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Balance: ankle & boot bits - Page 2

post #31 of 249
Thread Starter 
Originally posted by David M:
Back to the conducive environment issue.

Keep in mind that I did research with an instrumented device back in 1991 that allowed our team to acquire 3 D data on foot and leg pressures while skiing and compare this data with video and subjective skier input. Sofar as I know this has not been done by anyone else even today.

In some instances good skiers learned for the first what they were doing when they saw the data and video. It was often very different from what they thought they were doing. In addition, when they studied the data they always agreed with it. Keep this in mind. If I were to do the same thing with any one of you out there I would expect to get the same disconnect with reality. This is the main reason good skiers can't tell others what they are doing. Any explanation they try to give is usually miles from reality. It is also why they believe skiing is easy. For them it is. For the same reason such skiers ‘don’t get’ discussion threads such as this.

Otto: When Gleason in Taos asked me to come in and let him excavate my boots the other bootfitters stood around and marveled at what I did with an Exacto knife, heat gun, duct tape, pieces from my previous boots and chewing gum. After he got through with fitting my boots they fit just as before, only with a lot fewer pieces. I don't unbuckle or take them off all day either, why should I?
DM: Sounds like me at work. Most of the crap in ski boots does nothing but cause problems. I used to horrify boot fitters at my technical seminars with what I did to brand new boots. They just could not bring themselves to do the same thing for their customers. They tended to believe it was merely a matter of finding the right boot. Yeah right! I used to log 15 to 30 hours prepping a good pair of boots for a WC racer. And yes, you should not have to unbuckle them. In fact it is better not to.

Otto: One of the first things I do with all my boots is modify the forward flexibility to give me a range of mobility without putting tongue pressure on right away, that allows me to be in variable forward/aft positions, as in bump absorption, etc. without affecting the skis, unless I want to.
DM: If you are saying you want free space in the cuff to allow for ankle/shin movement before you begin to compress the tongue then I agree. This is absolutely essential. The worst thing you can do is to cinch the cuff around your leg to as the saying goes “transfer energy”. You will certainly do this -- right to your knee, hip and back.

Forward Lean
If you are also saying that you take some forward lean out of the cuff then we may be related. In the majority of cases boots have way too much forward lean for the angles of the joints essential for postural responses driven by the stretch reflex. If this is indeed the case you don’t have less stretch reflex you have none. The best way to tell if a skier is using the stretch reflex is to watch them ski through crappy rough snow, the kind the usually tosses skiers all over the place. If they look like they are skiing groomed hardpack they are probably using the SR.

Liners can and often do cause more problems than boot shells. The Lange World Cup liners with flow packs I can rip out in 2 seconds are my favorites. My wife’s boots are Head shells with lots of modification with Lange liners and a Dachstein tongue. What is the best way to fit most liners?? In the garbage can.

post #32 of 249
Thread Starter 
Originally posted by David M:
...Here is where we are at so far. The boot lower shell or tub as I call it has to provide adequate 3D space for your foot to function within under the maximal skiing loads. Interestingly lack of forefoot width was a big problem in footwear like running shoes until companies like Addidas began to fund research to see how wide the forefoot was just prior to propulsion. The guy who did the research at the Human Performance Lab in Calgary now lives in Whistler. I have copies of his papers that give the required width for various shoe sizes. Traditional 'lasts' as they are called were way too narrow. If you saw these lasts you would know why the ski industry is in the dark ages.

When I compared the loaded width requirements to the widths of typical ski boots a size 9 boot shell (without the liner) was about 1 cm too narrow (that's about 1/2 inch for you Americans). Not bad you say. Actually, it is very bad. The liner is about 1/2 inch thick. Now we are too narrow by 1 inch for an average width men's size 9. Imagine the situation for a wide forefoot. There is no way your foot can function with this constraint. This is not open to debate.

What if the boot is wide enough in the forefoot but does not provide enough clearance for the inner ankle bone and bones of the midfoot?
In this case the inner aspect of the foot hangs up preventing the sole from 'touching down' on the boot base. Sure some of you can still ski but it takes brute strength because you can only use concentric muscle contraction. What you may not realise or accept is that your foot is still trying to adapt to the terrain. It has not developed the tension and balance it needs in its truss like arch structure to set up the buttresses requied for the muscles of the foot and keg to pull against in a COORDINATED manner.

Force applied to the foot by COG acts on the downhill side of the inside edge of the outside ski. Anyone good at mechanics and physics in the group should know this is not a good thing. In such situations saying "watch me ski" doesn't impress me. You are driving a delapidated wreck of a car.
post #33 of 249
Thread Starter 
Originally posted by David M:
Ott: There is a story in the premium section in which I desribed my first ski boots in 1944. I went to the shoemaker, he stood me on some cardboard and traced my feet, took some wooden lasts off the wall, measured and eyeballed and when the boots were ready they fit like a glove, or shoe.

In the 56 years since, wearing many, many different boots, I never rested until I got a fit that, dancing the polka at the tea dance after coming off the mountain, I felt no need to unbuckle.

DM: The reason skiers like Ott "just ski" is because they can. From the very first time they skied they had a conducive environment for function. It may not have been perfect but it was good enough to ski in. Ott had 'been there' in terms of this environment long before he clamped his feet in hard shell stiff plastic boots. Once he had been there he knew what a conducive environment should feel like. Ski boots that are the most comfortable shoes you can wear.

