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Stance width and the movements of a turn

Postby SkierSynergy » Tue Jul 13, 2004 11:47 pm

An earlier post tried to work though the relationship between stance width, flexion, and ski tipping angles:

?Harald has always advocated relatively narrow stance as more functional. So how come he can carve and get deep angles? Is he defying physics or something?
I believe the trick is in relaxing the parallel shins restriction. In the upper C part of the turn aggressive free foot tipping will increase the gap between skis (inside ski will follow a shorter radius arc) and thus allowing impressive angles, however before transition into next turn skis have to come together again, so a little A frame is unavoidable.?

You are probably correct that there will sometimes be forces that will tend to separate the skis. Most of the time it is probably different edge angles or some knee steering on one of the legs. However, I?m not sure that there is a necessary increase in separation because the inside ski is following a shorter arc. I would guess the issue is more one of body movements (kinesiology) rather than one of the movements of bodies (physics). I?ll take a shot at working through the issue from what I know of PMTS and then anyone else can add or subtract what they will.

Let?s get back to an analysis of tipping and stance in terms of primary movements and see which movement(s) widen the stance during a turn. Just for a reminder: the three primary movements are 1) inversion / eversion of the feet 2) plantar and dorsiflexion of the ankle and pulling the feet back in general, and 3) flexion / extension of the legs. There are of course secondary movements of the hips, shoulders and arms, but they can be ignored here.

1) Rolling the skis onto a set of edges (inversion/eversion of the feet) will not necessarily widen the stance width. It moves the CM to the inside of the turn and if no other movements happen the body begins to tip into the turn. If the skis are put onto the same angle at roughly the same time they should follow parallel arcs of the same radius. Further, even if that is not the case, one can always adduct to pull the legs and skis together. .
2) Of course pulling the free foot back through plantar flexion of the ankle and the more general actions of pulling the leg back will enable one to invert the free foot to a higher angle, but if the angle is matched by the stance leg (as it does easily because internal rotation of the stance leg is so much easier), then this should not lead to a wider horizontal stance either.
3) Where we should begin to see an increase in the stance width is when the free foot leg is flexed. However, this is a widening of the stance vertically; not horizontally. The free foot glides up the stance leg in proportion to the angle of tipping and the level of pressure needed for the conditions. Flexing is done to allow a greater range of external rotation of the femur and open up a deeper range of angles that can be achieved by the movements in the foot and ankle. Flexion also allows the upper body to resist merely banking into the turn by allowing more separation in the movements of the lower and upper body.

Just as an exercise, place your hand on a wall and lean into it with fairly stiff legs. Now, without flexing your legs a) check to see how much inversion is available just through foot movements 2) see how far you can tip your upper body out over the stance leg. Then, let the imaginary free foot ride up the stance leg until it reaches knee height. Try seeing how much easier it is to a) get a greater tipping angle and b) get your upper body out over the stance leg. These two effects is what gives the ski ?bite.?

Vertical separation is functional and comes through flexion. Excessive horizontal separation is not functional and is either produced by leg steering and mismatched ski angles (the cause of this could be a either a movement deficiency or an alignment/equipment issue), or actively induced through abduction.

As Harald has posted elsewhere on the forum, there are some very good biomechanical studies looking at the negative effects of a wide stance (either parallel or wedged) on both ski performance, knee stress, and injuries rates. If anyone is interested, I can post a recent reference from a presentation at this year?s international congress on ski safety by North America?s leading researcher on these issues. From his results, he advocated abandoning the wedge and adopting a narrower stance with the emphasis on free foot tipping toward the little toe edge.

Two additional points. First, I am not advocating a ?tall in the transition? and flexed in the end of the turn rhythm. I only started my example with relatively stiff legs to first isolate the inversion / eversion movements from the other movements involved. The more flexion one can have at the beginning of the turn the greater range of movement and control one will have throughout the turn. Second, I am not advocating locked boots. The feet should be horizontally wide enough to work independently, but not so far apart to inhibit a functional release. Within the parts of a trurn the stance may tend to be the narrowest at the release because the narrower the stance and the more the flexion, the esaier it is to invert the old stance foot into a release. So, it would make sense to see a more definite narrowing at the release/transition.

