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Different plantar pressures in skiing

Discussion in 'Biomechanics, Sports and Foot orthoses' started by NewsBot, Jan 7, 2016.

  1. NewsBot

    NewsBot The Admin that posts the news.

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  2. NewsBot

    NewsBot The Admin that posts the news.

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  3. efuller

    efuller MVP

    The guy may know his skiiing, but he doesn't know his physics. We had the following discussion



     
  4. He wants to be "The Boss" of his own website, Eric. He will likely soon stop publishing your replies and your points about the inaccuracies of his statements.

    I did have to chuckle, though, when he told you to read Inman...... classic!.;)
     
  5. NewsBot

    NewsBot The Admin that posts the news.

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  6. NewsBot

    NewsBot The Admin that posts the news.

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    NewsBot The Admin that posts the news.

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  8. Linchpin5

    Linchpin5 Active Member

    Postural responses utilize GRF. So where is the ground when the foot is on a platform with GRF on either the medial or lateral border? Let's start by framing the context in Newton's Laws. Then we can have the basis for a meaningful dialogue.
     
  9. efuller

    efuller MVP

    I'm not sure what you mean by postural responses.
    I'm not sure what you are asking "So where is the ground when the foot is on a platform... "

    I agree that this discussion should use Newton's Laws. A good place to apply them is in a free body diagram.

    Are you familiar with the concept of center of pressure? It is essentially the average point of force and Ground reaction force can be considered to be acting at the center of pressure.


    Linchpin5, are you related to the author of that piece that I quoted?

    Eric Fuller, DPM
     
  10. terigreen

    terigreen Active Member


    Ski Orthotics and Ski Biomechanics: The typical down hill skier's lower extremities never goes through a complete gait cycle. They ideally should have limited pedal mechanics between midstance and the beginning of propulsion, with the knee in flexion during the entire contact phase. When initiating a turn a skier will maintain their control by directing the downhill knee medially and transferring the load into the foot over the inside edge. This is done by internally rotating their tibia, causing a closed kinetic chain pronation of the foot, which transfers the pressure through the boot onto the ski edge. In ski boots subtalar joint supination and pronation are responding to knee function. With ski orthotics the goal is to control the knee function by supporting the foot anatomy. A custom heat moldable ski orthotic from Atlas Biomechanics, allow the skier to turn quicker and with more power. The skier will have a more efficient transfer of energy and will have decreased overall foot and knee fatigue. A ski orthotic from can make you a better skier. Teri Green, www.atlasbiomechanics.com
     
  11. NewsBot

    NewsBot The Admin that posts the news.

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  12. Linchpin5

    Linchpin5 Active Member

    Hello Eric, Please accept my apologies for the delay in responding to your comment. First off, I am one and the same as the author of the quoted piece. I started my blog in 2013 with the intent of stimulating objective discussion founded in principles of applied science. So your comments are appreciated

    In the context of skiing postural responses are CNS mediated responses to perturbations in GRF that tend to disrupt skier equilibrium.

    I am familiar with the concept of center of pressure as the point center of GRF. This assumes a contiguous source of GRF from ground or an extension of ground such as is present on the uppermost storey of 100 storey building where the foundations are ultimately founded on ground. However in ski maneuvers, the only time there is a contiguous source of GRF under the entire foot is at ski flat between edge change or when a skier is standing at a bar hoisting a few drinks acres ski. Although I have used the term COP in my blog posts, I am in the process of revising existing posts so indicate the load imposed by the weight of a skier as W. Here are two posts that are relevant to my posts currently listed.

    THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: WHERE IS GROUND? - http://wp.me/p3vZhu-1V0
    THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: TIMING OF EDGE CHANGE - http://wp.me/p3vZhu-1Y9
     
  13. efuller

    efuller MVP

    For example, the skier choosing to shift weight from one ski to the other. And many more.


    I have to disagree with your assertion that the only time there is contiguous source of ground reactive force is when the ski is flat and not on edge. There is a contiguous source of ground reactive force whenever the ski is in contact with the snow. When the ski is in the air there is no contiguous source. You can't make a turn in the air. You need to have force from the snow to make a turn. Newton's first law. objects stay at rest, or in constant motion in a straight line unless acted upon by an external force. If there is force acting on the ski, there will be a center of force.