The problem with skiers today is that they have never been where those such as Ott have been. They have no clue as to what a boot should feel like. They have to learn this after the fact.

I am confident that I can alter a ski boot so that even God himself would ski so badly as to make a rank beginner look like a World Cup star. And there is nothing he would be able to do about it.
post #34 of 249
Thread Starter 
Originally posted by David M:
If you go back and read my earlier discussion on the events of walking you will note that after the triad of arches of the foot have made full contact with ground the ankle switches from plantarflexion to dorsiflexion. This movement begins to stretch the soleus muscle. What happens next represents one of the most critical events in both walking and the transition to an upright posture. As the head of the tibia passes in front of the ankle the soleus switches to isometric contraction. It stops lengthening and arrests further dorsiflexion. If it were not for this event three things would result; 1) the truss-like arches of the foot could not be tensioned by COG exerting a downward force to compress them, 2) the knee could not extend to lever COG up and over the ankle joint to arrive in front of it and, 2) the body would fall over.

The most important function of the soleus in arresting dorsiflexion is that the foot needs this to happen to become structurally competent in order to support COG. The balance system needs the arches to become tensioned before they can act as a proprioceptive receiver for critical information pertaining to balance. In short the foot needs the soleus in order to function period.

In addition the angle of ankle joint at which dorsiflexion is arrested is critical to the use of eccentric contraction in the chain of muscles that extends form the heel of the feet (or foot) to the pelvic girdle. If the angle of dorsiflexion of the ankle is too great the COG can not be levered over the ankle since it would have to move too far forward for the eccentric contraction of the gastroc to arrest it. The result is that we stay erect by squatting.

What are the implications for the ski boot? The angle of the cuff or shaft of the boot has to be compatible with the critical angle at which the soleus switches to isometric contraction. This should be the angle where the calf of the leg rests against the rear cuff of the boot. In a small percentage of skiers the boot shaft may be compatible with this angle. In most cases it is far too great. Rarely is it too straight.

The reasons for excessive forward lean are numerous. First and foremost is that the current wisdom in the industry is that this amount of forward lean is essential to skiing. This fit with the paradigm that pressure can be effectively applied to the fore body of the ski by the skier applying force to the front to the cuff with their shin. While this is true the physiologic consequences are severe.

The only possible way significant force can be applied to the front of the cuff by the shin bone is through the release of contraction in the soleus. This has two immediate consequences; 1) the foot becomes non-functional and the soleus can no longer sustain isometric or eccentric contraction. The effect is as if the skier has suddenly become disconnected from the ground from the knee down. Far more serious is that the tibia is now unprotected in the event of a forward fall. When in eccentric contraction the soleus contraction in well positioned to intervene to oppose stress on the tibia. This is no longer possible.

The mechanics work like this. The soleus relaxes allowing the weight of COG to act downward on the tibia. This forces it to dorsiflex against the cuff. There are 2 components of force here. One presses down and is resisted by the ground. The other acts horizontally towards the front of the boot. The reaction is down and back where it is resisted by the heel of the skier. As the cuff is progressively flexed pressure applied by the foot to the sole of the front of the boot decreases until the only force applied by the foot is under the heel. In effect the forefoot is now 'floating' about the ankle joint.

After the shin strikes the cuff and compresses the padding it starts to bend the shaft forward. This actions sustains the relaxation of the soleus muscle. How? Gravity normally acts to pull COG forward (i.e. dorsiflex the ankle). The soleus pulls back in the opposite direction (i.e. it plantar flexes the ankle). This is the normal balance equation where an internal force opposes an external force. Using the shin to flex he boot adds a new external force, the force in the bending resistance of the boot cuff. This third force has one very serious problem. It is on the wrong side of the equation. Think about this. Do the math.

I’ll be back.
post #35 of 249
Thread Starter 
Originally posted by David M:
Boot flex continued

Balance results when 2 equal and opposite forces are aligned in opposition to each other on a common path or vector.

In skiing a vertical force or a quasi-vertical force (i.e. behavioral vertical) pulls the skier down and the skier uses GRF to generate an opposing force with which to push back. The skier is in balance when the sum of all moments of force acting across the joints of the ankle complex and ascending upward through the knee at any given time equal zero. In the equation an internally generated force (i.e. muscularly generated) opposes an external force that tends to disrupt the skier’s equilibrium. The resulting equation looks like this:

1. F internal = F external

Since rotation of joints is the key issue in this balance equation the opposing forces can be thought of as clockwise and counter clockwise rotation. If in the external force is trying to rotate the ankle clockwise (into dorsiflexion) within the physical reference then soleus will oppose this force by rotating the ankle counterclockwise (into plantarflexion).

If the bending resistance of the cuff or shaft of the ski boot is opposing dorsiflexion of the ankle what direction is it acting? Counterclockwise. Therefore it is aligned in concert with the soleus. Now the equation looks like this:

2. F internal (muscle) + F2 external (ski boot) = F1 external

If you compare equation 1 with equation 2 you get:

1. F internal = 2. F internal (muscle) + F2 external (ski boot)

Therefore 1. F internal – 2. F internal = F2 external (ski boot)

CONCLUSION: Adding the bending resistance of the boot shaft into the equation forces the soleus to reduce its level of contraction.