By the way, I found Elvis on the mountain top last weekend:

http://web.pdx.edu/~petersj/HoodCamp/Elvis.htm

Enjoy
Jay

PS Hood still has great skiing left. Come on out!
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Postby piggyslayer » Wed Jul 14, 2004 6:49 am

Jay,

I am not getting how the flexing free leg BY ITSELF is resulting in carved vertical separation.
Yes sure, you lift your leg you get vertical separation, but the ski is not carving when you are JUST lifting it. If you look at both skis carving the snow, lifting action is equivalent to unweighting the inside ski. This simply will reduce how much the ski is bent into an arc, and if anything, should reduce not increase the curvature of the inside arc (the more you bent the ski the smaller the radius or bigger the curvature)!

If you look at the skis on the snow and assume that the skis are carving all the time the two clear contributors to the curvature of each arc are:
(1) edge angle of given ski
(2) how much the given ski is bent into an arc
So to carve and increase the separation between feet (if you lean this separation will be ?vertical? as opposed to horizontal I argue that you GOT to increase curvature on the arc left by inside ski. Whatever else you do will not be carving. You can ?slide? the ski to separate, ?skid?, lift up and place in a different spot, but all of this is not carving.

I believe carved way to increase the separation can ONLY be accomplished with aggressive tipping of the inside leg which is not matched by the stance leg (thus shins are not parallel). This is the reason I replied to earlier posts suggesting that strict parallel shins are the measure of skier ability. I agree with Bode/Bodie that they are not.

I agree that the more aggressive tipping needs to be accompanied by flexing the inside leg, but such flexing by itself (without extra LTE) will not lead to carved separation.

I think you simply "lighten" the inside leg and LTE tip hard to achieve carved separation.
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Postby *SCSA » Wed Jul 14, 2004 7:40 am

Morning Jay,

Would you mind posting that reference you mentioned?
Thanks in advance,
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Postby Ott Gangl » Wed Jul 14, 2004 8:16 am

Hmmm, don't you think that increased pressure to the shovel of the inside ski, achieved by pulling that ski back under you thus pressuring the front of the boot which transferres it to the tip of the ski which will bend and bite more and describe a shorter radius, will do the trick?. (Howzat for a sentence?)

If done judiciously so as not to lose the tail of the ski in a washout it will do the trick in recreational skiing.

....Ott
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Postby piggyslayer » Wed Jul 14, 2004 10:07 am

Hmmm, don't you think that increased pressure to the shovel of the inside ski, achieved by pulling that ski back under you thus pressuring the front of the boot which transferres it to the tip of the ski which will bend and bite more and describe a shorter radius, will do the trick?. (Howzat for a sentence?)

I think it MUST be part of it. But to achieve extreme angles, I think, more is needed.

And the more interesting thing, how the "vertically" separated skis end up together at the end. Is my A-frame "theory" at all viable?
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Postby milesb » Wed Jul 14, 2004 11:10 am

Mr. slayer, when you lift the inside ski alongside the stance ski, it shifts a significant amount of weight to the center of the turn. This causes the body to fall more into the turn. You can see this by doing it with one hand on a wall. get your feet on edge, and lift the inside foot up to the level of your outside knee. Notice the increase of pressure on your hand?
Maybe it's just me, but I have found that tipping the inside ski only gets my skis onto about a 30 degree angle. If I want more, I can either move my outside knee into the turn (big bad A frame) or flex the inside leg aggressively. Guess which works better! Note that it is pretty scary to do this move early in the turn on a steep slope.
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Horizontal Seperation and flexion

Postby SkierSynergy » Wed Jul 14, 2004 11:54 am

Let me clarify the purpose of my last post. There had been some discussion about whether the physics inherent in a carved turn (the inside ski taking a tighter radius ? keep in mind I disagree with this description) tends to produce a horizontal separation of the skis during parts of the turn. This didn?t seem correct to me ? see my post for why. So another question to ask is: ?Which, if any, of the primary movements produce separation of the skis and what type of separation they produce?? I am attempting to bring things back to an analysis of the primary movements used within PMTS.

