THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE – PART 11


In my preceding post I said that after a thorough investigation and analysis of the forces associated with platform angle mechanics I reached the conclusion that rotational (steering) force should be applied to an isolated area of the inner shell wall of the ski boot by the medial aspect of the head of the first metatarsal. The reason I conducted a thorough investigation and analysis of the forces is that as a problem solver this is my standard protocol. Protocol aside, the need for a thorough investigation of every aspect affecting athletic (skier) performance was known as far back as 1983.

….. quality teaching – coaching of neuromuscular skills in physical education should always be preceded by an analytical process where the professional physical educator synthesizes observations and theory from scientific and technical perspectives……

There are many sports skills which require that sports objects, implements, equipments, and apparatus be utilized. (implements such as ski boots and skis)

All factors must be studied in terms of the skill objective. If problems are noted in the performance of the skill, where did they originate? Within the performer? Within the sport object? Both? What precise changes must be made to obtain the skill objectives?

The directions for improvement given to the performer must be based on scientific and technical analysis of the total skill.

Analysis of Sport Motion (May 1, 1983): John W. Northrip

Planes of Forces

The ability to conduct a thorough investigation and analysis of the forces associated with platform angle mechanics and biomechanics requires as a minimum, a basic understanding of the engineering aspects of the associated forces. In the case of platform mechanics and biomechanics, knowing the plane or planes in which a force or combination of forces are acting is essential.

The Force Plane in the Perfect Fit

The objective of achieving a perfect fit of the foot and leg of a skier is create a perfect envelopment of the foot and leg of a skier with the rigid shell wall of a ski boot so that force is applied evenly to the entire surface of the foot and leg to create a unified mass with the ski so that the slightest movement of the leg will produce edging and steering forces. In this format force(s) applied to the base plane by the leg will be distributed to a broad area with limited ability to apply coordinated forces to a specific area of the ski. Sensory input is also limited by the uniform force applied to all apects of the foot by the perfect fit format creating what amounts to the skiing equivalent of the Bird Box.

Platform Planes

In the mechanics and biomechanics of platform angle there are potentially three horizontal planes in which forces can be applied as shown in the graphic below. The left hand image shows the rotational force applied to a torque arm plane elevated about the base and plantar planes. In the perfect fit format in the right hand image the leg is shown as a rigid strut extending to the base plane where rotational force is applied.When the foot and leg of a skier are perfectly fit within to the rigid shell of a ski boot any force applied by the leg can only applied to the base plane of the ski where the force is distributed over a broad area. Steering and edging forces applied to the ski by the leg lack precision because they cannot be applied to specific areas or applied in a coordinated manner.

In the above graphic the whole leg rotational effort applied to the base plane by foot in the two examples is shown with no resistance. In my next post I will discuss what happens when resistance is added that opposes the rotational force applied to the base plane.

11 comments

  1. Thanks David.

    Just been re-reading your post on Anna Fenninger/Veith and the “Austrian move” onto the new inside ski before the fall line, with the tail of the new outside ski in the air. In my own attempts to adopt this technique, and also in my attempts to perform the more traditional technique of loading the outside ski before the fall line, I found it hard to observe pressure and loading from ski to ski and eventually settled for moving as a body in space and allowing the ski pressure to look after itself on a turn by turn basis. That is, I’m more interested in the path i take over my skis that in what the skis are doing. (That may make me a terrible skier, I add it by way of explanation.) In doing so, I may have stumbled upon an answer to the question you pose:

    “Returning to Fenniger, one has to provide a rational explanation for her movement wherein she lifts the outside ski off the snow”

    In my attempts, I find a conscious effort to lift the tail of the outside ski very difficult (without standing up on the inside ski). I also find it difficult to consciously move onto any given ski. However, slow motion and still photos show that one ski or part of it will be airborne during some turns, usually without me being aware of it. I presume this is due to rebound. Sometimes, if I get a lot of rebound as I move into the new turn, the combination of rebound and inclination will mean that I land on the new inside ski before the new outside ski gains contact with the snow, and carve the top of the turn solely on the inside ski, and this is pronounced enough for me to be well aware of it as it happens. If i try to do this, it becomes an awkward endeavour, but without conscious effort, it can simply happen. Assuming I am correct in identifying rebound as the cause and landing on the inside ski as the effect, and assuming that as a rubbish skier I might still venture a tentative comparison with a leading athlete, is it possible that in taking a direct route of the body into the new turn, Anna Fenninger is not lifting the outside ski at all (other than by virtue of being attached to it as she moves towards over the inside ski)? That is to say, the raised tail is an effect, and the path of the body and resulting lack of pressure on the new outside ski are the cause. Any good?

