3-D motion capture comparison of elite WC SL skiers

3-D motion capture comparison of elite WC SL skiers

Postby theorist » Fri Dec 20, 2013 1:20 am

I try to keep on top of the sports science literature on skiing. Most of it does not have direct application to ski technique, but here’s one that does:

R. Pozzo, A. Canclini, G. Baroni, D. Benedetti and S. D’Ottavio. 3-D kinematic and kinetics analysis of slalom in elite skiers at the Bormio World Cup ski finals in 2008. In E. Muller, editor, Science and Skiing V (Meyer & Meyer, London), 2012, pp 355-363.

This paper was presented at the Fifth International Congress on Science and Skiing, which was held at St. Christoph am Arlberg, Tyrol, Austria, in December 2010.

The full paper is not available online, but the first five pages (355-359), which contain most of the key results, are available through Google preview:
http://books.google.com/books?id=zZvxTQ ... 08&f=false

What makes this study so interesting is that the authors weren’t simply asking how WC racers ski. Rather, they used 3D motion capture and timing to determine what movement characteristics separate the best-of-the-best from the best. Specifically, they set up a set of cameras covering five gates in the middle of the first run at the 2008 World Cup Finals in Bormio, and analyzed the performance, over that section, of the top 8 skiers in that run (top eight for the run, not top eight for the race). In order of run placing, from first to eighth, they were: Manfred Moelgg, Felix Neurether, Reinfreid Herbst, Baptiste Grange, Marcel Hirscher, Marc Bethod, Ivica Kostelic, and Benjamin Raich. [Incidentally, the three top overall SL skiers for ’07-’08 were Moelgg (531 pts.), Grange (512 pts.) and Herbst (450 pts.).] It turns out that two fastest skiers over that section (Neurether and Moelgg), as well as the two slowest over that section (Raich and Kostelic), were also the fastest and slowest for the run, suggesting that this section was representative of overall run performance. [I was unable to determine if the run finish order for all eight was the same as the performance order in this section, since the authors don't provide numerical velocity values for all eight, just a bar graph -- the paper is not as well-presented as it could be.]

Using their motion-capture system, the authors measured (or calculated from those measurements) the following over the entire section: the velocity and path of the skiers' centers of gravity (CG); the angles of inclination of both legs of each skier (defined by the lines from ankle to hip joint) to the snow surface; and the angles of both knee joints. Here are their key findings:

1) The CGs of the fastest skiers tended to show less vertical displacement (i.e., less up-and-down motion -- only 0.5 m – 0.6 m between the highest and lowest points, which presumably are the transition and the point of maximum angulation, respectively). Relatedly, their CGs also had slower lowering velocities, i.e., the rate at which their CGs moved closer to the snow between the transition and the point of maximum angulation. The latter result is expected, since their CGs, having smaller vertical displacements, had less distance to travel. But another possible contributing factor (the authors didn’t comment on this) could be that the faster skiers also spent more time in the first phase of the turn (from initiation to the point of maximum angulation).

Interestingly, the rates at which the skiers raised their CGs, in coming out of the turn, didn’t show a consistent pattern.

2) The CGs of the faster skiers tended to have the largest minimum turning radii (i.e., they didn’t go straight at the gate and turn sharply) (note this is the turning radius of their CGs, not of their skis). Correspondingly, they experienced the lowest maximum centripetal accelerations. Likely because of this, their skis had the lowest maximum ground reaction forces (GRFs). E.g., Neurether and Raich didn’t differ much in their minimum GRFs (1.97 BW vs. 2.17 BW, respectively; BW is body weight), but their maximum GRFs were 3.33 BW and 4.04 BW, respectively. I.e., Raich was briefly pulling 4 g’s. This value is consistent with measurements found in earlier studies. A lower GRF makes for less frictional losses and a ski that is less likely to break out of its carve. It also puts less physical demand on the skier.

3) The knee angle measurements were presented in a somewhat confusing manner, but the faster skiers had a more extended stance leg (less bend in the knee) at the midpoint of the turn (i.e., when the turning radius was at a minimum), at least for the two turns where the slope fell away to that side (these two turns also showed the most leg inclination). They also measured angular velocities of the knee joint (i.e., how fast the knee flexed and extended), but didn’t appear to find a clear trend.

