LIGETY AND SHIFFRIN: HOW I KNOW WHAT THEY ARE DOING


In order to know what to look for, one has to first have a theory that explains what is happening. In order to see what I see, one needs to know something about my theory and how I validated it. With all due to respect to Ted Ligety, in order to fully leverage the potential of the technique he is using, it is necessary to know the basic theory and especially the working principles behind it. Although I could wrong, based on the NY Times video, it is questionable whether Ligety fully understands all the relevant issues associated with the technique he using. Here is one thing that  he gets full marks for.

Ligety: “I step on the new (uphill) ski while it is still on the edge it was currently on in that turn (ergo the uphill edge).”

The question is why is this so important? The answer is that Ligety needs to initiate what I call the pronation flow when his uphill ski is still on its uphill edge. In addition, he needs to initiate a movement sequence that rotates his uphill foot into a foot flat position on the slope. In other words, Ligety (and Shiffrin) need to momentarily literally stand on the slope of the hill. Three things make this possible, 1) extension, 2) pronation and, 3) momentum. Momentum is tending to keep Ligety’s C0M travelling along a fixed path. Gravity is tending to pull Ligety’s CoM down towards the centre of the earth. Ligety and Shiffrin both push back against momentum and gravity by extending their uphill leg with muscle force.  While it may not be obvious from the camera angle of the photo below, Shiffrin is literally standing on the slope of the hill. Although the photo is blurry, you can see that she has started to pivot her left ski across her trajectory. Shiffrin is approaching maximum extension of her left leg. She will extend it further in conjunction with the pivoting of her foot in order to maximize the loading of forces into the turn. Ligety loads his ski more than Shiffrin through the timing of his extension.

Shiffrin initiation

There are several reasons why no one has figured out what Hess, Ligety Shiffrin and in fact all the world’s best skiers have been doing for decadeas. The main reason for the inability to see is that everyone has been focussing their attention on where the action is, at the apex of the turn where the outside ski is maximally loaded. Once the prevailing wisdom saw the downhill ski of the turn as the outside ski they were blinded in terms of the ability to see what the best skiers were doing. The prevailing mentality became to maximize the loading of the outside ski at the apex of the turn. The video below of Guenther Mader in 1987 shows the maximum loading technique. Mader literally pounces on his outside ski and uses the reaction force to catapult him into the next turn.

 

Loading the outside ski of a turn was the right idea. But the mechanism was wrong. It could not possibly work because the Centre of Pressure under the sole of the foot in the technique Mader is using is on the outside turn aspect of the inside edge of the outside ski. In this configuration, loading the outside ski will tend to push to foot to the outside of the turn and rotate it and the leg (more on this later) away from the turn.  These issues led to further serious errors. Trying to fix the foot in neutral and attempts to use the leg as a lever to hold the ski on edge. Today, a whole industry is working 24/7 trying  to make it impossible for skiers make the same Ski Move that Ligety and Shiffrin make.

By the time I saw Hess in 1987, I knew that the world’s best skiers were making a move that  pushed the Centre of Pressure on their uphill foot from under the heel to under the ball of their foot and kept it there through the apex of the turn.  The only mechanism with which to move Centre of Pressure to the ball of the foot is pronation. The pronated configuration associated with the Ski Move allows the leg to absorb energy while enabling the injury defence mechanism to be active.

By 1990, I had formulated a hypothetical model of the mechanics, biomechanics and physics of skiing that predicated that, among other things, the world’s best skiers would move Centre of Pressure to the ball of the foot of their outside ski prior to pivoting the ski and that if the associated movements were impeded technique would be degraded. In order to test this hypothesis we did studies in 1991 using the instrumented data capture device shown in the photos below. The device replaced the conventional ski boot. It recorded the 3-dimensional internal forces applied by the foot and leg of the skier to the interfaces of the device during actual ski maneuvers as well as degrees of cuff movement. Data was acquired through 17 data channels. After only a few runs, a technician in a tent could assess the quality of the turns of a skier was making based purely on  real-time, unfiltered data flow. In a future post, I will present and discuss the data. For now, it is suffice to say that the studies confirmed my Ski Move hypothesis.

Bird Cage Sensors

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This is the device Steve Podborski’s used. Birdcage

That is Steve Podborski’s foot in the device in the lower right hand photo. Testers, who ranged from World Cup Champions and Olympic medallists to neophytes, reported that the device was the best ski boot they had ever skied in. US Patent No. 5,265,350, a document of almost 200 pages, was based on the results of these studies. The focus of this patent, which was granted internationally, is structures of a ski boot that permit the user to assume a dynamically balanced based of support on one foot and use it to move to a dynamically balanced based of support on the other foot. In other words, to allow the average recreational skier to make the same efficient, effective Ski Move that Hess, Ligety, Shiffrin and other elite skiers use.

