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Taking a stand on the stability issue... - Page 2

post #31 of 60
Wear the fox hat: Not the snow, the SKI. I was talking about the skier and the ski, not the skier and the snow.
Am I explaining this wrong, or am I just getting the idea wrong?

DM: Sounds good to me Fox.
[img]graemlins/thumbsup.gif[/img]
post #32 of 60
CREATING INTERNAL STABILITY

If internal (dynamic) stability is required to manage external instability it follows that the key to managing external instability is the ability of the skier to create internal (dynamic) stability by bringing the stability of the snow up to the pelvis.

Hypothesis: The state of inherent (dynamic) stability of the joint system supporting CM is a reliable indicator of internal (dynamic) stability.

If this hypothesis is true then we need to concern ourselves with the factors that affect dynamic stability of the joints. There are three interrelated factors:
1. The position of the joints as it relates to the position and relative angle of the joints.
2. Compression force acting in a joint as it relates to the magnitude if force and its distribution. This is directly related to item 1.
3. The number of muscles acting across a joint and the degree to which their actions are coordinated.

Joint Stability
To acquire an awareness of item 1, try the following. Sit with your torso erect in a straight back chair with your knees bent at 90 degrees and your feet approximately hip width apart. Although either foot can be used start by holding the head of your right femur on either side with your hands so you can sense movement of the femur. Now slowly rotate your right foot clockwise without letting your femur move. You should be able to rotate it about 35-40 degrees. Note what happens to your foot as you rotate it. Now try rotating your right foot counterclockwise. You should only be able to rotate it about half as much as you could in the other direction. Again, note what happens as your foot reaches the limits of its range of rotation.

Now either support your leg so the knee is straight or sit on the ground to create this alignment. Do not let the femur rotate. Try the same tests again and note the result. If you wish to experiment further try these tests with knee angles in between 90 and 180 degrees.

What conclusions can we arrive at in terms of the alignment of the knee joint and its effect on stability of the knee?
post #33 of 60
Tog, heed the following:
Si: Unless we include dynamic and kinematic considerations in any model of skiing I think it is very likely that we won't be very successful.
DM: You got my vote Si.
post #34 of 60
[quote]Originally posted by Tog:
[QB]DavidM, I was only talking about physical distance that his body moves relating to the assumptions fox made that we only move very little in relation to our feet.

Your court case is a good question. I think Bode is allowing his feet to go way outside and using the force that pulls his body down the hill. So I don't think he "jumped". I see no platform from where he could have "jumped" from plus I see a lot of momentum to his skis shooting out to his left and the outside of the new turn.

Fastman: Tog, I empathize with your confusion on Fox's statement that the CM doesn't move much in relation to the skis. In the Bode shot sequence his hip is clearly well inside the vertical plane of the feet when he's at the apex of the turn. As I am sure, Fox, that you see that, I can only assume that you are refering to something else. Rather than the feet in relation to the vertical plane, are you refering to the feet in relation to the line of centrifugal force directed through the hip? That would put them more in line. As far as striving for a quiet upper body, I'm right with ya kid, just don't post to many pictures of Bode because he's doing his damndest to prove that goal as unimportant!

Tog, there is a platform to jump. In the completion phase of a turn the skier is, hopefully, in a state of dynamic balance on his uphill edges resisting the combined forces of centrifugal force and gravity. Those engaged edges can also be used a stable base of resistance to pull against. In other words you can use your leg muscles to pull your body up and over the skis. Just as the edge resists your pushing on it during the turn, it can resist your pulling on it during the turn transition phase. David's question (I think) is do we do that, or do we just relax our resistance and allow gravity to do the work for us. Good question. I would suggest that we utilize a variety of a combinations of the two. In relaxed free skiing just a very subtle pull to initiate the gravitational fall is needed. In situations where the glide phase between turns must be eliminated and a new turn must be started quickly, a strong pull to expediate the transition can be used. I would think race turns such as the one in the Bode shot sequence would often require a quick strong pull transition. Comments appreciated. :
post #35 of 60
FastMan: I think Bode is allowing his feet to go way outside and using the force that pulls his body down the hill. So I don't think he "jumped". I see no platform from where he could have "jumped" from
DM: Hmmm. Are you sure? Maybe I need to go a bit farther with the internal stability mechanism. If we all agree that an effective skier manages instability are we inferring that Bode is not an effective skier? If this is not the case then we are inferring that Bode manages instability. To do this he has to maintain internal (dynamic) stability. How does he do this? That is the question.