Is this a good thing? We will see in my next post.
post #36 of 249
Thread Starter 
Originally posted by Lisamarie:

Scroll down, there are a few charts of the lower leg. Even though I do not see any copyright info, we better cover our a$$es and just put up the link. Besides, there are way too many pictures. A suggestion: If you want to refer to a specific picture, copy and paste the url of that picture. That way we will be scrolling less.

post #37 of 249
Thread Starter 
Originally posted by David M:
If someone wants to start a new thread it is fine with me. So long as we keep the perspective that all the segments relate to balance.

More boot flex/forward lean issues

There is one more interesting effect related to boot flex. When COG is supported on the base of support at your feet the weight is carried under the heel and the balls of the feet (COP is the center of the pressure or weight). In terms of balance the base of support under your feet is what your balance system uses to help determine which way is up.

As explained in my last post when the soleus is relaxed and the shin begins to bend the cuff the pressure under the forefoot decreases until it is only under the heel bone. As this happens the plane of reference for the balance system switches from being the plane defined by the heel and the balls of the feet to a plane defined by the heel and the center of pressure (force) of the shin on the cuff. (You connect the 2 points with a line to find the plane of reference). To the balance system it is as if the ground suddenly tilted up from level to 45 degrees.

If you have too much forward lean in your boots and you hit a bump or mogul and your shin suddenly loads the cuff will this panic the balance system? Of course. Will you pitch forward wildly at the waist in response? Probably. Any females out there with moderate to large calves? Do you have too much forward lean in your boots? Odds are that you do. Did you recognize the sensation I just described. Yes? OK, it wasn’t you. It was your boots.

So, why do ski boots have the forward lean angle they do? The first reason is because they have to look good. And boots don’t look nearly as good if the cuffs are straighter. The second reason is that someone has decided that you need this lean angle to put pressure on the front of the ski. So, we have that, what do they call it…….., oh yes that “aggressive forward lean”. We’ve already sunk that theory.

What determines individual forward lean requirements? First and foremost is the critical angle at which your ankle joint switches to isometric contraction. This angle should be shaft angle or lean of the boot where your calf rests against the rear cuff. You need some free space to flex forward from here. We will get into this later.

What determines the lean angle of the shaft of the boot? Manufacturing considerations. Remember that the boot maker has to have an economical way to make a boot. No matter what angle he choses the boot maker has to fix the cuff to the lower shell so the two components act as one. At this point the boot is a bent plastic tube with a seam up the middle starting at the toe so you can bend the flaps open to get in.

But, doesn’t that big rivet on the side of the cuff allow it to hinge? I think not. The resulting plastic structure would not be stable enough. Here’s a bit of trivia. The stiffest plastic you can make a boot out of that won’t break like glass in the cold is about 1/20th as stiff as steel. Plastic boots generally rely on wrapping tightly around your foot for their stability.

There has been some inference in posts by others that I believe boots should be soft. I never said that. Did I say that? No. If a boot is set up right I believe they need to be very stiff on the sides of the leg but not to apply leverage to the ski edge. This is one of those external force on the wrong side of the balance equation issues. The shaft of the boot has to be stiff to provide a stable vertical reference to help you align internal forces through your foot. It is like training wheels for your balance system.

The process by which the boot maker establishes and fixes forward lean is at best arbitrary. It can be all over the map from one boot to another.

The big problem for the skier is that the forward lean of the boot is decided behind your leg where the calf muscle is. The forward lean angle you require for skiing is determined by the angle your shin is at when the soleus switches from eccentric to isometric contraction – i.e. the critical angle. However the ski boot typically over rules your body because the rear cuff will push your shin forward based on what? Based on the size of your calf muscle where the boot cuff wraps around it.

What does this all mean? Well if you have a large calf muscle you are in big trouble. If standing up for more than 5 minutes is exhausting do you think there is any hope you can ski? Not likely. Even if you have a small calf you may still be in trouble if you have small feet. Why? Because although boot makers allow for some variation as they shrink shell sizes down they still use the same forward lean angle on the cuff. Why? Looks, looks. The boot has to look good. Looks aside the other reason is “because that’s what we do”. In other words, “don’t ask us”.

Is forward lean easy to change. Sometimes. More often than not it can be a nightmare. The reason for this is that the molded interfaces can make it very difficult to put the cuff at an angle that is much different than the boot maker designed it to be at. In many instances I have not been able to straighten the shaft enough. A brand new pair of boots I bought for my wife 2 years ago went into the garbage because I could not make them upright enough for her calf.

One more issue. Assuming you can successfully align the cuff the next nightmare is the liner. Remember, it was formed to the shape of the shell the way it came from the factory. Am I saying forward lean can be a tough problem to correct. Absolutely.

This pretty much wraps up the boot flex issue. Please post any questions you have on this issue before I move on. Thanks
post #38 of 249
Thread Starter 
Apologies in advance in case anyone is offended at my cut & paste job
post #39 of 249
What an effort Disski. I actually put all the rellevent posts into word yesterday for my own reference and study. Thanks a bunch. [img]graemlins/thumbsup.gif[/img]
post #40 of 249

A bit about my experiments of 1991. The device that replaced the ski boot was a minimalist aluminum frame instrumented with 18 force sensors strategically placed so as to obtain 3 dimensional data on the forces being applied by the foot and leg. The device had provision to apply localized effects to specific aspects of the foot and leg in order to study their effect. For example, plates on either side of the balls of the foot could be repositioned to prevent the foot from spreading fully under load or to reorient the foot across the long axis of the ski (i.e. to turn the forefoot outward or inward).