4) By contrast with 1-3, above, the authors found that the maximum angles of inclination of both the inside and outside legs, which they presumed corresponded to the angles of the skis to the snow, did not vary significantly within this group.

Given that they captured the inclination angles and knee joint angles of both the inside and outside legs, I assume they also have the video data to measure stance width at the transition. It’s too bad they didn’t report this – it would have been interesting to see if it showed a correlation to performance. Maybe I’ll see if I can track down one of the authors and ask them this.

It should be emphasized that none of these athletes were skiing poorly – note that even the worst of this lot was the eighth best in the world on that run! From that perspective, they all skied at an extraordinarily high level. Yet, as is obvious from the race times, even among such an elite group one finds important differences.
User avatar
theorist
 
Posts: 277
Joined: Mon Mar 18, 2013 9:57 pm

Re: 3-D motion capture comparison of elite WC SL skiers

Postby h.harb » Fri Dec 20, 2013 7:01 am

I was at two of the congresses, prior to this one where the Slovenians had done this same study with Kostelic as their model. I've been through 10 days of presentations at the Congress and presented there three times, over the last ten years. Unfortunately, much of this work doesn't lead to innovations is learning or teaching skiing. I have definitely never seen anything of value that has translated into what a regular skier can benefit from for their skiing.
User avatar
h.harb
 
Posts: 7047
Joined: Sat Feb 03, 2007 2:08 pm
Location: Dumont, Colorado

Re: 3-D motion capture comparison of elite WC SL skiers

Postby theorist » Fri Dec 20, 2013 11:19 am

Hi Harald -- thanks for your reply. I agree completely that "Unfortunately, much of this work doesn't lead to innovations is learning or teaching skiing." Indeed, as I wrote myself, "Most of it [the sports science literature on skiing] does not have direct application to ski technique."

But I'm afraid I'm nevertheless a bit confused. You've often said that PMTS is science-based, and that it's based on WC SL technique, so I thought you'd appreciate a study that gives a science basis to some aspects of WC SL technique. But if you don't find value in scientific studies like this, then may I ask what scientific studies are out there, that you do like, that serve as the scientific basis for PMTS?

Based on my limited understanding, it seems there are three key components to PMTS:
1) What outcomes the WC SL skiers generate that are also of value to recreational skiers (e.g., reduced vertical motion, beautiful balance, clean carves).
2) What movements they use to generate those outcomes (this, I understand, is the heart of PMTS).
3) How to communicate those movements to students -- i.e., how to enable the students to do those movements themselves.

I'm not saying something needs to be science-based (i.e., directly supported by published peer-reviewed studies that have gained general acceptance within the scientific community) in order to be a valid teaching system -- often such studies haven't been done. But you've said PMTS is science-based, so could you please explain in more detail what you mean by that? To be fully science-based, it seems you would need scientific studies in support of all three components listed above. There are a limited number of studies, like the one I cited, for no. 1. For no. 3, I suspect there are some general scientific studies of teaching methods, but I don't know if there are any specifically supporting the teaching of the phantom move, etc. (if you could provide me with any references, that would be great). But the heart of PMTS is no. 2 - the idea that WC SL racers themselves do the PMTS essential movements (tipping of the inside ski; flexing of the stance leg; pulling back the feet to maintain fore-aft balance; CA; and CB). This is also the area that generates the most controversy, since while outcomes are visible, the underlying movements often aren't. To do a scientific study showing that the racers actually use the PMTS movements, you'd probably need to combine motion capture with real-time EMG measurements -- so you could tell exactly which muscles are or are not being activated -- and specifically look for the PMTS essentials. I've not seen such a study so, again, if you could provide any links to relevant articles that would be great.

As before, any clarification you could provide to alleviate my confusion would be appreciated!
User avatar
theorist
 
Posts: 277
Joined: Mon Mar 18, 2013 9:57 pm

Re: 3-D motion capture comparison of elite WC SL skiers

Postby Max_501 » Fri Dec 20, 2013 2:55 pm

theorist wrote:As before, any clarification you could provide to alleviate my confusion would be appreciated!


