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knee "safety zone"

post #1 of 20
Thread Starter 

Hi guys, 

 

I'd like to figure out the "safety zone" for your knee ligaments when pivoting. (I refer here to pivoting as the femur turning in the hip socket. Applied equally in carved or smeared turns.) Here's a pic SkiDude referenced in an earlier thread where the skier's well outside of the safety zone: 

 

 

Now some people may be saying "that's because he's a-framing!" But my understanding is that even if both legs matched the angle of his outside leg, he'd still be structurally weak, as that kneecap's getting loaded. 

 

There's a lot of emphasis on coiling these days - i.e. allowing the lower joints to turn while resisting with the upper body, which winds the skier up like a spring. I'm wondering if that turning of the lower joints past a certain point endangers the knees of our athletes. 

 

So for this guy below, when is he in danger? (sorry for the grain--using a video camera.)

 

Here's the guy neutral. 

 

 

Tipping onto edges - looks aligned from foot through to hip

 

Not aligned--turning knees. 

 

Not aligned - outside knee is cockeyed and tipped inside. 

 

 

Hey, it doesn't look so cockeyed now that the inside knee's matching... but is it actually stable? I actually appear to have moved my hip inside as well... perhaps that's why it looks more stable as well... Also, I know I'm not showing ski school definition of pivoting. I tried walking through a pivoting turn and it looks clunky and stupid--gravity's a b!tch. But I'd like to think we can still explore the danger zone through some of these shots, plus any others that people can contribute. 

 

Is there a simple rule of thumb we can use here? e.g. keep the outside knee out from under the body? 

post #2 of 20

I think the first thing I would say is "don 't pivot any more than you have to." If you're in a race course, you have to steer, pivot, stivot, whatever you want to call it when the vertical distance between gates is less than the sidecut of your skis and/or the horizontal offset is...well, pretty offset. This is Ron LeMaster's discussion of pivoting to get to the "effective steering angle" (I wish he'd have called it the "effective carving angle."). As you say "I tried walking through a pivoting turn and it looks clunky and stupid--gravity's a b!tch."

 

If you're on something super steep and/or hard, you don't have to windshield wiper. Just get the turn started before the fall line, continue through the fall line, release as soon as you exit the fall line. Or, if you want to back your speed down, pull the finish of the turn more across the hill and out of the fall line.

 

I don't think of carving as something you only do on a race course or hard snow. I carve turns in powder because it still makes sense to get the skis up on edge and make them bend in all that soft stuff. That way, the skis turn you rather than you trying to muscle a flat ski around in powder. Similarly, I think the best turn in bumps is a carved turn.

 

If I look at your still shots, there's no pivoting, but there are various degrees of knee angulation. There's a theory that says knee angulation is bad...it stresses the knee, and the leg should be in a straight line, letting the angulation happen only at the waist. Maybe, maybe not. The knee's a pretty fragile structure anyway, and some knee angulation can be a good thing. As in, as LeMaster noted in one of his fall presentations, race coaches are talking about "early outside knee angulation" as a way to get bigger angles soon up above the fall line. It's also a way to increase edge angles once you're in the fall line without risking tipping in.

 

So...I dunno about rules of thumb, but I guess mine is "Be as kind to your knees as you can be...but it's skiing, and if you're knees aren't doing good things, your turns won't, either...":cool

post #3 of 20

Pivotting your skis at 80km/h on a WC course is kind of out of my range of thaught but if we back off a bit I can relate to it a little bit better. I dont think that your knees would be in danger when steering/pivotting your turns. The danger steps in when you carve because if you pivot and skid in case something happens you probably will just skid some more. But when you carve your skis dont give you any remourse. They can cut in under you and if you are A-framing a lot your knees can indeed be at danger. So yes, keep your knees out from underneath your body. Thats a good rule.