    Eric
     
  14. efuller

    efuller MVP

    I realized I may have missed the crux of your point. I still disagree, but it comes down to this:

    Why do think Cop does not happen when the ski is on edge?

    Eric
     
  15. The use of center of pressure (CoP) does not assume a "contiguous source of GRF". Rather, CoP allows the summation of all reaction forces acting on the plantar foot or ski surface to be reconciled into a point location. Skiing on a ski edge still allows the CoP to be determined, just in a different location than when the ski if flat on the snow.
     
  16. Linchpin5

    Linchpin5 Active Member

    Skiing is different than gait in that skiing is essentially static squating. While squatting you still have to keep your center of mass under your center of pressure or you will fall over.

    Seriously? Static squatting? I have a whole series of posts on my blog on stance based on foot to core to shoulder sequential fascial tensioning.
     
  17. Linchpin5

    Linchpin5 Active Member

    Fair enough. I agree. But for the sake of technical continuity and the definition of CoP needs to be revised to mean the point center of reaction force. The use of the term GRF when there is no ground under foot which is the case when a wide ski on edge is misleading.
     
  18. Linchpin5

    Linchpin5 Active Member

    See my post FAT SKI SYNDROME - http://wp.me/p3vZhu-I7
     
  19. Linchpin5

    Linchpin5 Active Member

    Yes, you did miss my point. See the graphic in my post - WHERE IS GROUND? - http://wp.me/p3vZhu-1V0
     
  20. Linchpin5

    Linchpin5 Active Member

    "The typical down hill skier's lower extremities never goes through a complete gait cycle."
    >Correct. But skiing does involve load, unload and swing leg phases.

    "They ideally should have limited pedal mechanics between midstance and the beginning of propulsion, with the knee in flexion during the entire contact phase."
    > In the few studies that were done, the researchers found little variation in the angle of the shank in the elite skiers regardless of the stiffness of the shaft of the boot they were skiing in.

    "When initiating a turn a skier will maintain their control by directing the downhill knee medially and transferring the load into the foot over the inside edge. This is done by internally rotating their tibia, causing a closed kinetic chain pronation of the foot, which transfers the pressure through the boot onto the ski edge."
    > How about generating some free body diagrams showing how this works.

    "Internally rotating their tibia?"
    > How could this work?

    "With ski orthotics the goal is to control the knee function by supporting the foot anatomy."
    > How can an external intervention that limits the natural dynamics of healthy foot control knee function?

    "A custom heat moldable ski orthotic from Atlas Biomechanics, allow the skier to turn quicker and with more power."
    > Compared to what? This claim is meaningless.
     
    Last edited: Mar 29, 2017
  21. Linchpin5

    Linchpin5 Active Member

    Actually, I read everything that interests me anything that even remotely interests me. Massive amounts of diverse of information combined with less cognitive filtering than most people's brains is what leads to creative insights. This is why I have been awarded 2 patents for original thought, not just unobvious innovation. My recent patents discloses things about the foot in footwear environment that no one seems to have thought of, not even podiatrists. The Scientific American Mind has a special edition called The Mad Science of Creativity that contains some excellent articles on the creative process.

    I mostly spend time just reading and acquiring information. The reason I have become active on Podiatry Arena of late is that I see an opportunity for objective narrative, one that considers the big picture.
     
  22. efuller

    efuller MVP

    I read your post and I don't see which graphic that you are referring to that is supposed to answer my question.

    Linchpin wrote:
    I am familiar with the concept of center of pressure as the point center of GRF. This assumes a contiguous source of GRF from ground or an extension of ground such as is present on the uppermost storey of 100 storey building where the foundations are ultimately founded on ground. However in ski maneuvers, the only time there is a contiguous source of GRF under the entire foot is at ski flat between edge change

    Eric asked:
    Why do think Cop does not happen when the ski is on edge?