This is not rocket science. Straight forward bio-mechanics and simple physics is the science that PMTS is based upon. If you start with page 1 of Book 1 your confusion should be alleviated.

BTW, the Essentials can easily be seen in all of the top WC racers.
User avatar
Max_501
 
Posts: 4124
Joined: Thu Mar 10, 2005 7:39 pm

Re: 3-D motion capture comparison of elite WC SL skiers

Postby jbotti » Fri Dec 20, 2013 3:49 pm

theorist wrote:Hi Harald -- thanks for your reply. I agree completely that "Unfortunately, much of this work doesn't lead to innovations is learning or teaching skiing." Indeed, as I wrote myself, "Most of it [the sports science literature on skiing] does not have direct application to ski technique."

But I'm afraid I'm nevertheless a bit confused. You've often said that PMTS is science-based, and that it's based on WC SL technique, so I thought you'd appreciate a study that gives a science basis to some aspects of WC SL technique. But if you don't find value in scientific studies like this, then may I ask what scientific studies are out there, that you do like, that serve as the scientific basis for PMTS?

Based on my limited understanding, it seems there are three key components to PMTS:
1) What outcomes the WC SL skiers generate that are also of value to recreational skiers (e.g., reduced vertical motion, beautiful balance, clean carves).
2) What movements they use to generate those outcomes (this, I understand, is the heart of PMTS).
3) How to communicate those movements to students -- i.e., how to enable the students to do those movements themselves.

I'm not saying something needs to be science-based (i.e., directly supported by published peer-reviewed studies that have gained general acceptance within the scientific community) in order to be a valid teaching system -- often such studies haven't been done. But you've said PMTS is science-based, so could you please explain in more detail what you mean by that? To be fully science-based, it seems you would need scientific studies in support of all three components listed above. There are a limited number of studies, like the one I cited, for no. 1. For no. 3, I suspect there are some general scientific studies of teaching methods, but I don't know if there are any specifically supporting the teaching of the phantom move, etc. (if you could provide me with any references, that would be great). But the heart of PMTS is no. 2 - the idea that WC SL racers themselves do the PMTS essential movements (tipping of the inside ski; flexing of the stance leg; pulling back the feet to maintain fore-aft balance; CA; and CB). This is also the area that generates the most controversy, since while outcomes are visible, the underlying movements often aren't. To do a scientific study showing that the racers actually use the PMTS movements, you'd probably need to combine motion capture with real-time EMG measurements -- so you could tell exactly which muscles are or are not being activated -- and specifically look for the PMTS essentials. I've not seen such a study so, again, if you could provide any links to relevant articles that would be great.

As before, any clarification you could provide to alleviate my confusion would be appreciated!


So the real question is whether you can learn PMTS and become a better skier using PMTS without first finding a peer reviewed scientific study. If the answer is no, then you might ask yourself why and if you are really here to learn and to become a better skier or if you have another agenda. If the answer is yes, then clearly you don't need references or links to a peer review study and you can get on with what all of us trying to accomplish which is to become a better skier using PMTS.
Balance: Essential in skiing and in life!
User avatar
jbotti
 
Posts: 2184
Joined: Fri Nov 28, 2003 10:05 am

Re: 3-D motion capture comparison of elite WC SL skiers

Postby h.harb » Fri Dec 20, 2013 4:38 pm

Theorist, thanks, yours is a well thought out post, , however the reason I don't value the (results) studies, (I value the efforts) other than for Academic maturation, is for the very reason I stated earlier, that is, the studies don't advance ski learning or instruction. I used the studies, yes, but only to evaluate and substantiate what PMTS was "already capable" of doing, the science and biomechanics were already in PMTS, studies or not. Efficient mechanics or movements is what PMTS already taught at every level, and it was acheiveable.. Now that is shown, (through thousands of results with participants) and it's success is obvious, let's move on to movements skiers can learn, can perform, that gives them confidence and balance, enough to approach the skiing efficiency that World Cup skiers achieve.

PMTS is now, a much larger, more focused, and more relevant study of skiing progress, than any study that any University can produce.
User avatar
h.harb
 
Posts: 7047
Joined: Sat Feb 03, 2007 2:08 pm
Location: Dumont, Colorado

Re: 3-D motion capture comparison of elite WC SL skiers

Postby jclayton » Sat Dec 21, 2013 9:31 am

Theorist , I see why you have that nickname .