post #4 of 20

#3, sort of.  The way A-framing is used in high performance skiing doesn't involve a very tipped nor pressured inside ski.  It is waiting to be used.  In the old days it was because the equipment required us to be downhill ski focused and when that blew you had something to stand on.  The side cut was minimal, the skis where long and straight.  When you did put that foot down it was already on edge under your foot (centered) and you were in a countered position to balance against the forces and could "cut the corner"so-to-speak to change your turning angle quicker.  Today I think it can still be used to 'catchup' or to avoid something quickly (think racing and someone cutting you off on a groomer)

 

#3 is dangerous because the ski is tipped.  It doesn't look like you have too much pressure or counter though.  So, if your DIRT is good enough to avoid the intricacies of the terrain, you might get away with it (edit because I meant to add and if the inside is tipped and pressured quite a bit).  If you add counter to pic #3 it is very bad, especially if your DIRT is off. 


Edited by Crud Buster - 12/4/13 at 7:48pm
post #5 of 20

IMHO all of those are well within normal range. Nothing to be really concerned about.

Although, if the difference between femoral IR and tibia ER increases than yes it could be dangerous. Your knee, believe it or not is supposed to go into valgus. 

post #6 of 20

One important point is how fore-aft affects the moment in the knee.

 

The sideways bending moment in the knee is approximately given by the distance between the plane spanned by the ankle, knee and hip joint and the force vector concentrated under the ski.

 

What this means in practice is that if you are balancing on your heel (aft) the knee is less sensitive to A-frame. If you are very much fore the sideways moment becomes larger.

 

One example is Vonn's accident. She landed in A-frame and the ski cut into the snow so making the force very much fore. Very sensitive position

 

Also it doesn't really matter what causes the aforementioned distance, knee angulation and hip angulation both put stress in the knee.

 

IMO it is not advisable to knee-angulate more than the limited amount that comes from tipping the feet, i.e. no knee-drive.

post #7 of 20
Quote:
Originally Posted by jzamp View Post
 

IMHO all of those are well within normal range. Nothing to be really concerned about.

Although, if the difference between femoral IR and tibia ER increases than yes it could be dangerous. Your knee, believe it or not is supposed to go into valgus. 

 

 

Quote:
Originally Posted by Jamt View Post
 

One important point is how fore-aft affects the moment in the knee.

 

The sideways bending moment in the knee is approximately given by the distance between the plane spanned by the ankle, knee and hip joint and the force vector concentrated under the ski.

 

What this means in practice is that if you are balancing on your heel (aft) the knee is less sensitive to A-frame. If you are very much fore the sideways moment becomes larger.

 

One example is Vonn's accident. She landed in A-frame and the ski cut into the snow so making the force very much fore. Very sensitive position

 

Also it doesn't really matter what causes the aforementioned distance, knee angulation and hip angulation both put stress in the knee.

 

IMO it is not advisable to knee-angulate more than the limited amount that comes from tipping the feet, i.e. no knee-drive.

and yet many people ski with valgus without pain or knee issue.

 

Both jz and Jamt are pointing to the same thing.  The amount of joint torque, the sum of the strain in all the related tissues surrounding the knee. We can often look at people and feel something needs correcting, but it is the quality of their sensation that matters more than.  Teaching people to become aware of multiple options to change the sensations in their knees is more important than directing one alignment over another. There are to many considerations that impact how people use themselves, giving them the contrast of what feels like more torque/strain vs less would be a better learning tool IMO

post #8 of 20
Quote:
Originally Posted by Metaphor_ View Post
 

 

 

 

 

 

First pic above - hip angulation - good for long radius turns.

Second pic - knee angulation - good for short radius turns. (I've always heard that knee damage comes from falling back and low while the ski remains engaged, not while skiing along with knee angulation.)

Somewhere in between - good for medium radius turns. 

 

Learning to ski with knee angulation is an eye-opener.  It delivers power to the turn.  

One focus for making turns this way is to keep the middle of the inside shin (top of boot) under the belly button/hips, not out to the side.    