Relevant to the present thread is that the cuff of the boot rotated about an axis. The cuff could be adjusted around the skier’s leg so that there was no front to back slack. This was done without compressing the calf muscle (although we could certainly do this too).

The forward lean position of the cuff could be adjusted with a backstop. The degrees through which the cuff could rotate with essentially zero resistance was adjusted once the forward lean was set. We could also allow the cuff to be completely free hinging in that it could rotate all the way back until it was almost horizontal as well as rotate forward well beyond the normal range of motion of the ankle. The device had provision to permit various forms of resistance to be introduced at the end of the free play. We could also vary the constraint that would prevent the midfoot from articulating properly. One very important feature was the neither anklebone would ever be impeded in its horizontal transverse movement.

The results of the experiments were enlightening. What we found was that by varying the constraints of the cuff function we could play the soleus muscle like a musical instrument. For example when we set the forward lean too upright we would witness multiple attempts by the balance system to get COG over the top and ahead of the ankle. But the most amazing thing we did was to have skiers attempt to ski with the cuff completely free hinging. When a skier suggested this I thought he had gone insane. I was proven wrong when suddenly he was skiing beautifully right before my eyes.

Most interesting of all was that the tech guy (a biomedical engineer) monitoring the incoming data was set up in a tent and could not see the ski hill or the skier. Although he was not a skier he was giving us very accurate information within a day as to how well the test skiers were skiing just by looking at the incoming pressure data -- no video that was added later! This completely blew me away.

Once I figure out how to import drawings I will try and put a picture of “The Gadget” as I named it.

One thing to note is that experiments on the effects of footwear done at the Human Performance Lab in Calgary, Alberta have shown that when constrained in footwear the human foot will always begin by trying to create its barefoot function. It will then negotiate an increasingly compromised solution until it has found the level where it can work within the frame of reference. This is in essence what we were researching with our experiments.
post #41 of 249
What David is saying rings a bell. When I was in the men's boots I definitely was in dorsiflexion at turn's end: 1) stiff race boot, sized down; 2) high boot, large calves; 3) the aggro forward lean was adjustable, the rear spoiler removable, but the boot shaft was strangling my calves, causing my feet to fall asleep on the chair, and I needed to install boot heaters to keep my formerly severely frostbitten toes alive. In the junior Dolomite Sintesi, I have not felt the need to heat the boots ever, with only dance tights on my feet. This alone improved my performance. How can you ski with numb feet?

I was a woman in mortal combat with her boots. No wonder women drop out at a higher rate than men. How do I know that? The statistics tell me so.

[ November 25, 2002, 07:17 AM: Message edited by: nolo ]
post #42 of 249
disski has done a great job, and all of you are amazing! What a phenomenal thread! Should this be put into such form that it can go into the "Premium Article Collection"?
post #43 of 249
With me also Nolo. I hate to wear out the Tai-Chi connection, but the structural awareness I got working through grounding my COG and maintaining my balance in movement through the forms by focusing on developing my root, or full foot contact, and everything working up from that point plays right into this discution. How my newest boots allow a better feeling of being connected at the foot, by allowing more natural movements. Just can't wait to get out on the snow.

Nolo, I liked how you expanded Horsts's qoute in the other thread. This is all about teaching as far as I can see.

DM, Thanks, and keep going.
post #44 of 249
Thread Starter 
Originally posted by David M:
Here is something to think about while you are waiting for my next post on walking.

The foot has 3 loading bearing points situated at the lower ends of its bow-like trusses: the heel bone (the calcaneus), the ball of the big toe (the head of the 1st metatarsal) and the ball of the small or 5th toe (the head of the 5th metatarsal). They represent the mechanical references of the foot.

In skiing the ball of the big toe is the most important of the 3 mechanical references because it represents the furthermost point into the center of a turn from the center of the outside ski that the foot can apply force. Under the right circumstances and with a functional foot COP can be driven to the ball of the big toe.

NOTE: A functional foot in the above context is a foot capable of loading the ball of the foot by levering the 1st metatarsal.

The current theory is that edge control results when the force applied by the foot somehow aligns itself over the inside edge of the outside ski. To date no one has explained the mechanism by which this alignment takes place. This issue aside, the theory is flawed in itself because it ignores the effect of sidecut.

The sidecut of a ski effectively increases the width of the ski as measured at the waist. Therefor an alignment of the force applied by the foot with the edge of the ski at its waist would still result in a misalignment of opposing forces. This will cause the foot to move into supination or away from an edge set. Even if the theory were valid it would still not produce a stable base of support which is essential to initiate movement from. According to Newton’s Laws “for every action there must be and equal and opposite reaction”. You can only push as hard as the snow pushes back. If the base you are pushing against shifts you will lose the balance equation because balance results from the alignment along the same vector path of 2 opposing forces.

Does anyone know the solution to this problem?

post #45 of 249
Disski, You absolutely rule for doing this! Thank you. The thread becomes much more coherent in this format. Appreciate it.