As the guys say science is used as a base for PMTS . Biomechanics etc which do have peer reviewed studies .

A peer reviewed study of PMTS itself is hindered by the fact that Harald has no peer !! . ( World cup skiers sure , but they do not have the science based knowledge or inclination to start up a system , a strong probability and my assumption of course )

The best , nearly peer , review is by the number of expert skiers who have been through the camps and system and demonstrably show their huge improvement and visibly outski 99.9999% of skiers on the slopes .

I have yet to see a peer review study of Pilates , Yoga etc etc but unlike religion one can see visible benefits .
skinut ,among other things
User avatar
jclayton
 
Posts: 1019
Joined: Wed Oct 29, 2003 12:37 pm
Location: mallorca ,spain

Re: 3-D motion capture comparison of elite WC SL skiers

Postby cheesehead » Sat Dec 21, 2013 9:57 am

theorist wrote:The CGs of the fastest skiers tended to show less vertical displacement (i.e., less up-and-down motion) ...
Their CGs also had slower lowering velocities....
The CGs tended to have the largest minimum turning radii (i.e., they didn’t go straight at the gate and turn sharply) ....
Their skis had the lowest maximum ground reaction forces (GRFs)... A lower GRF makes for less frictional losses and a ski that is less likely to break out of its carve. It also puts less physical demand on the skier....
The faster skiers had a more extended stance leg (less bend in the knee) at the midpoint of the turn....

Well, yes, I would say all of those things are consistent with the foundations of PMTS. No extending, no edge sets, smooth movements throughout the turn.
--- aka John Carey
Madison, Wisconsin
cheesehead
 
Posts: 274
Joined: Thu Feb 19, 2009 3:42 pm

Re: 3-D motion capture comparison of elite WC SL skiers

Postby theorist » Tue Dec 24, 2013 5:54 pm

h.harb wrote:Theorist, thanks, yours is a well thought out post, , however the reason I don't value the (results) studies, (I value the efforts) other than for Academic maturation, is for the very reason I stated earlier, that is, the studies don't advance ski learning or instruction. I used the studies, yes, but only to evaluate and substantiate what PMTS was "already capable" of doing, the science and biomechanics were already in PMTS, studies or not. Efficient mechanics or movements is what PMTS already taught at every level, and it was acheiveable.. Now that is shown, (through thousands of results with participants) and it's success is obvious, let's move on to movements skiers can learn, can perform, that gives them confidence and balance, enough to approach the skiing efficiency that World Cup skiers achieve.

PMTS is now, a much larger, more focused, and more relevant study of skiing progress, than any study that any University can produce.

Thanks Harald – I did put a lot of thought into those posts, so I really appreciate your saying that. And thanks also for the above clarification – I can now better see where you’re coming from.
User avatar
theorist
 
Posts: 277
Joined: Mon Mar 18, 2013 9:57 pm

Re: 3-D motion capture comparison of elite WC SL skiers

Postby theorist » Tue Dec 24, 2013 5:59 pm

jbotti wrote:So the real question is whether you can learn PMTS and become a better skier using PMTS without first finding a peer reviewed scientific study. If the answer is no, then you might ask yourself why and if you are really here to learn and to become a better skier or if you have another agenda. If the answer is yes, then clearly you don't need references or links to a peer review study and you can get on with what all of us trying to accomplish which is to become a better skier using PMTS.

Please note that, while I find them of value, I never said I required a peer-reviewed study in order to learn skiing; rather, I wanted to know what Harald meant by science-based, which he nicely answered.