Getting this to happen is complex, involving turning the femur in the hip socket, flexing the inside knee just the right amount, focusing on the inside ski's LTE pressure, and paying attention to where the middle of the inside shin is relative to the belly button/hips.

 

Once a feeling for knee angulation kicks in, it's a power trip.  Keeping that inside shin beneath the hips strengthens medium radius turns as well.

Fast-tiny-wiggly-carved-turns directly down the fall line are possible. Fun!

 

Rule of thumb:  keep the inside shin as close to under the hips/belly button as possible given the turn forces you are managing.


Edited by LiquidFeet - 12/5/13 at 7:21am
post #9 of 20

There is some subtle distinction to make Meta.  If your femur is turned inwards just a bit so that the knee is not 90 degrees square to your hips, it matters which direction you are leaning too.  So in your photo you are leaning exactly 90 degrees sideways from the direction your pelvis is facing and you're turning your foot inward from both the square position of your pelvis as well as the the direction of lean (and the direction of force vector).  

 

Now imagine that if you have some hip counter, your knee in fact is not facing square to your pelvis, it will be pointing a bit in, (or the pelvis is pointing out, depending on how you want to look at it.)

 

Do the same lean in your house with the lean corresponding to 90 degree angle from the direction your knee is pointing.  In other words line up the force vector down your stacked leg with the knee forward on that force vector, stacked, not turned in with knee angulation.  But have your pelvis facing slightly out.....countered.  The relationship of knee to pelvis will not be square and will be like your last photo, but the knee will be stacked 90 degrees to the force vector.

post #10 of 20
Thread Starter 

So if I had to average out the responses from this thread to form some thoughts on preserving your knees: 

 

If you're engaging edges by allowing your feet to tip and allowing the COM to move into the turn on its own (without force), you're probably aligned

If you're square to your skis and riding like a robot, you're probably aligned

 

If you're engaging edges by driving your knees into the turn, you're probably driving force through the outside of the knee and aren't aligned 

 

Here's what I don't understand: 

Driving Keeping the CoM driving through the heels is stronger, allows us to safely handle more load. e.g. when doing squats, you always keep your CoM over your heels. However, my understanding of ligament injuries, as LF noted, is most tears happen when the skier gets back and inside, rather than forward and inside... so what's the disparity?

The artificial countered, butt-twisted-inside-the-turn stance is weak and great way to explode a ligament. Is this true? 

How much pivoting, or turning of the femur in the hip socket, can we create before we're loading the knee? Assume you're allowing separation to take place. 

 

I've just been watching some CSIA clips looking for what looks out of alignment. And I'm having trouble finding some examples. So here's what I could come up with... 

Sorry it doesn't show up well static as in motion... but it appeared like the knee of the front skier has tipped farther inside the turn. The rear guy looks aligned. Now if the front skier's inside knee were tipped equally, I probably wouldn't even notice this...  but is this safe? 

post #11 of 20

again nothing unsafe in some valgus collapse. 
The chances you'll tear the ACL by tipping knees/ankles are basically none.
The ACL will tear is there is an external force that drives the tip of the ski outwards while the femur points in (kinda like phantom foot) or there is an uncontrolled medial collapse in which the femur rotates in and the tibia out (this could happen upon an hard landing)

post #12 of 20

Safe seems highly contextual, its a cool picture though. The CNS is keeping both skis engaged in both skiers. The redundancy of our movement options is a wonder. The front skier seems to have a pelvis rotated more to the right(right pelvic bone behind the left) compared to the skier in the back. Because the pelvis isn't more forward the hip has to make up the difference with more internal rotation to allow the foot the luxury of remaining more neutral and not having to over pronate to set the inside edge. 

 

The front  skier is demonstrating nice hip mobility. With respect to the earlier posts, if someone has lost this mobility over time due to joint imbalances then it is not so safe as the more distal joints need to make up the difference. Since the ankle is strapped into a ROM reducing boot, you can imagine the knee's world, increased strain. Ligament damage is a result of loss of absorption, max out the tissue surrounding a joint and the energy gets displaced to next supporting structure.

