- instead we should just figure out which subset can most effectively convert VO2 into forward-motion power, and focus only on those few. But if (B) the performance limit for most of us is something like Lactate Threshold with a strong "peripheral" component, then looking for more muscle groups to engage is good, even if somewhat less effective (provided they don't interfere with the more effective subset). My current analysis: It may be that elite XC ski racers are limited by their central cardio-vascular capacity (heart chamber volume, max rate, central artery-vein capacity) -- but most of the rest of us are not operating anywhere near our personal genetically possible VO2max, and certainly not in a 50 km marathon race. Lots of authorities say that the aerobic limit for most of the rest of us is strongly influenced by how well-trained our body is at transporting that oxygen out to the specific muscles that need it. And that "peripheral" capacity depends on lots and lots of tiny capillaries out there right near those specific muscles -- also on how good the specific muscles are at quickly and effectively using the oxygen that arrives, and quickly and effectively transporting bad byproducts out ("lactate" seems to be the prime offender that the authorities mention). My reading says that (B) is the condition for most of us. So spending time building those peripheral capacities makes sense, and all the programs I've seen in books devote lots of training hours to exercises for that. And to me, finding clever ways to use more muscles make sense -- and when I look carefully at videos of Alsgaard and Swenson skating, I see them making very precise use of a large number of muscles. I don't see how the elite XC ski racers could have _trained_ themselves to such a high VO2max central cardio-vascular capacity without having figured out how to recruit a larger muscle mass. Same for the rest of us: the more muscles we learn to engage in our skiing, the more stimulus we can provide to raise our central CV capacity to higher levels. That's my current theory anyway -- if somebody's got corrections or improvement, I'd be glad to hear them, so I can do better on my technique and training choices. Ken |
#27
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Skate technique USST two cents
Derick Fay wrote
if I twist my torso to increase propulsive force by an average .5%, this may be offset by a 2% average loss on the d/p from being in a less stable position . . . or the figures might be reversed. We just don't know. That's why it's important to delay the twist of the torso until _after_ the main part of the pole-push motion -- in both V1 (poling-side only) and V2. So even with no force numbers, it's a reasonable guess that the interference with the effectiveness of the pole-push is small -- though of course like Derick says, it's not proof. Then I look at several elite racer videos with single-frame-step, and I see precise confirmation of that guess: The racers delay the twist of their torso in both V1 (poling-side only) and V2, but on the non-poling side of their V1, the elite racers start the torso twist immediately (because there's no pole-push to interfere with). Next I try it myself on rollerskis, and on snow, and the delayed torso-twist move just _feels_ stronger than "quiet upper body". After that, if I still needed more proof, I would do measured personal time trials (not for this move, but I did some for the "forward step"). In the absence of these kinds of measurements, isn't on-snow performance a better means to evaluate technique recommendations than theoretical deduction? Sounds good to me. So where has anyone displayed any sort of controlled time-trial measurements to justify the "quiet upper body" religion that seems to be sweeping much of American coaching right now? Now I'll offer some controlled time-trial experiments on "quiet upper body": the winners of all the World Cup skate races. The fastest skater on the US National team in actual World Cup races. The QUB experiment fails on every criterion. Using physics is not about relying solely on "theoretical deduction". It seems to me that any discussion of physics without some measurement of the magnitude of the forces involved isn't too helpful. I put some estimates of force magnitudes up on the web at http://roberts-1.com/xcski/skate/power_model Since then I've thought of some improvements to the model, and perhaps a significant correction -- just haven't gotten around to it. I'd love to see someone publish something better, but it's a start. Ken |
#28
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Skate technique USST two cents
I have to say that you both have right in this question.