It would probably help if I explained what motivates my questions: I’m a perpetual student of all the sports I do. I’ve found I progress fastest when, in addition to doing regular, focused training, I also gain a good big-picture understanding of whatever system I’m following -- its origins, underpinnings, philosophy, range of application, and so on. This big-picture understanding gives me a framework that helps guide and focus my practice. My coaches consider this a strength – they say it makes me a better student, since ultimately you have to teach yourself (as Harald wrote in 2008, “I can only give you the mechanics, it has to ultimately come from you, from your inside, if you want to become a great skier.”). For me, therefore, the two approaches are synergistic, not mutually incompatible.
User avatar
theorist
 
Posts: 277
Joined: Mon Mar 18, 2013 9:57 pm

Re: 3-D motion capture comparison of elite WC SL skiers

Postby theorist » Tue Dec 24, 2013 6:00 pm

cheesehead wrote: Well, yes, I would say all of those things are consistent with the foundations of PMTS. No extending, no edge sets, smooth movements throughout the turn.

Yes, that was my observation as well, which is why I thought it might be of interest to forum members. Note, however, that this work by Pozzi et al. is a study of outcomes, not of what movements the racers are using that give rise to them.
User avatar
theorist
 
Posts: 277
Joined: Mon Mar 18, 2013 9:57 pm

Re: 3-D motion capture comparison of elite WC SL skiers

Postby theorist » Tue Dec 24, 2013 6:06 pm

jclayton wrote:Theorist , I see why you have that nickname .

As the guys say science is used as a base for PMTS . Biomechanics etc which do have peer reviewed studies .

A peer reviewed study of PMTS itself is hindered by the fact that Harald has no peer !! . ( World cup skiers sure , but they do not have the science based knowledge or inclination to start up a system , a strong probability and my assumption of course )

The best , nearly peer , review is by the number of expert skiers who have been through the camps and system and demonstrably show their huge improvement and visibly outski 99.9999% of skiers on the slopes .

I have yet to see a peer review study of Pilates , Yoga etc etc but unlike religion one can see visible benefits .

I should first clarify that what I was asking after in my original posts (sorry if this wasn’t clear) was not a study of the effectiveness of PMTS as a teaching system. Rather, it was a study confirming the movement patterns and muscle activations that elite racers use to achieve their results. As an ex-racer, that’s of more direct interest to me.

A great practical value of established scientific results is that they settle controversies. Serious thinkers argued for millennia about whether a perpetual motion machine was possible, until modern thermodynamics settled that in the mid-19th century (except for those at the fringe). As for ski racing, as Harald remarked:
h.harb wrote:Some people are commenting on my technical analysis on my Facebook page and it's obvious how diverse the understanding of skiing for this level of skiing is.

As with the perpetuum mobil, if studies were done to establish a scientific consensus about these movement patterns, this diversity of views would largely disappear, which would have the effect of focusing the entire ski instruction community more in the same direction (not entirely, because there would still be controversy about how to best teach the movement patterns to students, but at least the arguments about what movement patterns the racers actually use would be quieted). It’s also important to note that having a broad-based consensus within the scientific community is qualitatively different from having an argument based on biomechanics, since in the latter case someone could make their own biomechanics-based argument and come to a different conclusion, thus perpetuating the controversy, which is what we see today.

Finally, at the risk of getting OT, yoga has, in fact, been the subject of extensive scientific study. Granted, you don’t need such studies to practice, and benefit from, yoga. Nevertheless, there’s a movement today towards achieving a convergence between east and west. If we were to gain a scientific understanding of yoga (and of other eastern bodies of knowledge, like the Chinese internal martial arts, which I practice), that would deepen our understanding of these arts, and also deepen our scientific understanding of the human body.
Last edited by theorist on Tue Dec 24, 2013 6:11 pm, edited 2 times in total.
User avatar
theorist
 
Posts: 277
Joined: Mon Mar 18, 2013 9:57 pm

Re: 3-D motion capture comparison of elite WC SL skiers

Postby h.harb » Tue Dec 24, 2013 6:09 pm

Some people analyze outcomes better than others, and the best analyze and ascertain why and where certain outcomes result. In either case, you still have to convey accurate movements to the student for their demonstrated weakness.
User avatar
h.harb
 
Posts: 7047
Joined: Sat Feb 03, 2007 2:08 pm
Location: Dumont, Colorado

Re: 3-D motion capture comparison of elite WC SL skiers

Postby Max_501 » Tue Dec 24, 2013 6:33 pm

theorist wrote:It would probably help if I explained what motivates my questions: I’m a perpetual student of all the sports I do. I’ve found I progress fastest when, in addition to doing regular, focused training, I also gain a good big-picture understanding of whatever system I’m following -- its origins, underpinnings, philosophy, range of application, and so on.