DM, a question, where in the process of transferring the stability of your balance structure to the shin and heel does support for the rear of the knee become critically compromised by the soleus needing to become less active in the balance process? Is this a progressive, or an on/off type of loss of support. Is the entire knee compromised or just specific directions. Is the point where the soleus turns on and off during walking and flexion the same point as when it is placed in a 'correctly' fitting boot or does the boot change the point. Thanks for your knowledge and time.

post #46 of 249
David, I have a couple of questions.

1. What are the drawbacks (performance and safety) to using boots designed to your criteria?

2. Have you done similiar research on snowboarding?
post #47 of 249
Thread Starter 
Originally posted by David M:
In my last post the heel and the outer (small toe) aspect of the lead foot had made contact the ground. At this point the foot had been plantarflexing (pushing down like pushing on the gas pedal of a car) to drive the foot into compliance with the ground. As the lead foot swung forward the pelvis began to rotate horizontally about the support leg. If the lead leg were the right leg and you were looking down on the pelvis it would be rotating counterclockwise. This means that the pelvis would be rotated or adducted into the support leg and it would be rotated out of or abducted from the swing leg. This is consistent with the fact that the support leg will be in a pronated position at the start of the walking movement while the unloaded lead leg will be in a supinated position. As the heel of the foot made contact with the ground the anterior tibialis on the shin had gone into eccentric contraction while the soleus was being passively shortened.

When the heel of the foot made contact with the ground it established COP. COP then radiated along the outer (little toe) aspect of the foot as it made contact with the ground. On top of all this the foot has begun to reverse its supinated position and rotate in the direction of pronation. Something else important has happened. Walking is a forward fall. As the lead leg swung forward COG began to move forward and downward from its start position.

Does everyone have a good picture of this? OK. Here is where things start to get interesting. As the outer aspect of the foot comes into full contact with the ground the transverse aspect at the balls of the feet is also beginning to make contact. As it does COP swings across the foot from its outer aspect and begins to head towards the ball of the big toe. As the forefoot (balls of the feet) comes into full contact with the ground the ankle joint reverses direction and the foot starts to dorsiflex (i.e. pull up). As it does it begins to stretch the soleus muscle in the back of the leg causing it to go into eccentric contraction.

Something else is happening. Since the adapting foot is now pronating the pelvis is beginning to rotate about the adapting foot. As it does the side of the body with the adaptive foot is becoming the strong side because the pelvis is turning into the leg (it is becoming adducted). While this action has been unfolding the leg has been dorsiflexing with the soleus in eccentric contraction. As the knee passes over top of the ankle the soleus switches gears and goes from eccentric contraction to isometric contraction (i.e. muscle does not change length). I other words, it stops the dorsiflexion of the ankle dead in its tracks. Why?

Nolo: I don't know about which muscle fires or how, but when I walk I seem to be levering the COG forward from the first point of contact at the heel to the center of the foot, where I feel my weight passing through the talus to the foot.

The body needs to get COG back where it started – in front of the ankle joint. It also needs to get the leg back into eccentric contraction. Think about what would happen if the ankle didn’t stop dorsiflexing. COG would keep dropping. It is already below the height it started from. As nolo said it has to be levered back up somehow. It does this by stopping the leg from moving and then pulling COG up and over the ankle by using the quads in the front of the thigh in concentric contraction. Once COG has gone over the top and is in front of the ankle the gastrocnemius in the back of the leg begins to be stretched by the extension of the knee joint. This causes it to go into eccentric contraction. Now COG is back where it started.

The action just described is fundamental to effective skiing. What is especially important is the angle of the ankle joint that is present when the soleus changes to isometric contraction. If our ski posture is set up correctly (I will get to this) the forces of skiing will cause the soleus to revert to eccentric contraction in order to maintain this critical angle. Am I suggesting that consciously ‘flexing your ankle’ in skiing is a bad idea. Absolutely.

If you walk slowly you can feel the soleus and gastroc muscles kick in. The first thing you feel is your soleus changing gears and going from eccentric to isometric. This feels like someone slapping the calf muscle just below the knee. When the gastroc kicks in next it feels like something poked you in the hollow behind your knee.

Next I will discuss how this works in skiing. Although some of you are probably getting the urge to rush off to the ski hill and try this you should control this urge. A whole array of things can prevent you from using the mechanism I have just described. And unless you correct them there is no way you will overcome these limitations.

Think about all this. You need to have a clear mental picture in order to understand how to apply it in skiing. More to come (of course). RicB we are getting to your question re an environment conducive to balance.

Next I will respond to the recent questions.
post #48 of 249
Thread Starter 
Originally posted by David M:
First a few quick answers to nolo’s question after which I will start the jump to skiing since I know that is what everyone has been waiting for.

Nolo: David, would you go into this a bit more? It sounds like we can't line up forces on a shaped ski, and yet many experience "carving" on their shaped skis. What gives?
DM: Shape skis have been an important step in the right direction in many ways and a wrong step in many ways. It depends on the conditions and ski geometry as to whether you have a bad hair day or not. Your observations are relevant. The thing we need to consider is the type of snow. Hardpack and ice are solid-like. Powder is fluid-like. The mechanics are very different. Skis need to match the snow. More later.

