THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE – PART 12


At this point my discussion of the mechanics and biomechanics of platform angle is at what I can appropriately call the moment of truth. Moment in the context of the mechanics and biomechanics of platform angle means moment of force or torque; platform angle involves the ability of the CNS of a skier to control torques across the inside edge of the outside ski so the skier can stand and balance on the platform.

What is Balance?

That balance is the single most important factor in human movement, especially movement associated with athletic performance, is undisputed. In complex activities like skiing that involve movement in 3 dimensional space in a dynamic physical environment, optimal balance is critical. But what constitutes balance? In order to know if a skier is has optimal balance or is even in balance one has to know what balance is and what factors enable or compromise balance (i.e. postural) responses and  especially the factors that enable optimal balance.

The Balance Zone

A skier is in balance when the CNS is able to maintain the position of a skiers’ COG within the limits of a narrow band close to the inside edge of the outside ski during the load phase of a turn. The load phase of a turn occurs in the bottom of a turn when the force exerted on the platform by the COM of a skier must be balanced against the external resultant force of gravity and centrifugal force. In the load phase, the CNS must maintain COG within the forward limit of the Balance Zone within close proximity to the ball of the foot. When balance is challenged COG must not exceed the rearmost limit of the Balance Zone that lies just in front of the ankle joint. The Balance Zone and its limits are shown in the graphic below. If COG exceeds the limits of the Balance Zone shown in pink, the skier will lose their state of balance and with it dynamic control of the platform underfoot.  They will also suffer a lose of dynamic stability in the joint system of the lower limb.

The Balance Plane

In the ski system platform the plantar plane under the plantar aspect (sole) of the foot is the interface of CNS mediated balance activity. When the coordinated, concurrent forces are applied at the main force transfer point of the foot that I call the Center of Control, shown in the preceding graphic, the applied forces will manifest in more than one plane as shown in the graphic below.Force Fa applied under the head of the first metatarsal will be distributed over an area around its center.  When the force applied in the plantar plane is transferred through the structure of the platform to the base plane the center of force will maintain its position. But when the force area of distribution will increase as shown in the pink zones under the head of the first metataral and the base plane. In free rotation of the ski, resistance from the force of friction Ff will be minimal as will any force applied in the torque arm plane by the eccentric torque arm. Rotational force will be largely confined to the base plane.

The Missing Force Factor: Sidecut

In the free rotation, the effect of the sidecut of a ski is not a significant factor in terms of a source of resistance. But as the transverse aspect of the base plane of the ski acquires an angular relation with surface of the snow the resistance created by GRF acting at the  limit of sidecut at the shovel sets up an interaction between the rotational force applied to the inner wall of the boot shell adjacent the medial aspect of the head of the first metatarsal with the resistance created by GRF at the limit of sidecut at the shovel. In the graphic below I have connected the  2 dots of the platform ground effect problem with a line drawn between the two points.The graphic below shows a schematic of the mechanical aspects of the opposing moment or torque arms between the two dots that I connected in the preceding graphic. The inside edge below the head of the first metatarsal acts as a pivot in conjunction with the Center of Force applied 90 degrees to the transverse aspect of the base plane for the plaform to rotate about as the ski goes on edge.

As the base plane of a ski acquires an angular relationship with the snow the torque arm rotating the ski goes into what cane best be described as turbo torque boost. Whole leg rotational force continues to rotate the whole ski but the eccentric torque arm engages and applies a high torsional load that winds the body of the platform about the shovel. This mechanism has to be considered in the perspective of the of the inertia from the movement of the skier driving the cutting action of the shovel.  The graphic below shows the opposing how opposing torsional forces at the limit of sidecut and applied by the application of for by eccentic torque arm to the vertical shell wall by the medial aspect of the head of the first metarasal act to apply a upward force that extends to the outboard end of the plantar plane of the platform.  This is the mechanism that enables elite skiers to balance on their outside ski and initiate precise movement from from a dynamically stable platform.I first solved basic mechanics and biomechanics of the outside ski balance problem 30 years ago. The degree of difficulty was not great. Solving the problem took diligence and persistence in researching all the relevant aspects and identifying all significant forces and associated planes.

I’ll let the readers ponder the informaton in this for a while after which I will be happy to respond to questions and comments.