In that case you definitely need to start with Anyone Can Be an Expert Skier 1, the PMTS Instructor Manual, and the companion videos.
User avatar
Max_501
 
Posts: 4124
Joined: Thu Mar 10, 2005 7:39 pm

Re: 3-D motion capture comparison of elite WC SL skiers

Postby theorist » Tue Dec 24, 2013 6:39 pm

h.harb wrote:Some people analyze outcomes better than others, and the best analyze and ascertain why and where certain outcomes result. In either case, you still have to convey accurate movements to the student for their demonstrated weakness.

Agreed, of course.

Harald, I have just one more key question about the PMTS-racing connection I’d like to get an understanding of, but I hesitate to ask because you’ve already taken a lot of time to respond to my earlier queries. So I’ll ask, but I’ll apologize in advance for asking. I hope that’s OK!

The question is analogous to the one I had about Ligety’s wide GS transition stance, except it concerns the edge change sequencing I see in Hirscher’s slalom. You provided such a rich and clear answer about Ligety, which is why I’d be really interested to hear what you say about this feature of Hirscher’s skiing:

There are three basic edge change sequences:
1) LTE of new inside foot leads BTE of new stance foot => “O-frame”
2) Simultaneous => “Π-frame” (I’ve used the Greek capital Pi, signifying lower legs are parallel)
3) BTE of new stance foot slightly leads LTE of new inside foot => “Λ-frame” (Greek capital Gamma, signifying lower legs are angled medially; I’m not using“ A-frame,” because that refers to something different, where the skier is using the inside leg for support).

I did a photo montage reconstruction of the all the transitions in Hirscher’s second run at Zagreb in Jan. 2013 (http://www.flickr.com/photos/111386007@N06/11538860684/). I chose that run for two reasons – first, it was a great run – with it, he went from 0.01 secs. behind to winning the race by 0.57 secs.; second, his skiing in this run was particularly beautiful and relaxed (the course was set by his coach and thus optimized for Hirscher's strengths), making what I saw more likely to reflect his true technique rather than mistakes. I understand PMTS has good reason for advocating against no. 3 for developing skiers. However, in the montage, I only see the O-frame three times (one of which was the same one you captured in one of your recent blogs); IIRC at least one was used when Hirscher wanted to make a very rapid but small direction change. The remaining transitions are about equally divided between Π-frames and Λ-frames. So what’s going on with all the Λ-frames, i.e., where Hirscher has moved to the BTE of his new stance leg before the LTE of his inside leg? And it’s not just Hirscher. Since you’ve been to the ski science conferences, you probably remember a 2010 motion capture study by Reid of elite slalom skiers (though not as elite as Hirscher), in which he found that, on average, at the crossover point (CM crosses over skis), the skiers were already at a 5 degree BTE angle on their new stance ski while they were still on the BTE of their new inside ski (also at 5 degrees). I've also seen this in Ligety's skiing and (though it's unusual), sometimes in your skiing as well (http://www.flickr.com/photos/111386007@N06/11539047525/).

Is this specific type of BTE lead a mistake, or is it a specialized tactic that, while counterproductive for developing skiers, can be legitimately used by elite racers to increase their speed by getting them carving their stance ski at the earliest possible point in the turn (with the usual admonition that it works for them only because of the maturity of their technique and the high forces involved)? I should add that a Λ-frame necessitates a marginally wider transition stance, to make room for the legs to angle in.

I’ll also add it seems this particular instance of BTE-lead is not inconsistent with a phantom move. What Reid measures as increasing negative knee angulation (in the stance leg at the end of the turn) seems to be the phantom move (see http://www.flickr.com/photos/111386007@N06/11538824195/); the only difference here vs. a standard PMTS phantom move seems to be that, at around the time that the phantom move is starting, the old inside leg is moving off its LTE so fast that it sometimes gets to its BTE before the new inside leg moves to its LTE.
User avatar
theorist
 
Posts: 277
Joined: Mon Mar 18, 2013 9:57 pm

Next

Return to Racing

Who is online

Users browsing this forum: No registered users and 6 guests