Let me add one more truth:
"Effective skiers always try to keep one foot on the snow".

Fastman: Rather than the feet in relation to the vertical plane, are you refering to the feet in relation to the line of centrifugal force directed through the hip?
DM: Oooh, getting warm. Maybe Bode is somehow keeping CM acting through his feet.

Fastman: As far as striving for a quiet upper body, I'm right with ya kid, just don't post to many pictures of Bode because he's doing his damndest to prove that goal as unimportant!
DM: Maybe not. Let's wait and see what comes out of this thread.

Fastman: In the completion phase of a turn the skier is, hopefully, in a state of dynamic balance on his uphill edges resisting the combined forces of centrifugal force and gravity. Those engaged edges can also be used a stable base of resistance to pull against. In other words you can use your leg muscles to pull your body up and over the skis.
DM: You are right that he should have a base at his feet at turn completion. But completion is also initiation in effective skiing. Is there a more stable base available for Bode to use? Eralier I have said that an effective skier wants the most stable base possible to move off of.

Fastman: I would suggest that we utilize a variety of a combinations of the two.
DM: You got my vote on this one.

Fastman: In relaxed free skiing just a very subtle pull to initiate the gravitational fall is needed.
DM: Yawn...but you won't win speed events with this one. It is tooooo slow.

Fastman: In situations where the glide phase between turns must be eliminated and a new turn must be started quickly, a strong pull to expediate the transition can be used. I would think race turns such as the one in the Bode shot sequence would often require a quick strong pull transition.
DM: Or a power assist of some kind to either augment the existing external forces or create a force that is predominant.

Since one of Fox's isssue's is to ferret out the role of the outside and inside skis (feet) in a turn can we focus on the internal stability issue. Can anyone offer conclusions on knee stability and joint angles from my earlier post? I will add more information later today.

Comments appreciated.
post #36 of 60
[quote]Originally posted by David M:
[QB]FastMan: I think Bode is allowing his feet to go way outside and using the force that pulls his body down the hill. So I don't think he "jumped". I see no platform from where he could have "jumped" from

DM: Hmmm. Are you sure? Maybe I need to go a bit farther with the internal stability mechanism. If we all agree that an effective skier manages instability are we inferring that Bode is not an effective skier? If this is not the case then we are inferring that Bode manages instability. To do this he has to maintain internal (dynamic) stability. How does he do this? That is the question.

Fastman: Whoops david, I think you miss read my post. The above quote you attribute to me were not my words, they were TOGS which I later responded to with an explanation of how a platform in fact does exist.

Fastman quote: As far as striving for a quiet upper body, I'm right with ya kid, just don't post to many pictures of Bode because he's doing his damndest to prove that goal as unimportant!

DM: Maybe not. Let's wait and see what comes out of this thread.

Fastman: David, this is simply a tongue in cheek commentary on Bode's willingness (and ability) to abandon maintenance of a quiet upper body in favor of effecient edge control. He won't sacrafice the carve. I will be starting a thread on what makes Bode fast in a few weeks after I return from the alps, stay tuned, it will be interesting. :
post #37 of 60
As the knee joint straightens there is less rotary movement available at the foot. I would therefore conclude that the knee joint is more stable as it straightens. Is this correct?
post #38 of 60
HarveyD: As the knee joint straightens there is less rotary movement available at the foot. I would therefore conclude that the knee joint is more stable as it straightens. Is this correct?
DM: Yes. When the knee is flexed (bent) the interface of the joint opens to allow rotation to occur between the femur (thigh) and the tibia (leg). As the femur becomes more aligned with the tibia (moves towards 180 degree alignment of the joint) the amount of possible horizontal rotation in the joint decreases progressively to zero at close to 180 degrees. As other factors come into play the knee becomes very stable at small angles.