Nolo: My understanding of the mechanics is that the reaction force of the snow combined with the anatomical events you have detailed deflects the COG inside the turn, which is a "centripetal force" that counteracts or balances the movement of the foot into supination or away from the inside edge ("centrifugal force").
DM: How would this provide the stable source of GRF your balance system needs??. More on this important issue down the road.

Back to using the walking model in skiing. Although equipment is a huge factor affecting performance let’s leave this until later and cut to what I see as the fundamental issue. The first thing that comes to the surface is that the physics of skiing are different than walking. I agree.

Let’s start by assuming you are entering the 3rd phase of the turn in a strong ski posture (yes, this discussion is coming) and you want to initiate the next turn using the walking mechanics. The same forces don’t exist as in walking so you have to help create rhe required forces or at least increase the level force acting on you. Let’s use sailing as an analogy.

You are in the same position in the sailboat about to end one tack or jibe and start a new one. What forces are acting on the boat?

Gravity is pulling the boat down vertically – same thing in skiing.
Water buoyancy is pushing the boat up against gravity – snow pushes back against the forces applied to it by the skier.
Wind force is trying to push the boat downwind – gravity is trying to push the skier downhill.
The sailor is steering the boat into the wind – the skier is steering their skis into the hill against gravity and centrifugal force.

What is an knowledgeable sailor going to do to start the direction change so that forces come into play to turn the boat into a new tack?

Over to you nolo or anyone else who knows sailing and wants to tell us what should happen.
post #49 of 249
Thread Starter 
Originally posted by John Dowling:
Actually, to tack you haul the mainsail in (tighten the boom). This causes the boat to roll to leeward (away from the direction of the wind). When the boat is pitched over, the lift of the sail creates a turning force that rounds the boat up into and through the wind. As you find your new course, you ease the sail again to its optimum angle to the apparent wind.

To jibe, you haul in the sail as you turn away from the wind, putting the trailing edge (rear) of the sail to the wind. Eventually, the wind switches sides of the sail and slams it across the boat, often dramatically. Hauling the sail in as you jibe (or gybe) helps you keep a little control of it. Unless you are windsurfing, in which case you can jibe by spinning the sail away from the wind (putting the leading edge of the sail to the wind) as you turn.

So I guess I don't understand how this applies to ski turns.

post #50 of 249
Thread Starter 
Originally posted by David M:
Assume hardpack conditions. Your COG must be supported on your outside leg on a stable base of support (assume this exists – I will work out the details later). External force is trying to extend the knee of the outside leg and dorsiflex the ankle. This force is resisted by eccentric contraction. Your inside leg is flexed in isometric contraction with its foot higher than and slightly ahead of the outside foot. Although COG is supported on the outside leg the inside leg plays an intermittent role in helping to maintain balance. In this configuration your hips should be open to the fall line (counter rotated) so that the pelvis is passively adducted or turned into the outside leg and passively abducted or turned away from the inside leg. By passive I mean that the muscles in the pelvis that would actively abduct or adduct the leg are balanced through a parallelogram mechanism with the result that the net sum of opposing muscle tensions is zero.

The force of gravity is trying to pull COG vertically down towards the center of the earth. This force translates into a vertical force and a shear force acting down the slope. Because of acceleration COG has inertia. According Newton’s Laws COG will continue in motion in the same direction unless acted on by an outside force. Ignore forces like air resistance. The primary outside forces acting on COG are gravity and centrifugal force. They are both trying to pull COG down the fall line. While gravity just wants to pull COG down to the center of the earth. These are the natural forces in play. What is the only factor stopping these forces from changing the skier’s direction? The opposing force muscularly generated by the body applied through a stable base of support at the outside or downhill leg.

While the outside leg has some amount of flexion at the knee the forces acting on skier are pulling against the flexor muscles with the effect that the leg wants to fully extend. The gastroc muscle, which is in eccentric contraction, resists extension of the knee. (No quads anywhere to be seen LM)

Pause and think about the inside (uphill) leg. What is it doing? The knee is much more flexed than the outside leg. But, it is more or less passively flexed by its position and the force of the snow pushing up against it. If the body were vertical to the ground and supported on the uphill leg COG would be much lower to the ground than if it were supported on the downhill leg. This affords the possibility of pushing against the ground by extending the uphill leg at the knee.

Where is the lead (uphill) leg in relation to the walk sequence? What does the skier have to do to get COG over top of and in front of the ankle of that leg?

John Dowling: “To jibe, you haul in the sail”
DM: You reduce the pushing action against the snow by beginning to relax the muscles of the outside leg. Do you fall over? No. Why not? Newton’s Laws. The force of gravity is acting on COG to pull it into the fall line. But it first has to overcome the inertia of COG. Result. COG starts to drop vertically and arc into the fall line. As it does it eventually has to cross over the base of support at the feet. (i.e. the skier has to jibe). At the same time the muscles of the outside leg are relaxing the uphill leg is actively extending at the knee in concentric contraction. As COG begins to cross over the base of support both legs are extended at the knee with concentric contraction. For a brief moment the skier is able to stand vertically on the slope of the hill. This seeming defiance of gravity is possible because of the inertia of COG and the force applied to the snow by the legs with active extension at the knee. It is important that flexion or bending at the hip be maintained in order to keep COG over the feet.