 

So long as a person can displace the forces it is safe, increase the ground reaction forces and the joints need a more neutral alignment to better displace the energy and protect the ligaments.  Which is why I don't think comparing squat force management can be used a direct correlation to skiing, squatting is a symmetrical movement, skiing is not, apples and oranges IMO

post #13 of 20
Quote:
Originally Posted by chad View Post
 

Safe seems highly contextual, its a cool picture though. The CNS is keeping both skis engaged in both skiers. The redundancy of our movement options is a wonder. The front skier seems to have a pelvis rotated more to the right(right pelvic bone behind the left) compared to the skier in the back. Because the pelvis isn't more forward the hip has to make up the difference with more internal rotation to allow the foot the luxury of remaining more neutral and not having to over pronate to set the inside edge. 

 

The front  skier is demonstrating nice hip mobility. With respect to the earlier posts, if someone has lost this mobility over time due to joint imbalances then it is not so safe as the more distal joints need to make up the difference. Since the ankle is strapped into a ROM reducing boot, you can imagine the knee's world, increased strain. Ligament damage is a result of loss of absorption, max out the tissue surrounding a joint and the energy gets displaced to next supporting structure.

 

So long as a person can displace the forces it is safe, increase the ground reaction forces and the joints need a more neutral alignment to better displace the energy and protect the ligaments.  Which is why I don't think comparing squat force management can be used a direct correlation to skiing, squatting is a symmetrical movement, skiing is not, apples and oranges IMO

Could you expand on that please?

post #14 of 20

sure.

 

met is correct, in squats the focus is pressure through the heel, but it is done so as to minimize the torque on the knee, squat with weight on the front half of the foot and you can feel the shift into the kneecap and away from the larger hip extender muscles (gluteals), relying on smaller muscles=recipe for pain. Squatting is symmetrical in that the pelvis needs to be able to rotate over the hips equally throughout the movement. Skiing on the other hand is asymmetrical, the outside leg is in a internally rotated position, the inside more externally rotated with relation to the adjoining pelvic bones, as the skier compresses to manage the turn force the hips are having to negotiate that loading through different portions of the joint compared to a squat where they are essentially the same zones of the joint.

post #15 of 20

Thanks! Thumbs Up

post #16 of 20
Quote:
Originally Posted by Metaphor_ View Post
 

If you're engaging edges by allowing your feet to tip and allowing the COM to move into the turn on its own (without force), you're probably aligned

If you're square to your skis and riding like a robot, you're probably aligned

 

There is one more possibility, which I tried to explain earlier, but I guess I didn't very well.  Your pelvis can be countered outside of square, while the knee is pointing square to the skis.  

 

This is key, its not neccessary knee angulate in order to ski into counter.

 

People have been knee angulating for decades without tearing anything, but nonetheless it is still a weaker stack to ski on.   Further to that, when you use knee angulation to create edge angles, you don't end up with the CoM inside enough to sustain the large edge angle.   You will probably begin to release.  Further to that if you rotate your femur in order to knee angulate, there is a good possibility of twisting the tail out.

post #17 of 20
Quote:
Originally Posted by Metaphor_ View Post
 

 

Here's what I don't understand: 

Driving Keeping the CoM driving through the heels is stronger, allows us to safely handle more load. e.g. when doing squats, you always keep your CoM over your heels. However, my understanding of ligament injuries, as LF noted, is most tears happen when the skier gets back and inside, rather than forward and inside... so what's the disparity?