The absolute limmit is normal set by the cardiac output multiplied by the blood Hb value, but this is in optimal conditions. Even elite skiers have problems of producing maximal results if they switch between techniques as classic and skating. One example is Frode Estil that have tried to do better in skating or do as good as in classic skiing, measurements of Vo2 uptake differs a lot between the two tehcnique used, with the skating as the lower one for him. So clearly this is a more complicated issue than the "cardiac output answer". Doing sports in inefficient ways normally makes the O2 uptake lower, but sometimes the athlete have a body composition that make him more efficient in the O2 uptake point but bad in the mechanical point. This put some more variables in this technique question, namely the body efficiency in doing things in some ways and these ways is not the same for everybody and therefore thecnique is to a degree individual. I do agree with Pete that you have to try what works for you and not take it for granted that how you are doing it today is the best for you, eg experimenting or trying new ways of doing things is the way for progress. Technique is not a square rigid box of doing things, it is at multi fazetted ball which is elastic and suite all in it's own way but to know what's right for you, you have to try new ways of doing thing and by that get to know what works for you. Ken Roberts wrote: Maybe I'm just not understanding what is meant by "cardiac output" here, but most of what I've read about exercise physiology says it's more complicated than that. Mark Drela wrote aerobic power output is limited mainly by cardiac output, not by how much muscle mass can be recruited. If I'm understanding Mark right, this is a key issue in choosing among techniques. Because if (A) most of us are limited by central VO2max, then there's little point in looking for more muscles to use -- instead we should just figure out which subset can most effectively convert VO2 into forward-motion power, and focus only on those few. But if (B) the performance limit for most of us is something like Lactate Threshold with a strong "peripheral" component, then looking for more muscle groups to engage is good, even if somewhat less effective (provided they don't interfere with the more effective subset). My current analysis: It may be that elite XC ski racers are limited by their central cardio-vascular capacity (heart chamber volume, max rate, central artery-vein capacity) -- but most of the rest of us are not operating anywhere near our personal genetically possible VO2max, and certainly not in a 50 km marathon race. Lots of authorities say that the aerobic limit for most of the rest of us is strongly influenced by how well-trained our body is at transporting that oxygen out to the specific muscles that need it. And that "peripheral" capacity depends on lots and lots of tiny capillaries out there right near those specific muscles -- also on how good the specific muscles are at quickly and effectively using the oxygen that arrives, and quickly and effectively transporting bad byproducts out ("lactate" seems to be the prime offender that the authorities mention). My reading says that (B) is the condition for most of us. So spending time building those peripheral capacities makes sense, and all the programs I've seen in books devote lots of training hours to exercises for that. And to me, finding clever ways to use more muscles make sense -- and when I look carefully at videos of Alsgaard and Swenson skating, I see them making very precise use of a large number of muscles. I don't see how the elite XC ski racers could have _trained_ themselves to such a high VO2max central cardio-vascular capacity without having figured out how to recruit a larger muscle mass. Same for the rest of us: the more muscles we learn to engage in our skiing, the more stimulus we can provide to raise our central CV capacity to higher levels. -- Forward in all directions Janne G |
#29
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Skate technique USST two cents
In article , Mark
Drela wrote: Except nobody nobody can defy the rules of physics, no matter But, people can certainly misapply the rules of physics and get to the wrong conclusion. Any technique approach which aims to make you go faster must do it through in increase in NET power -- power produced minus power dissipated, or maybe through reduced air resistance. Ok, let's talk power ... Unless you finish at a higher elevation, or don't come to a stop after the finish line, the overall NET power is zero, regardless of how fast you went around the course. The time integral of power is energy, assuming you start from rest and finish lying collapsed on the snow like Bjorn Daehli always used to do, your energy is the same at the beginning and end of the race, thus, net power is zero. So, let's be careful with power. Now at various times, the net power may be non-zero. On a flat stretch at constant speed, net power is zero. Power produced (by legs pushing on snow) equals power dissipated (by snow and wind resistance). On an uphill, there must be positive net power (i.e., you have to do work to raise your body up the hill). On a downhill, there is a net negative power (i.e., you don't do any work, but you do lose energy to friction). In either case, the net power is non-zero because elevation (and hence gravitational potential energy) and speed (and hence kinetic energy) are not constant. Now assuming our skis are waxed, we've chosen appropriate clothing for the day's event, we have to follow the set course and we can't change the viscosity of air, there's not too much we can do about the dissipative forces, so maybe it's