JD: as you turn away from the wind, putting the trailing edge (rear) of the sail to the wind.
DM: As the skier extends at the knees the action of the extensor muscles rotates the legs into a neutral position with the pelvis (i.e. neither abducted or adducted) which is open or counter rotated into the fall line. This action turns the skier away from the forces. As the feet rotate under the pelvis the old outside leg comes under COG and the new inside (old outside leg) becomes the new lead foot in the walk sequence. The effect is due to extension having created the up and over levering action required to move COG in front of the ankle joint.

JD: Eventually, the wind switches sides of the sail and slams it across the boat, often dramatically.
DM: To complete the process the skier continues to rotate the legs simultaneously to bring them across the fall line. This establishes the same position COG in relation to the base of support at the new outside that it had with the old outside leg. The walk sequence is complete. Creating the abducted, adducted relationship of the legs with the pelvis by actively rotating the legs produces the same result as the passive process associated with walking.

One last thing. Having established the foot position under COG the skier must relax and settle onto the outside foot. Why? To allow the forces to turn on the stretch reflex by stretching the muscles in the back of the legs and the back (as we will see when we get to stance). You have just jibed on skis.

This is the physics of how the forces are used. However, a whole array of obstacles stand ready to prevent you from actually performing this feet. Simple things like too much forward lean in your boots for example very common) will make it impossible to make the moves described as well as preventing you from using eccentric contraction in any kind of useable way.

Any questions?
post #51 of 249
Thread Starter 
Originally posted by David M:
LM: "We as skiers are capable of moving in multiple planes, so by equalizing the forces in any given plane, we can use this to our advantage. We can remain semi- relaxed, able to create our next move. For if we are not relaxed, and the muscles are already active, they can not do anything else until they do become relaxed.
This is a very common issue with recreational skiers- they do not understand that muscles can only do one thing at a time, then must be relaxed prior to their next operation. Instead, they try to go from one movement to another without relaxing, resulting in involving larger muscle groups to overcome the resistance of the original muscle."

DM: Lisa Marie is it ever great to be in such good company! A few other issues to consider:

1. It is not practical (not to mention possible) to consciously orchestrate muscle activity to create balance. Yet many articles on ski instruction suggest processes that would pretend to do exactly this. Utter nonsense. The writers of such gibberish should spend some serious time in medical libraries.

2. If a muscle is in eccentric contraction other muscle in the same family can not be in concentric action at the same time; i.e. muscles can not be lengthening and shortening at the same time!

3. If you are fighting the forces you are most certainly using concentric contraction to ski. Unfortunately, widely read ski magazine articles are suggesting that the ideal skier should be a WWF wrestler.

LM: Muscle synergy. Don't go skiing without it!!!
DM: I second that. FYI I helped launch a human performance company called synergy. I chose the name for a very good reason.
post #52 of 249
Thread Starter 
Originally posted by David M:
LM: Sarah Schlepper. The first time I watched her race, was on OLN, a WC event which also featured Janica kostelic. What a contrast. Kostelic looks like she is barely working, but her race results prove otherwise.
DM: Very astute observation LM! I am truly impressed. If you watch video of a Kostelic in slow speed you will see her make exactly the walking movements with the associated eccentric muscle action I have just described.

I concur with your comparison. Assuming Schlepper and Kostelic were athletic equals how much of an advantage do you think Kostelic's movements give her. 100, 200 or 300%. How about more?

post #53 of 249
Thread Starter 
Originally posted by John Dowling:
Nolo- In windsurfing, at least at higher skill levels, no one ever tacks. The small boards do not have enogh momentum to move through the wind. When you jibe, it's easier to spin the sail so that the mast (instead of the trailing edge of the sail) face the wind. That way the the wind changes sides of the sail calmly as in a tack, even though the boat is actually jibing.

DavidM-I guess I don't like this metaphor because my view of sailing is that you deliberately power up before turning, sometimes to the point of putting the boat nearly out of control (as in power tacking a race boat) while in skiing the power comes as you release the turn you are finishing. I understand all those descriptions of ski turns, but it doesn't relate to sailing to me.

post #54 of 249
Thread Starter 
Originally posted by David M:
John DowlingavidM-I guess I don't like this metaphor because my view of sailing is that you deliberately power up before turning, sometimes to the point of putting the boat nearly out of control (as in power tacking a race boat) while in skiing the power comes as you release the turn you are finishing. I understand all those descriptions of ski turns, but it doesn't relate to sailing to me.

DM: John I don't think you understood all the descriptions.

This is not a passive process where you trickle into the fall line. You do deliberately "power up" to the point of losing control.

You start the movement into the fall line by relaxing the outside leg. But the forces required to power up your uphill leg so it is transformed into a stable base of support (it isn't at the this stage) are not sufficient in this environment. So you have to create the necessary forces by extending first on the uphill leg and then extending on both legs. When you do this you launch baby. And you don't just launch into the fall line either. You launch UP and into the fall line. And when you do you had better be supremely confident of the forces because you are sending your precious body to the place skiers fear the most.

If this isn't powering up to the point of losing control (serious injury or death comes to mind) then maybe we are speaking different languages.
post #55 of 249
[What determines individual forward lean requirements? First and foremost is the critical angle at which your ankle joint switches to isometric contraction. This angle should be shaft angle or lean of the boot where your calf rests against the rear cuff. You need some free space to flex forward from here. We will get into this later.]