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4 comments

  1. Thanks for that Dave and I agree that for readers (not just here but on other ski technique blogs) there can be confusion about where in a turn something is happening. For instance you mention 3/4s of the way through a turn certain things happen and some others might perceive in their mind it being later than that and so on . You say and I agree that it seems to make sense that “My position is that a turn starts at the point where the ski goes from the base being flat with the snow and begins to rotate into the snow” I can relate to that..
    But in transition from one turn to the next some transitions could have the skis flat for longer or shorter periods I assume… But anyway the tipping of what I call the new inside ski that I see in your stick diagram you are saying (I think) is not in the transition yet if I understand you correctly, but still towards the end of the last turn…just about to go into transition ? Am I correct about that?
    But to me when I look at those stick diagrams she has already finished that turn and is in transition leading into the new turn…Your stick angles show the new inside ski angled in more than the outside ski or leading it into the new turn. Sorry too for the long reply (too) but still trying be be sure where in the sequence she is. So in your stick diagram showing the angles she is still in her old turn ? Well yes without seeing the actual video I may be confused too about what I think I;m seeing in a freeze frame……So keep going with more blogs
    and it may sink in better….I admit to still not quite seeing it the same as you but it could be the freeze frames and how I perceive them.

    1. Where my perspective differs from others’ is that I am viewing issues in the context of the underlying mechanics, biomechanics and physics at play. In future posts I will provide some Poser sequences that illustrate the issues in the transition. One of the things that influences the movement sequence is that the boot cuff limits the angle and amount of flexion of the ankle joint. In your bare feet you can lift one foot and place it on a bench beside you in line with the other foot on the floor. But if you are wearing ski boots you will have to place the lifted foot in front of the other foot. In order for a skier to move their centre of mass forward from the position over the ball on the current outside ski to the new (uphill) outside ski they have to square their hips to their skis. This requires that they neutralize their turn and run straight. They have momentum. But there is no centrifugal force which is really tangental force from a curve. The ability to load very high torque forces into a turn means that Ligety or Shiffrin can delay the point at which Centre of Mass starts to move over top of the ball of the foot of their new ski. For this reason the duration of the transition will vary. But Ligety and Shiffrin are controlling this precisely because they can make such a short radius turn around the gate without slipping.

  2. Dave….looking at the first stick diagram of Shiffrin; what do you think about the early tipping of the inside leg ? …or do you see that as still the old outside leg? Just curious because some other ski analysts talk about the early tipping of the new inside leg as being cutting edge in slalom ? (not so much the weighting of it …more just the early tipping of it.,)

    1. Doug, there seems to be a lot of confusion about where a turn starts and ends. A good example is the frequently heard comment of Olympic commentators that a racer needs to get more pressure on their outside ski when in fact the racer has exceeded the effective edge angle limit of their outside ski due to the limitations of their technique. So for all intents and purposes the turn is over and the racer is in survival mode. I have also heard coaches remark that a racer is “finishing his or turn on their inside ski”. The reality is that the racer has either reached the edge angle limit of their outside ski or they are in the transition phase. The fact that only a small number of racers like Ligety and Shiffrin are actually able to make the Ski Move adds to the confusion. Most assume that a good racer can be taught to ski like Ligety or Shiffrin. From the perspective of athletic ability, the answer is unequivocally yes. But the problem is that for most skiers the ski boot is preventing the joint actions needed for them to ski that way. Some will take the position that a good athlete can adapt. That is like saying that Usain Bolt can run as fast with a 100 lb weight on his back or his feet tied together as he could without these impediments. Insofar as tipping the inside ski, as far as I can tell cues of this nature came about because of the erroneous perception that one can ski on two skis by simply rolling the knees into the turn, use their legs like 2x4s to tip the skis on edge and let the skis make the turn without skier input. I intend to demonstrate that this is a very bad idea. Bear with me. I can only go so fast as I think it likely I am deep into new territory for most readers of my blog. I apologize for the long answer to your question. My position is that a turn starts at the point where the ski goes from the base being flat with the snow and begins to rotate into the snow. BTW the two photos of Shiffrin are the same. In the photo she is within microseconds of the start point of her turn either way. It is not exact because of the frame rate of television. Her ski is ‘tipping’ into the turn because of an over-centre mechanism that is being driven by pronation in conjunction with a multiplier effect from the stack height of the equipment. This is about the same as the stack height effect of a hockey skate blade or about 7 cm. Hopefully, this will all start to make sense in the next 7 or 8 posts. Keep the comments coming. I need to know whether I am connecting with the reader and if not, what course correction is required,

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