best to think about how we can maximize power produced by the body (i.e., specifically power related to forces on snow, not production of heat which doesn't really get us anywhere). Easy enough, until we realize that we're not going straight down the trail, we need vectors and all that jazz. Any power used up to go side-to-side doesn't help us at all, we need to worry about power being directed down the trail (now this may sound a bit hokey because power is a scalar, but anyway). One of the key things that seems to be discussed relatively little is the angle of the skis, which should be a key variable. If you are pushing perpendicular to the ski (which you must be doing if you waxed right and have a "zero-friction" ski), the angle determines a bunch of things including: component of the force in the along-trail direction (which helps you), component of the force in the cross-trail direction (which just rocks you from side to side), component of velocity in the along-trail direction (which helps you), component of velocity in the cross-trail direction (which just moves you back and forth across the trail). It seems to me, based on physics, that for a given frictional force there is likely an optimal angle of skis that maximizes velocity down the trail. What is it? Happy skiing, Brian |
#30
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Skate technique USST two cents
Below is a more detailed writeup I cobbled together
which touches on the points which have been raised. I don't remember seeing anyone separate the power-producing (good) and power-dissipating (bad) muscle action. The latter plays a big role and is crucial in efficient free-motion propulsion like skating. And the potential payoff is very large. People worry about improving some particular propulsive push from 90% to 91% by some subtle technique change. I'm talking about changing from -50% to +90% or whatever. Negative efficiency IS present in the form of the power-dissipating muscle action, and it's important to identify it and to minimize it. That's where the payoffs are, I think. A lot of these points have come up on other forums, in particular on rec.bicycles.science, and later on the "Hardcore Bicycle Science" email group. Free-motion skating propulsion often came up as a contrast to the forced-motion pedaling propulsion. Optimum skating angle also came up. A number of the people there were accomplished physiologists, elite athletes, engineers, etc, so it was all fairly well grounded in reality. ---------------------------------- An Approach to Improving Cross-Country Skiing Techniques Mark Drela January '04 Here's a different take on the problem of efficient skiing technique. Almost everyone focuses on increasing muscle power production. But equally important is reduction of muscle power dissipation, and I think here where technique can pay off the most. First some definitions... Muscle power production: Mechanical power produced by a muscle which exerts a contraction force while decreasing in length. This is called a "concentric contraction". Muscle power dissipation: Mechanical power absorbed by a muscle which exerts a contraction force while increasing in length. This is called a "eccentric contraction". Power is also dissipated by ski friction and air drag, but these can be treated separately. First of all, it's not clear that using more muscles, or using them in some more clever way (e.g. faster or slower turnover, shorter or longer stroke, etc) will increase mechanical power production. In aerobic sports, the main limitation on power is cardiovascular output rate. The heart and lungs can put out oxygenated-blood fuel only so fast, and in steady state this can be easily consumed by only a small fraction of all the muscles available for power generation in skiing, running, biking, rowing, or whatever. One consequence if this is that as long as muscles are well-coordinated and don't fight each other, the distribution of the power generation between muscles doesn't seem to affect the net power output very much. For example, measurements have shown a wide variety of force distribution around the pedal circle between different elite cyclists. Changing from push-only pedaling to a combination of push and pull won't significantly change your steady-state power output in a time trial (try it!). The naive argument will claim the pulling up will surely increase power, because "additional muscles" are used. But it doesn't work that way. Similarly, aerobic power output does not seem to be too sensitive to muscle contraction rates. Time trial pedaling cadence among different riders might vary between 75 and 95 rpm. Clearly, the optimum is very flat. In skiing, the consequence of all this is that the details of the power generation is probably not too important, assuming everything is well coordinated. For example, if the arms are made to put out more power, the legs will have to put out less. A common flaw I see with almost all ski technique arguments is that they ignore this "zero sum" cardiovascular constraint. Likewise, a small change in turnover rate is not likely to significantly affect power production. So where to look for problems and improvements in skiing? As I mentioned in the beginning, one place is to reduce power dissipated (wasted) by eccentric muscle contraction. If this is achieved, the net power available for overcoming ski friction and air resistance HAS TO increase in one way or another, and you'll go faster. The law of conservation of energy works. The most obvious way in which the body dissipates energy is by using the eccentric contraction to arrest and reverse body motion. An extreme example is a shadow boxer. He puts kinetic energy into his arm and glove