I can ski with my boots unbuckled (very soft flex), but I have never found free space to the front (or back) a useful feature. I want "progression" and free space gives a bang (dual rate) when I get to the boot. That is why I choose to snug the "Power Strap" up against the boot liner and not the boot shell. 'More progression.

I was pleasantly rewarded while skiing Saturday with the responsiveness and solid traking (tip to tail) when I pressured the tips of my skis. "Dust on Crust" conditions. These old school methods really do work!

post #56 of 249
Thread Starter 
Originally posted by Ric B:
I've had my students describe this as climbing the stairs. you can give them this feeling of release and the natural tipping that follows on carpet if you want. Simply stand relaxed knees slightly bent, feet a narrow hip width apart, then have them just slightly pull one hip upward, straight up. The result is the transfer of COG to the other foot over the little toe side and a resulting rolling of the foot and movement of COG away from the contact point. Do this on snow, and direct the awareness down to the feet and light bulbs go off.

What you are doing by releasing with a relaxation and an inside extention is a constant movemnent to be long from the inside shouder to the outside foot through the middle of the turn. a strong skeletal stance. The hip is critical here. How can we direct our energy to the outside foot if our outside hip is higher. That's a contact point thats not equal to the challenge. for me, more important than rotation of the hip, is the hip being tilted down to the outside. This is one of the good angles everyone talks about in good skiers. It's a sublte amount of tilt most of he time, and it's there in all good skiers, and yes it is a very similar to the movement of the hip in walking. We can't find that "equilibrium of the forces" if one part of our skeleton is acting out of sequence. Out of the mouth of my Tai-Chi instructor last night "movement is started at the feet from your root, powered by the legs, directed by the hips, and felt in the body.

How do we find new ways to introduce this structural awareness and sensitvity to begginers and low level skiers? How do we help them recognize their natural balance and skeletal movements in the context of skiing and the equipment they're using? DM, I would still like to know what you did in your experimnets to find the "enviroment that is conducive to balancing". [img]tongue.gif[/img]
post #57 of 249
Thread Starter 
Originally posted by nolo:

Once a student experiences the sensation of standing up down the hill there's no keeping them in a traverse. Now they start experiencing the turn from FALL-line to Fall-line.

Love that hang time! It's the intermittent reinforcement that makes turning so much fun.
Also, I don't know if walking is as tangible an analogy as pedaling would be. The hardest sell with students is their perception that dealing with stronger forces must mean exerting more force. It is counter-intuitive to think that trying hard might be counter-productive. .
post #58 of 249
Thread Starter 
Originally posted by TomB:
DM said: You start the movement into the fall line by relaxing the outside leg. But the forces required to power up your uphill leg so it is transformed into a stable base of support (it isn't at the this stage) are not sufficient in this environment. So you have to create the necessary forces by extending first on the uphill leg and then extending on both legs. When you do this you launch baby. And you don't just launch into the fall line either. You launch UP and into the fall line.

Are you sure you want to emphasize the extension of the uphill/inside leg (soon to become the new outside leg) to that extent? Why all that effort to step up to the inside leg? Why not just let the body fall to the inside of the new turn by releasing the old outside leg. There are plenty of forces on you to pull you that way anyway. Are you trying put the skier back in that eccentric contraction? Do you really need to straighten that new outside leg to achieve that? :
post #59 of 249
Thread Starter 
Originally posted by John Dowling:
</font><blockquote>quote:</font><hr />Originally posted by nolo:
The hardest sell with students is their perception that dealing with stronger forces must mean exerting more force. It is counter-intuitive to think that trying hard might be counter-productive.
I sometimes explain that concept in this way: If you were a racer, the forces generated by your turns would increase with increasing speed. Therefore, the maximum speed you could travel through a turn must be limited by your ability to resist those forces (strength). If you are using all your strength to resist the centrifugal force of the turn, what can you do to contol yourself? The only thing that does not require more strength is to relax the outer leg, and allow momentum and gravity to take you out of the turn into the new one.
DM- I understand rebound and powering out of turns, but to me it's just not the same as powering into a turn in a sailboat.

post #60 of 249
Thread Starter 
Originally posted by Ric B:
I don't want to talk for DM, but I think the point is that relaxing the outside leg imediatly tranfers the COG and pressure to the new outside foot. It's the slow and deliberate extention of that leg that fully engages the edge. If there is no extention, if the leg muscle is doing isometric work, then you have to wait for the edge to fully engage because the body is already tipping into the turn. It's not a step up, as the COG is imediately moving away from the new point of contact, and the extention is helping the COG move in that direction, or simply put, the extention is to the side, because of the all forces at work. If you looked at the foot movements by themselves, suspended in the air, you would see the feet pumping up and down. The extention allows the long skelletal posture in the middle of the turn. Long and short, empty and full. How can you have an edge without first establishing contact? and how can you establish effective contact when gravity and centrifigal force are moving your COG away from that contact point. You have to grow your root and initiate from the new contact point, and you need to extnend that new leg from your contact point to achieve this. This is (and I'm on shaky ground here), the centripetal force that makes a turn. Have at it folks! [img]tongue.gif[/img]
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