In my imaginary example of a turn, both skis were kept weighted. There was no transfer of weight to the stance ski by lightening the new free foot ? free foot here, only designates the inside foot. Piggyslayer is correct that lightening and tipping the free foot can produce vertical separation -- the more one flexes to lighten the more separation However, this is also true when the free foot is kept weighted. That is why I used the more general movement term of ?flexion? rather than ?lighten.? Both skis were kept weighted in my example, then, three movements happened in order:

1) Inversion/eversion to tip the skis on edges; 2) dorsiflexion and pulling the free foot back to increase the tipping ability; and 3) flexion of the free foot leg and extension of the stance leg, to again increase the tipping ability and allow a better upper body position. Even at this point the free foot may still carry a lot of weighting, though the momentum of the turn will tend to passively shift the weighting to the stance leg.

My main points were first, that (given the other movements in the description) it was the flexion of the free foot leg that allowed the vertical separation of the feet/skis. Piggyslayer is correct that flexion alone will not produce a good PMTS carve ? though it could put the stance ski on it?s inside edge and produce a type of carved turn. But my example had a lot more going on than just flexion. In my example, if there wasn?t any flexion the body would simply lean or bank into the turn. I know this well because a common problem of mine is not having enough flexion on short hard carves. I pay for it in less hold and decreased ability to adjust to changing conditions. My second point was that none of the primary movements in themselves produce horizontal separation. Horizontal separation is produced by other things: alignment/equipment issues, dysfunctional movement patterns, etc.

I hope this helps clarify. I am really trying to bring the analysis of this issue back to looking at primary movements because at base, it is these movements (rather than bigger maneuvers) that are taught in PMTS.

I think Piggyslayer brings up another important issue though and that is exactly how pulling the foot back through dorsiflexion and how leg flexion contribute to a greater ability to tip using inversion, but that is for another person to do or another time.
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Function of pulling the free foot back

Postby SkierSynergy » Wed Jul 14, 2004 12:03 pm

Ott,
I don?t think that the function of pulling the foot back is to pressure the front of the boot and thus bend the shovel of the ski. Mainly, it controls fore and aft balance (which one could use to pressure the shovel of the ski, but I don?t think that is advocated in PMTS ? if your pushing boot plastic you are more likely leg steering or something).

However, pulling the leg back also works to increase the available tipping ability of the inverting free foot.

Next time you are on skis do thius static exercise. Extend what would be the free foot out in front of the stance foot (maybe have about ? boot length between the boots). Try to invert the free foot toward the LTE. You will have a definite blocking of your ability to invert.

Now pull the free foot back so that the toe of the free foot is behind the stance foot toe. You will notice a tremendous increase in the ability to invert toward the LTE -- given that your boots allow the proper articulation that is needed.

It is this increased availability of deeper angles that I find most intriguing.

The same happens with increasing your level of lexion. Proper inversion and pulling the foot back will take you to another level of edge angles, but then again you will hit a block. To get more available, you have to increase the amount of outward rotation available in the upper leg. This is what flexion does. Both actions increase the amount of tippping that is available and increase the ease with which it comes.

Jay
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More Angles

Postby Harald » Wed Jul 14, 2004 12:24 pm

There is more than one answer to, how do you get more angles? Let?s look at some situations first.

Upper body:
1. What happens if your upper body and hips are leading the turn or are creating turning energy? If you don?t have the awareness to bring the body back into a connected and aligned position to the forces, you won?t develop bigger angles.
2. If the upper body isn?t at least slightly counter balanced, you will lean into the turn causing angles to stop developing.

The reason I started with the upper body stems from the need to build pressure. If pressure under the foot of the stance ski is not building you won?t be able to get inside far enough to develop angles. High body angles depend on keeping the upper body balanced inline with the ski edge. If this gets disconnected or disrupted, angles stop building.

The lower body needs to relax to get into a high angle position. If your legs are steering you aren?t relaxing, because you are using you adductors and this causes the mass of the hip to move therefore not aligning to the ski edge. Steering rotates the bigger mass higher up, decreasing the body?s ability to create angles. There are cases when an athlete uses body rotation to accelerate the edge through the turn, but balance and counter must be set up perfectly and this is usually done on flat slopes where there is plenty of time to recover.