Good one Harvey! [img]graemlins/thumbsup.gif[/img]

Here's another related issue.
The transfer of rotation of the femur through the tibia to the foot is:
1. More efficient with the knee bent at a large angle, or
2. More efficient with the knee bent at a small angle?

Want to try and make it 2 for 2 HarveyD?
post #39 of 60
I'd say that the transfer of femoral rotation would be more efficient with the knee bent at a large angle. So, now what is the practical application of these observations? Start the turn with extension and finish flexed down to improve edging?
post #40 of 60
Nah I'm backing straighter leg - try turning leg(femur) while in a tuck....
post #41 of 60
David, I think some quick clarification may be in order so everyone understands your terminology.

I believe the angle you are refering to is the angle BETWEEN the femor and the tibia/fibula. A straight leg would represent 18o degrees.

As the knee is bent the angle progressively gets smaller until the angle would approach 0 degrees when the leg was completely folded with the thigh pressed against the calf. A straight leg represents a large angle, a bent knee produces a smaller angle.

# #
# #
@ 180 degrees @ 90 degrees
* *
* *

# Femor * Tibia/fibula @ knee cap

David, is this what you mean? Thought I heard some confusion in the posts replys. Saying a large angle could be misinterpreted as meaning a large bent in the knee.
post #42 of 60
Whoops, disregard the picture. It didn't come through as intended in the posting process. :
post #43 of 60
If we go by Fastman's definition then I'd change my answer. I was originally thinking in terms of actual knee angle rather than with conventional terminology.
post #44 of 60
2. More efficient with the knee bent at a small angle?
If small angle means straighter leg I'd go with that.
post #45 of 60
Isn't the efficiency of this transfer directly related to the angle of the femur to the hip? And don't we create this angle in large part with the bending of the knee, which draws the foot up under the hips? I see the efficiency as a curve, with the middle being most efficient. Staight leg = inefficient, and knee bent at 90 degrees = inefficient. There seems to be a sweet spot for skiing rotation for me that lies between those two extremes. At least as it relates to my skiing.

This sweet spot of knee bend for me allows for muscle recruitment that feels more efficient, or at least stronger. Is what I feel correct? I really don't know. [img]smile.gif[/img]
post #46 of 60
Oops, sorry for the confusion re large and small knee angles. (Obviously) I meant a linear relation of the femur and tibia as 180 degrees (net small angle at the joint) and a right angle relation of the femur and tibia (net large angle at the joint). Thanks for clarifying this Fastman and others. In order to avoid future confusion how about if we use ‘small angle’ to mean a joint that is extending or opening to create a more linear alignment of the bones on either side and a ‘large angle’ to mean a joint that is flexing to create a more angular relationship with the bones on either side? If anyone wants to suggest an easier reference please feel free.

RicB’s point is on topic. There is an ideal relation of the hip, knee and ankle joints that reflects a strong skier stance. Obviously the knee must have some amount of flexion in a strong stance. Less angle at the knee is generally better in terms of stability. But there is a limit when other factors come into play.

After our little experiment I think the case is strong that stability in the knee will increase as the alignment of the femur and tibia approach a linear relationship. When supporting the weight of the body (CM) the compression force resulting from stacking CM more vertically over the knee can increase stability in the knee by up to 400%. Based on this it is reasonable to infer that as the femur and knee become more aligned in a ski stance compression force in the knee will increase and with it stability. Compressive loading of the ankle joint has a similar effect.

So what are the implications for skiing? Let’s first look at Wear the Fox Hat’s issue as it relates to the outside and inside skis in a turn. The real issue as I see it is the outside and inside feet and legs. Here are two things to consider.