    I did see a graphic where Center of Pressure was mislabled as center of mass. The graphic with 3 feet in different positions with a pressure plot under them. You did correctly label bodyweight W and is a downward force applied to the top of the foot as you showed. However, what you labeled as COM should be an upward force from below (footbed, ski, ground, whatever). The force from below is a reaction force to the pull of gravity on the center of mass. The center of mass is an imaginary point and imaginary points cannot apply forces. The center of pressure is also an imaginary point, but it is the sum of the applied forces that you see in the plantar pressure diagram in your picture. So the CoP can be considered to be an average of where those forces are applied. This average can be used to explain what is happening while skiing or in gait.

    Trying to apply the gait cycle to skiing is getting away from physics. You can use physics to explain both skiing and gait without confusing people with the notion of having a swing phase of gait in skiing. Yes, you can unweight a leg/foot in skiing, but that is not swing phase.

    So to use those diagrams in both skiing and gait. When only the heel is in contact with the ground, the center of pressure will be an upward force posterior to the ankle. Body weight is applied by the tibia to the top of the talus and will be a downward force. Those two forces create a force couple that will tend to plantar flex the ankle joint. Or, if for some reason you wanted to put your weight on your heel in a ski boot, you could use your anterior tibial muslce to create a dorsiflexion moment at the ankle that could prevent the foot from plantar flexing when ground reaction force is on the heel.

    When the center of pressure is anterior to the ankle joint the combination of body weight and ground reaction force will tend to dorsiflex the ankle. Tension in the Achilles tendon can prevent that dorsiflexion. Or another way to look at that, tension in the Achilles tendon will shift the center of pressure forward under the foot. That further distal of center of pressure will tend to cause arch flattening and increase tension in the plantar fascia.
     
  23. Linchpin5

    Linchpin5 Active Member

    I'm not sure which graphic you are looking at. I understand the differences between COP, COM, COG etc and the fact that COM is an attractive force and does not in itself apply a physical force as the experts in ski industry appear to believe when they show balance in skiing as the result of a simple alignment of opposing alignment of COM and GRF at a point on the inside edge of the outside. This ignores the fact that GRF is acting along the entire running surface of the edge of the ski in contact with the snow. The fact that eminence in the ski culture takes precedence over evidence presents a challenge in trying engage in objective discussions where positions are defendable with the use of principles of applied science and current studies. Any party who attempts to engage in such is typically on the premise that this will adversely affect skiing by making things unncessarily complicated. This is like positing that knowledge of human brain is unnecessaary so long as one is skilled with a scalpel.
    While COM does in itself a force, it drives the moment arm resulting from the central load transfer mechanism that tensions the plantar aponeurosis through a progressive exponential moment arm. When the PA reaches peak tension the soleus arrests shank movement by going into isometric contraction. This post shows a simple model I made ub 1993 to illustrate this mechanism - http://wp.me/p3vZhu-20U. It is only recently that the mechanism of Achilles/forefoot load transfer has begun to be appreciated.
    There is an abundance of information on my blog, much it as far as I can tell constituents orginal thinking. If you tell me which search words or phrases you are using I can add tags to my posts.
    I will respond to the other issues you raised later.
     
  24. NewsBot

    NewsBot The Admin that posts the news.

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    Influence of slope steepness, foot position and turn phase on plantar pressure distribution during giant slalom alpine ski racing.
    Falda-Buscaiot T et al
    PLoS One. 2017 May 4;12(5):e0176975. doi: 10.1371/journal.pone.0176975. eCollection 2017.
     
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    NewsBot The Admin that posts the news.

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  26. Craig Payne

    Craig Payne Moderator

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    oh dear! I don't think the words "reverse windlass" mean what he thinks they mean ...
    So many factual errors on that page and so much contradicted by the actual scientific evidence. Lots of 'wishful thinking' - (ie make something up and wish it was true)
    Love the comment that " Shoes with elevated heels, cushioning and toe spring" inhibit what they think is the 'reserve windlass' ummmm .... nope .... the opposite is the case.

    and "Footbeds and Insoles" also inhibit it!!!!! Dunning-Krugar personified!
     
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