10 comments

  1. I would like to check my understanding of your position on inclination. Is it something achieved by pronation of the foot upon landing in the rise line, or does inclination already take place during the float? Let’s take the extreme edge angles of Ted Ligety for example – are you proposing he achieves those angles solely through the roll over, or is the roll over an aligment upon landing that allows him to take advantage of and continue extreme inclination during the drop? In short, is it pop, drop, land, pronate and incline, or pop, incline while dropping, land with pressure and keep building it? (Currently playing with it on snow. Have the over it feeling and the rebound, but the angles aren,t happening. Also find I have to push hard with the outside leg to pressure the ball of the enough to bend the ski, pronation alone doesn,t quite seem to do it.) I suspect what I might be trying to do is not what you are actually advocating.

    1. Also find I have to push hard with the outside leg to pressure the ball of the enough to bend the ski, pronation alone doesn,t quite seem to do it.)
      REPLY: I am about to post on new studies I found recently that provide insights on why you are having difficulty pressuring the ball of your foot and why this problem is common. The mechanism involves a rapid, very short duration peak impulse force that gets the forces (actually torques) going into the turn. Once the mechanism is engaged there is very little perceived effort. This is why elite skiers make it look easy. To them it is easy. They don’t know how it works and have no incentive to try to figure it out. They also probably believe that anyone can do what they can do. It’s just a matter of practice. It isn’t. New information I found confirms what I suspected back in the 1970s.

      1. Having re-read post 12 in conjunction with our recent discussion, it does indeed seem that what I was missing was this:

        “As the base plane of a ski acquires an angular relationship with the snow the torque arm rotating the ski goes into what can best be described as turbo torque boost. Whole leg rotational force continues to rotate the whole ski but the eccentric torque arm engages and applies a high torsional load that winds the body of the platform about the shovel. This mechanism has to be considered in the perspective of the of the inertia from the movement of the skier driving the cutting action of the shovel.”

        In other words, the info was there to be found. However, it is so different to what you will read and hear about carving that I didn’t get it, probably due to assimilating it to my prior understanding. If the ski is kept flat, the result is the tail washing out as the ski spins under an upright skier. Hence people will generally say “You mustn’t use rotation in carving.” But if performed with inclination and the resultant high edge angle during a drop following rebound, the rotational movement redirects the unweighted ski into the fall line, and on contact with the snow the rotational force drives the ski tip into the snow and continues to hug the edge to the hill throughout the arc (i.e. provides ‘grip’). Have I finally got it?

        And it looks like this?: https://youtu.be/oakBu3DR3oU

      2. Yes, you have it now. I do rotational (steering) exercises when I start skiing each time with minimal edge angle and just enough inclination to get a feel for the snow texture. Then I increase my inclination as my skis come across the fall line to close the kinetic chain and translate the rotational effort into rotation of the ski into a locked edge carve.

        Racers like Shiffrin rotate their skis as they approach the rise line above the gate usually when she is airborne. When she makes contact with the snow the rotational effort translates directly into edge set angle. The external forces create torques that tend to rotate the outside ski downhill or out of the turn. Closed chain rotational effort applied to the foot by the internal rotators (glutes) in the pelvis counter the external torques. Sidecut provides a source of resistance to the rotation muscle effort. For some reason terms like open and closed (bio)kinetic chain rotation are absent from the narrative of skiing.

  2. David,

    Thought you might like to know that this post changed forever the way I ski! I’m 63, been skiing since I was 6. But I’ve never been able to carve on ice – until now. The trick for me was applying torque through the outside of the first metatarsal – what a difference!! Thank you! Thank you!

    By the way, I got to your site by doing research on why my new boots (this year) made me ski terribly and hurt my feet so much that I didn’t even finish the first day of skiing on them. I went back to my old boots.

    Then I found your site and started reading. The first thing I did was to take out the insole the ski shop sold me to ‘improve performance’, and loosened the power strap and the shaft buckles. I tried the new boots again – better! After some further custom shell fitting (my boot’s shells were designed to be molded with heat), things hurt less (and are easier to get on and off). Still have some work to do on the tongue…and probably ramp angle as well.

    Your careful, thoughtful, and analytic approach is truly unique – and wonderful! Keep it coming!!

    With gratitude,

    Jeff Bruckner

  3. Hi David,

    I have always assumed that ski design engineers knew or should know these concepts. K2 Skis used to publish a small book about ski design but, I have not seen it and don’t know if it’s still available. How can engineers design without understanding the forces acting on the device? Is it the same scenario as the ski boot “design without function” model we are so familiar with?