1. Since the inside leg is bent at the knee and ankle more than the outside leg then the inside leg will be less stable under load than the outside leg based on joint angle alone all other factors aside.
Agree. Disagree

2. Rotary forces are most effectively applied to the skis during active extension (i.e. extension with the use of concentric muscle contraction).
Agree. Disagree

Hopefully we are moving towards a position on the role of the outside and inside feet in a turn. Since the functional limitations of the feet are a significant factor in the system I will throw out some issues later today or tomorrow.
post #47 of 60
Dm, I was confused with your 180/0 degree terms. I had to think of the knee straight or bent. Sorry to jump in here this deep into the conversation, but I found it interesting. Any way,

#1 agree.

#2 not so sure. What I would say is that rotation is best acompanied with some extention or flexion, depending on the situation. Looking at it from a short leg, long leg view, then rotation would be best added to the lenthening of the shorter leg, or rotation to a more stable structure. Of course in skiing, there are always exceptions. Sometimes we need to rotate as our leg gets shorter. So is the most efficient use of a skill always most appropriate when blended with other skills? [img]smile.gif[/img]
post #48 of 60
Thanks RicB. From a perspective of knee stability the mechanics favor the outside foot and leg in a turn as being dominant. But this depends on other factors.

In the lower limb system the hip joint is the most stable because it is a ball and socket joint. The knee is the most predictable in terms of stability and instability because this is closely tied to the angle of the joint. The mechanics of the foot favor the outside foot because the inner aspect of the foot is designed to carry much larger loads than the outer aspect which is more like an outrigger or training wheel on a bike. It helps to steady the structural arch of the foot.

The key issue is the stability of the foot. If for whatever reason the outside foot is unstable then the inside foot and leg can be more stable as a unit in some situations all things considered. So the answer to Fox Hat's question lies in the stability of the foot.

In terms of the application of rotary forces there is an opportunity at the initiation of a turn to extend on both legs. This affords a soft start to rotary forces so that as extension progresses and the knee becomes more stable the application of rotary force becomes more positive. This is what I was thinking of when I posed the question.
post #49 of 60
David, could you elaborate on what you mean by the term "soft start"? It seems that the extension of both legs is good technique in short turns as can be seen on Steve Smart's short turn video as well as on the website. Does this mean it's more of a pivoted start? I guess what i'm trying to reconcile is that I thought we determined that rotary was enhanced with somewhat more flexion.

[ January 27, 2003, 11:44 AM: Message edited by: HarveyD ]
post #50 of 60
I just confirmed that rotary is more responsive during active extension by trying it on my 8Board. Thanks for stating that for us, David.
post #51 of 60
Thanks HarveyD. By soft start I mean that the rotary action of the femurs engages the legs and feet (and skis) progressively. It is like feathering the clutch of a car as opposed to coming on hard. This is due to the progressive closing of the interface in the knee that accompanies extension. This allows rotary and edging to inetgrate as opposed to being separate events.

The movement pattern at the end old turn/start new turn is important because it allows one to rotate both legs simultaneously.

Steve Smart, good skier. Was a Whistler boy until recently.
post #52 of 60
Thanks, David M. This last discussion facilitates my understanding of the Smart video, where he indeed does integrate rotary and edging to initiate a carved short turn.

[ January 27, 2003, 12:24 PM: Message edited by: HarveyD ]
post #53 of 60
OPENING COMMENTS for this thread started by Wear the Fox Hat
1.Wear the Fox at: In another thread, DavidM said, “My issue is the ability to develop the most stable platform available under foot.”
2. I do know that maximum stability of any stationary object occurs when the CoG acts through the centre of the area of support,
3.David has mentioned in several threads about weight transfer from one ski to the other, and (correct me if I'm wrong, DM), would be more of the opinion that turning involves using the outside ski in the main, whereas I would have thought that using both would be better.

DM: The 2nd statement is the key because it centers on where the center of support can be in relation to the outside and inside skis. Specifically, does the center of support have to fall within one foot or can it be somewhere between the two feet. It is my position that the location CoG is driven by whatever position results in the most stable possible platform under foot. This is turn depends on which foot can develop the most stability.