using concentric contractions with each thrown punch, only to absorb it momentarily later with eccentric contractions. Jumping up and then landing does the same thing. There is a definite aerobic cost, but he isn't doing anything that's normally considered useful. All his aerobic power is dissipated as heat in his own muscles, rather than in an opponent's chin. In skating, similar losses occur when the outward motion of the leg is stopped and reversed immediately after the ski is lifted from the snow, and then reversed again just before the ski is set down again. Similar losses occur with poling arm motions, and upper body motions during Double Poling. Some rules for minimizing these losses can be deduced from basic mechanics. The time-average power dissipated by any oscillatory motion, whether skating or classical, is proportional to P_waste ~ moving_mass * velocity^2 * cycles_per_second Reducing any of the three things on the right will reduce wasted muscle power dissipation. Classical skiing is inherently more dissipative than skate skiing, simply because the maximum leg velocity seen by the skier is equal to the skier's forward velocity. In contrast, the lateral leg velocities in skating are much slower, so skating wastes less power by this mechanism. However, unlike the direct push of classical skiing, skating propulsion has some loss associated with its partly-sideways propulsive force, which is then partly canceled by friction. An analysis shows that the skating propulsive efficiency is 1 - Cf/tan(theta) eff_skate = ----------------- 1 + Cf*tan(theta) where Cf is the effective friction coefficient, including both friction and snow compaction, and theta is the angle of the ski from the direction of travel -- small angles when going fast, large angles when going slow uphill. The Cf depends greatly on the snow, but can be measured as the downhill slope required to maintain a slow steady velocity. My rough guess is Cf = 0.02 old well-packed snow Cf = 0.05 fresh soft snow Here are some calculated efficiency numbers: theta eff_skate eff_skate (Cf = 0.02) (Cf = 0.05) ------ ----------- ----------- 5 deg 0.77 0.43 10 deg 0.88 0.71 20 deg 0.94 0.85 30 deg 0.95 0.89 40 deg 0.96 0.90 50 deg 0.96 0.90 From these it's clear why we're forced to use a bigger skating angle on high-friction snow -- the efficiency is very bad if Cf is big and the angle is too small. The skating efficiency is maximum when theta = 45-50 degrees, depending on the exact Cf value. Such angles are used only on the steepest climbs. So why don't skiers use such angles all the time? The reason is that large skate angles at high speed on level ground would require high lateral leg and ski velocities and fast turnover, which will unacceptably increase the "shadowboxing" power loss from the rapid motion reversals. So the optimum skate angle and associated turnover is a balance between good skating efficiency and acceptable "shadowboxing" losses. This will vary with snow conditions -- high-friction snow favors larger angles and hence a faster turnover. If we look at the P_waste dependency above, we can deduce how one might reduce this power loss regardless of the snow condition. Here are some simple rules: 1) Minimize the mass of the fastest-moving parts (skis, boots, poles). Light equipment helps. This is somewhat obvious. 2) Once a good skate angle and corresponding lateral velocity is chosen, try to MINIMIZE turnover rate by bringing each foot as far in as possible, and extend it as far away sideways as possible. This is not so obvious. The revolutionary clap skate in ice speedskating clearly gives an advantage with rule 2. With the blade still on the ice, the heel can be lifted off the skate and a longer push can be achieved using just the toe at the very end. From photos it's pretty clear that good XC skaters likewise lift the heel and push with the toe at the end of the skate stroke. From the longer leg extension one might conclude that more power is being produced, but this is unlikely if the skier is already maxed out aerobically. The real benefit is more likely to be reduced power dissipation. 3) When arresting motion, try to make the effort do useful work. For example, during DP, try to give a sharp additional push on the poles when the torso is nearly at the lowest position. This will decelerate the torso and transfer some of its kinetic energy into propulsion, leaving less energy to be wasted by eccentric muscle contraction when the torso motion is reversed. I've heard a similar technique should work in rowing -- a sharp final tug on the oar at the end of the stroke will put some of the energy of the moving upper body into propulsion, leaving less energy to be dissipated when the upper body is returned for the next cycle. 4) Rather than using wasteful eccentric muscle work, learn to use gravity to reverse motion whenever possible. For example, the leg at the end of the classical kick should be relaxed, so it's free to swing back and up against gravity and then swing back down and forward. Likewise, let gravity reverse the arm motion as much as possible by relaxing muscles in the cycle where appropriate, so the arm coasts up and then back down. Same goes for DP arm motions. In summary, the ideal situation is when muscle power is applied only when the limb is moving in the propulsive direction, and is relaxed and swinging like a free pendulum at other times. Good skiers surely use gravity like this instinctively, but it's good to be aware of what's happening from a physics viewpoint. Some degree of self-diagnosis and technique correction can then be performed, which is likely to be more effective than pure trial and error. |
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