    I believe you stated that the foot is rotated by the hip when weighted as in when standing. What role does ankle rotation play in this movement. I can easily rotate my ankle below the knee without moving my femur or hip when seated and when standing with a flexed knee. When the knee is fully extended the ankle rotation seems very limited as if there is a locking mechanism engaged in that position. I also notice that there is an inversion/eversion tendency at the limits of ankle rotation. How does this figure into this discussion?

    Also, pressuring the first met downward must, of course, also cause an upward force on the outside or lateral edge of the foot. So the foot is also rotating along its long axis as we pronate and supinate inside the boot. Constraining this movement without restricting it too much is one of the challenges I am finding as I fit my boots. Is the upward force you describe at the end of this part related to any of these factors or more to external mechanical actions/forces of the equipment?

    I remember being amazed after following a very strong skier across clear ice that he had left a very thin line, barely visible on the surface with no indication of slippage! He was long gone and I knew he was the first one down that trail on morning patrol. This series seems aimed at explaining that mystery. Thank you.

    Excellent series. Regards,

    Herb Jones

    1. How can engineers design without understanding the forces acting on the device?
      ANSWER: They can’t anymore than engineers can design a plane without a working knowledge of the principles of aeronautics.

      Here’s an excerpt from a 2013 paper called Materials, Designs and Standards Used in Ski Boots for Alpine Skiing: “The development of alpine ski equipment has been done, in the beginning, mainly by trial and error, using on-snow tests. Despite the large market of ski equipment, not many scientific papers have been published on this subject in the past”.

      How did this happen? My read is that people with little knowledge of the issues started experimenting. I was one of them when started. Like others, I tried (in vain) to get a tight connection of my foot by packing more and more padding inside my liner. I was convinced it made sense right up until my efforts at achieving perfect fit reduced elite world cup downhill racer Dave Murray to the level of a beginner. That’s when I began to realize how little I knew and how much I needed to know. But by that time the perfect fit story had taken root and been blindly accepted as a scientific truth that quickly elevated to the status of Holy Grail of skiing. The perfect fit was assumed to be not correct but the best possible option.So it escaped scrutiny even by engineers.

      I remember being amazed after following a very strong skier across clear ice that he had left a very thin line, barely visible on the surface with no indication of slippage! He was long gone and I knew he was the first one down that trail on morning patrol.
      COMMENT: When I started skiing I quickly advanced to an intermediate/expert level. But I could not hold an edge on ice no matter what I did or how hard I tried. One day I watched a mature female skier navigate her way down a blue run on sheet of glare ice with perfect edge grip in low cut leather boots. This made no sense to me. When I asked elite skiers and ski even pros about how she did this the typical response was “It just takes practice”. But even they could not hold an edge on ice like this mature female. They could not do what they told me I could learn. Right then I knew the industry was overlooking something that was fundamental to skiing.

      I all address your other issues later and add them to this response.

  4. As the elite skier tips the outside ski onto its inside edge, placing maximum pressure on the MTP1 [center of control] to help transmit force to the cutting edge of the shovel, two other factors in addition to sidecut must be considered in the resultant efficiency of the long torque arm effect on the inside edge of the ski:
    1) sidecut
    2] ski stiffness
    3] ski camber

    Skis with added stiffness and camber may result in significantly greater shovel [and tail] pressure on the inside edge, since the GRF applied at the center of control [MTP 1] may be greater at tip/tail with too much camber and/or ski stiffness. The result could be a ‘super turbo boost’ tip grab and under-foot slippage. Increased sidecut would also augment this effect as the ski is placed on edge.

    Consider the example of a stiff ski with increased sidecut and camber tilting progressively onto its edge. The downward force exerted directly under the MTP1 is impaired progressively as the ski is tipped. The primary contact with the snow plane will initially be at the tip and tail, and only with increasing force will the ski edge under the center of control [anterior heel-MTP1] be engaged.

    Is this a correct deduction?

    Bob Drake

    1. The issues you raise do not consider the fact that the CNS can and will regulate forces in 3 planes at a subconscious level and will only apply as much force as required to effect global control in what is called automaticity. This level of control is impossible at a conscious or executive level. In this context it is a graphic statement of what they don’t know when participants technical discsussions of skiing talk about pressure, steering and edging as separate skills that a skier must blend (at a conscious level????).

      Matching the ski to the skier and conditions is important to performance especially race performance. But in the order of things skier performance is the first level of priority.

      Hopefully it is no longer a mystery as to why increasing stand height or ski width affects platform mechanics. This is (or should be) grade school level mechanics and physics.

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