There are 3 possible scenarios. Here is the first one. Assume Hard Pack conditions (not ice)

Scenario 1 – Outside foot is dominant
The outside foot is dynamically stable. By ‘dynamically stable’ I mean that any change in the orientation of the platform under the outside foot about its long axis during a turn will be consistent with the flow of the movement of the joints of the body (Let’s leave the ‘how’ until later.) The outside foot will be pronated (i.e. rotated about its long axis into the hill). The outside leg will be extended and rotated into the hill (this is consistent with the rotation of the foot in pronation). The pelvis will be rotated into the outside leg (towards the outside of the turn) and the outside leg will be rotated as a unit about the ankle. This in combination with bending at the waist raises the inside hip and moves the head of the inside femur in front of the ankle of the inside foot. These actions are possible for two reasons; 1) the stability of the ground has been extended from the snow through the foot through the knee to the pelvis and, 2) the heel and ball of the outside foot provide resistance for the outside leg to rotate about the ankle.

Hypothesis
During a turn the pelvis acts as a bridge between the legs.

What about the inside leg? The inside knee and ankle are more flexed than the outside knee and ankle. The difference will increase as the skier comes across the hill in the 3rd phase of the turn. So the connection between the femur and tibia of the inside leg will be ‘loose’ in terms of the ability to apply rotary force to the foot or to resist rotary force applied to the knee by the snow. The tension in the pelvis resulting from its position with the outside leg will cause the femur to move away from the pelvis (i.e. into the hill). If this were the swing foot in walking then the foot would be supinated.

What about the inside foot? Weight applied to it by CM acting though the inside hip will cause the foot to pronate. Don’t I mean supinate? No. How can the inside foot pronate if the outside foot is pronated? The outside foot will be pronated because the position of CM will cause the external force to drive the foot into pronation. The inside foot is supported on its outer border. So any weight applied to it will cause it to pronate. The looseness of the connection at the knee limits the type and amount of force that can be applied to the inside ski by the foot. This same mechanism allows the femur to move in a direction normally associated with a supinated position of the foot while allowing the foot and tibia to rotate in the opposite direction.

Control of the inside ski is limited to three factors: 1) the amount of weight (force) applied to the ski, 2) the fore/aft distribution of weight on the ski and, 3) the edge angle. The mechanism is the position of the pelvis and inside femur. The position of the pelvis in rotation about the ankle of the outside foot is used to control the fore/aft distribution of pressure on the ski by controlling the position of the head of the femur in relation to the inside foot. It also regulates edge angle by controlling the position of the femur into or away from the turn. The amount of force applied to the inside ski is regulated by a combination of rotation of the pelvis and bending at the waist. This regulates the position of CM either away from or into the turn.

It is not possible to effect the same degree of control of the inside ski that can be achieved with the outside ski because of the looseness that exists in the inside knee. In the process described above sufficient weight is applied to the inside foot to tension the knee by allowing the foot to pronate and then using the femur to move the tibia into or away from the turn to control edge angle.

This is a lot to assimilate. So I will hold off posting the mechanics of a dominate inside foot until later.
post #54 of 60
I can attest to the tendency of the inside foot to pronate, since my efforts to balance on the uphill ski's outside edge in one-footed skiing have so far been frustrated despite my attempt to roll the hips into the hill. I'll be anxious to learn how to achieve more dominance with the inside foot.
post #55 of 60
Take up inline skating.
Work on parallel turns (on corresponding edges) over off season.

Your skiing edging(& balance) will improve out of sight.

Hint: get some lessons from an IISA (??) qualified instructor.
You want the rear knee almost directly behind the front one when doing these.

We do them on a bike track about 3 feet wide on a long downhill - using the turns for speed control. Other site we use we take witches hats & turn around a course.