Once you release from the previous turn the inside leg must flex freely early in the high ?C? part of the turn. This moves the CG (Center of Gravity) inside the turn. If the skis are too wide, the tendency is to lose pressure on the DH or outside ski. As the outside leg lengthens to maintain contact with the snow, the ankle must be tipping to increasingly higher angles to maintain edge grip. The ankle controls the knee?s position under the CG and hip. If knee drive is applied edge function is reduced. Relaxing the hip and inside leg from the high ?C? to middle of the turn, can only be achieved and increased if balance and pressure are perfectly aligned to the ski edge. This alignment begins from the shoulders, goes through the body, all the way down to the big toe edge of the stance ski.

If you watch Bode?s wilder turns you will see his upper body far advanced into the lean for the coming turn. He can sometimes get away with this as he gathers it together and puts himself into the ?traditional? angulated position before the pressure load phase of the turn. By traditional I mean hip, knee, ankle and shoulder angulation. He and everyone else have to get to this point at sometime in the turn or they will not be able to hold the edge. You must be very relaxed to position the body in this way and as soon as the pressure is needed you have to contract the muscles in the legs, hips and torso to withstand the forces.

I experiment with this type of turn using the Von Greunegin as it gets the upper body started to the new turn more aggressively than a two footed release. In a difficult situation on steeper slopes or on bad snow, I prefer to keep my CG more closely connected to the pressuring edge from the top of the high ?C?. So that?s a start but not the complete answer. Have a go with that one.
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Studies on stance width and knee issues.

Postby SkierSynergy » Wed Jul 14, 2004 4:42 pm

One good researcher on issues of stance width is Tom Andriacchi from Stanford University. He is one of the top researchers on knee injuries and does a lot of work with these issues in bike racing. It turns out that the relationship between flexion, stance width, and lower leg totation are really important for injury issues in bikers.

More recently he gave a presentation in the Biomechanics section of the International Congress of the International Society of Ski Safety 2003. His analysis of knee mechanics in deep flexion during skiing have some interesting conclusions. the following are my interpretations of his stuff.

1. Andriacchi's data on deep flexion concludes that lower leg rotation is to some extent inherent in the action of flexing the knees. When the knees are flexed the tibia naturally rotates a bit. the extent increases with the deepness of the flex (just how much also varies among individuals). In a stance no wider than the hips, the affects are minimal. However, the effects increase dramatically as the stance widens or moves into a wedge. Flexion with either a wide stance or wedge greatly increases the rotational force while also putting most of the force on the tail of the ski.

I read this as an inevitable recipe for skidding at some point in the turn - probably the end. In order to reduce these effects, Andriacchi suggests a narrower stance with emphasis on little toe edge tipping. As far as I know he is not familiar with PMTS.

2. More importantly, his work also concludes that either a wide stance or a wedge with flexion dramatically increases stress on the ACL. Again, a narrower stance with emphasis on little toe edge tipping generally reduces stress on the knee. In addition, contracting the leg muscles to pull the leg back also reduces the stress on the ACL by reducing anterior translation of the tibia (stabalizing the head of the tibia in it's more natural position).

Considering that knee injuries of various types account for approximately 30-40% of all alpine ski injuries ? and that there are probably even more complaints about sore or tired knees (especially from older skiers) one should take this advice seriously.

On this research, the traditional advice of a horizontally wide stance with flexing and inward driving of the knee is the worst thing you could do for either edge hold or knee stress; and an move toward a horizontally narrower stance with emphasis on little toe edge tipping enhances both carving hold and reduction of knee stress.

You can also see Dr Mike Langran's summary of the Congress at:

http://www.ski-injury.com/lrn.htm#Biomechanics

I don't think Andriacchi has published the ski paper in any journal, though he is at Stanford. I have several of his other papers dealing with these issues in bike racers. I will look for them. Originally I think I got them while doing searches under his name on the Stanford website and the sites of the international Society for Biomechanics in Sports.

Hope this is interesting.

Jay
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Postby piggyslayer » Thu Jul 15, 2004 9:39 am

Jay:
Hope this is interesting.

Very!
Interesting enough for a separate thread.
Thanks for the earlier post as well, I agree with it.
My basic question is: can strict parallel shins requirement be relaxed in expert skiing.

milesb:
I agree with you. I just tried to look at what is causing the "carved" separation of skis. How come both skis carve arcs and these arcs are first close together and then far apart and then again close together. I concluded that "uneven" tipping between LTE and BTE is happening.
This whole thing was triggered by earlier post suggesting that strict parallel shins are a must in expert skiing and I think they are not.

Still reading Harald's post.
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