 

 

 

Quote:
Originally Posted by jzamp View Post
 

again nothing unsafe in some valgus collapse. 
The chances you'll tear the ACL by tipping knees/ankles are basically none.
The ACL will tear is there is an external force that drives the tip of the ski outwards while the femur points in (kinda like phantom foot) or there is an uncontrolled medial collapse in which the femur rotates in and the tibia out (this could happen upon an hard landing)

 

 

I think jzamp answered your question metaphor. It is about the force direction. If you fall backwards and the edges hook up in different directions the force will point outwards and will be way outside the plane I described above. The same will happen irrespective if you are fore or aft but most falls are to the aft. In Vonns accident the ski cut down and got stuck making it very fore, but in her case the injury happened before she fell.

 

Also, we should not confuse twisting forces in the knee with sideways loading forces in the knee. I suppose ACL injuries are primarily associated with twisting forces. If you knee drive you will have both twisting and sideways loading. Not a good combo I think.

post #18 of 20
Quote:
Originally Posted by Jamt View Post
 
Quote:
Originally Posted by Metaphor_ View Post
 

 

Here's what I don't understand: 

Driving Keeping the CoM driving through the heels is stronger, allows us to safely handle more load. e.g. when doing squats, you always keep your CoM over your heels. However, my understanding of ligament injuries, as LF noted, is most tears happen when the skier gets back and inside, rather than forward and inside... so what's the disparity?

 

 

 

Quote:
Originally Posted by jzamp View Post
 

again nothing unsafe in some valgus collapse. 
The chances you'll tear the ACL by tipping knees/ankles are basically none.
The ACL will tear is there is an external force that drives the tip of the ski outwards while the femur points in (kinda like phantom foot) or there is an uncontrolled medial collapse in which the femur rotates in and the tibia out (this could happen upon an hard landing)

 

 

I think jzamp answered your question metaphor. It is about the force direction. If you fall backwards and the edges hook up in different directions the force will point outwards and will be way outside the plane I described above. The same will happen irrespective if you are fore or aft but most falls are to the aft. In Vonns accident the ski cut down and got stuck making it very fore, but in her case the injury happened before she fell.

 

Also, we should not confuse twisting forces in the knee with sideways loading forces in the knee. I suppose ACL injuries are primarily associated with twisting forces. If you knee drive you will have both twisting and sideways loading. Not a good combo I think.

The ACL is composed of 2 "strings"/bands that wrap around each other. 
A pure lateral force will not stress the ACL unless taken to such an extreme that the MCL completely ruptures and than keeps separating the tibia and femur.
What really stresses the ACL is the combination of femoral IR, tibial ER, and tibial anterior translation. This motion increases the stress on the fibers and when a lateral force is applied it becomes enough to rapture at least one of the bands. The lateral force doesn't have to be very significative, in fact most non contact ACL injuries happen without compromising the MCL.

 

This is a great animated video that shows how it happens.

 

***THERE IS SOME ARTHROSCOPIC SURGERY VIDEO IN IT, SO WATCH AT YOUR DISCRETION*** 

 

post #19 of 20
Quote:
Originally Posted by chad View Post
 

sure.

 

met is correct, in squats the focus is pressure through the heel, but it is done so as to minimize the torque on the knee, squat with weight on the front half of the foot and you can feel the shift into the kneecap and away from the larger hip extender muscles (gluteals), relying on smaller muscles=recipe for pain. Squatting is symmetrical in that the pelvis needs to be able to rotate over the hips equally throughout the movement. Skiing on the other hand is asymmetrical, the outside leg is in a internally rotated position, the inside more externally rotated with relation to the adjoining pelvic bones, as the skier compresses to manage the turn force the hips are having to negotiate that loading through different portions of the joint compared to a squat where they are essentially the same zones of the joint.

 

How about getting on a Smith machine, both feet out to one side, in order to do lifts?  One can put those feet out to the side of the COM when using a Smith machine, but ski boots would be more supportive than collapsible running shoes, which I use.  The machine supports this asymmetry, while this type squat activates different muscles than a symmetrical, normal squat.  This lift feels more similar to skiing.  

Anybody do this for skiing?

post #20 of 20
It'd rather train regular squats, in conjunction with tri-planar single leg exercises.
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