Warning - take care on your first ski run - you will tend to have an opposite ski tip lead for the first few turns...
(skating the inside skate leads on outside edge)

Worked so well on me I know a couple of people taking up skating this summer. I pretty much started the last winter season ahead of where I finished - instead of behind.
post #56 of 60
HarveyD, what I feel when I'm doing outside edge turns is that I'm leveragng my boot shaft to acheive edge angle by movng my leg from the hip, or as DM puts it, using the femur to control the lower leg shaft. I also have to fight getting in the backseat when on the inside edge, which is because my hips don't have the natural movement of an inside edge turn. A small forward leveraging extention helps me stay forward. This feels quite different from the other edge which is focused on the foot, and has the natural feel of a regular one footed turn.

DM, if I'm undrestanding you right, I can't find anything to disagree with here. It's just as I feel it. The ball of the outside foot is where I feel my stability come from in the type of turn you desribe. Even in a tele turn, the stability comes from both feet being on the balls. Take this to snowboarding (which I don't do) and this probably translates to the difference between toe side and heel side stability. It's all pretty interesting. :
post #57 of 60
Yes you are right RicB in what you describe. [img]graemlins/thumbsup.gif[/img]

The looseness in the knee of the inside leg is the key factor limiting the ability to develop much stability on the inside ski of the turn. The only way you can effect any degree of control on the inside ski in the first scenario is if you are able to develop a stable base of support on the outside foot and leg. This allows the pelvis to be used as a bridge to transfer stability to the inside hip so you can control the femur of the inside ski. Awareness of this mechanism can help you to fine tune the use of the inside ski.

I compare the process to learning to play a musical instrument. One learns the basics of the instrument and then learns exercises like playing scales unil they achieve a basic level of competence. After this they begin to learn simple music. Eventually they become a technician. But one does not become a musician until they acquire rythym and feel. Even after they reach his level good musicians always continue to hone their technical ability by playing scales. One does not buy an instrument and then simply join an orchestra or do solo recitals. They acquire competence first.

In my view activities like skiing would benefit from using a similar model.
post #58 of 60
Before I proceed with the next scenario I would like to put forth the following hypothesis for consideration.

The ski boot affects the angle of the ankle which in turn affects the angle of the knee. It follows then that there is a direct relation between the forward lean angle of the ski boot and the stability of the knee of the outside leg of a turn.
post #59 of 60
Quote:
Originally posted by David M:
Before I proceed with the next scenario I would like to put forth the following hypothesis for consideration.

The ski boot affects the angle of the ankle which in turn affects the angle of the knee. It follows then that there is a direct relation between the forward lean angle of the ski boot and the stability of the knee of the outside leg of a turn.
Is there more or did everyone give up?
post #60 of 60
Quote:
Originally posted by Wear the fox hat:


So, that’s my theory, and I expect the main point of debate will be over the phrase in stage 2:
“the best option is to spread the weight evenly over the two feet”.

The question is, “Is this correct?”

Well, I’m not sure. I do know that maximum stability of any stationary object occurs when the CoG acts through the centre of the area of support, but this does not mean that we, as sensate beings obey that rule. Perhaps our brains override the best solution with one which fits our world view, so we no longer spread the load evenly and put more weight on one support than the other.

Where do you stand on this issue?

S
I think it's more simple than that. According to definition, "Weight is a quantity representing the force exerted on a particle or object by an acceleration field". When you are standing, you feel your weight, because you are interacting with whatever it is you are standing on. When you are jumping down a cliff, you are feeling weightless, even if you weigh 300 lbs.

Conversely, when you are carving a turn, you body experiences the gravity pull, the centrifugal acceleration, which observers cannot see from the chairlift, because it only acts on YOU, and the reaction of the snow to both of these forces. Therefore, when you let both of your feet go away from underneath your body, your weight is still balanced (unless you overedge) over your skis FOR THE CONDITIONS OF THE TURN, even if in the picture it looks like you are about to fall.

The opposite is true. If you force yourself to place your CM always over the point between your boots as the VERTICAL line indicates, you will most likely do a faceplant the moment your skis accelerate into the turn enough to create a centrifugal acceleration.
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