    1. “Returning to Fenniger, one has to provide a rational explanation for her movement wherein she lifts the outside ski off the snow”

      In my attempts, I find a conscious effort to lift the tail of the outside ski very difficult (without standing up on the inside ski). I also find it difficult to consciously move onto any given ski. However, slow motion and still photos show that one ski or part of it will be airborne during some turns, usually without me being aware of it. I presume this is due to rebound.

      ANSWER: Yes, it is due to rebound (actually recoil) from what is called the catapult mechanism.

      There are 2 situation where the inside can be used to advantage. In one situation it can be used to achieve a higher line in certain gate setups. But the main reason skiers like Hirscher and Shiffrin use the inside is to control inclination to get enough edge angle before they load the outside ski. In this application they use their outside ski to steady their position on the inside ski. As they incline as the start to cross the rise line below the gate the shovel of the inside ski will diverge from the shovel of the outside. Once they have sufficient inclination they steer (rotate) their outside across the gravity line and increase the torsional force in conjunction with vertical loading of the inside edge. This mechanism cannot be employed without first establishing inclination. The sequence is incline, steer/pressure-torsion load.

      Clarification is needed on ‘the top of the turn’ issue. There is the end of a turn characterized by the transfer of weight to the inside ski and a release of energy created by a loading sequence that resulted in dynamic stability of the platform that supported and resisted the load from the weight of the skier. This is how the catapult mechanism is loaded. Transferring weight to the inside ski initiates a controlled free fall. Skis are positioned in a float phase so they can be steered across the rise (fall or gravity) line so they end up between the skier and the force of the pull of gravity where they can be loaded. There is no such thing as the top of a turn because a skier is in a controlled free fall. I call the move ‘drop and pop’.

      Please let me know if this explanation is not clear

      1. Many thanks for the detailed response. I used ‘top of the turn’ to mean the phase between transition and the fall line, i.e. when the skis are on their downhill edges, after the point most coaches would refer to as the ‘transition’, which if I’ve followed your stuff correctly is initiated by the roll over – unless you use the inside ski before the fall line, in which case the move must be surfier. The only difficulty I have here is that the inside ski has already curved into the fall line by that stage, in which case the turn must have already started, no? The portion of the track on the snow between the transition and the fall line is curved, and follows a different direction to the previous turn, and hence is itself part of the new turn, even if there is low pressure here. Or when you say ‘steered’*, do you mean horizontally twisted, i.e. you’re not talking about an arc to arc carve so much as a floating pivot followed by a carve in the fall line?

        “But the main reason skiers like Hirscher and Shiffrin use the inside is to control inclination to get enough edge angle before they load the outside ski.” – is this a conscious intention, or merely a benefit?

      2. “But the main reason skiers like Hirscher and Shiffrin use the inside is to control inclination to get enough edge angle before they load the outside ski.” – is this a conscious intention, or merely a benefit?

        ANSWER: It’s a learned pattern that has become automatic (automaticity). Watch how Shiffrin Hirscher bank into the landing of their intended line and feed their skis into what amounts to a slot. With repetition the CNS is able to calculate a flight path that includes a banked turn onto at fixed landing spot.The key is to be able to rapidly acquire dynamic stability of the platform which becomes a launching pad from which to initiate precise movement. The real race is to what is called ‘time to stabilization’.

      3. Might be easier if we have a clip and times. At 0.22, the skier is in free fall but the skis are curving – the turn has begun, right? Or do you have it later than that?

      4. Adam Zampa is killing it (I mean his inertia). If you want to go slow this will do it. But loading the skis before crossing the rise line will mess up the loading sequence. Watch this video of Hirscher. If you can slow it to 0.25 speed it is better. Watch how he lands and loads on his line towards the next gate. – https://au.eurosport.com/alpine-skiing/world-championships/2018-2019/skiing-news-breathtaking-marcel-hirscher-goes-1.7secs-ahead-of-field-in-slalom_vid1164601/video.shtml#au-fb-sh

      5. I see. If you pivot the top of the turn, there is no top of the turn. Also quite pronounced in SL here: https://youtu.be/oakBu3DR3oU Unless to your more exacting eye Hirscher is doing something different.

        It looks to me that when there is space on flatter terrain, Hirscher does carve before the fall line. Elsewhere you have said that pure carving is slow. Ligety has said a number of times that pivoting the top of the turn is an outcome of having to make tighter turns in courses and that in free skiing GS, he is trying to carve arc to arc and that is what you should be trying to do whenever possible in training. Unfortunately there is next to no footage of Hirscher freeskiing. However, in the little I can find, the tails seem to be following the tips with no pivot: https://youtu.be/l3Ei43eOgwE

        Now I am confused though as to how the roll over comes into play as an initiation move. I can see how this is used to initiate a cross over in arc to arc, pure-as-physically-possible carving. Is it also being used to get a pop to land and carve in the fall line? Again, I’m being exacting here because I want to go out and try it and to do that I need a clear idea of the initiation.

      6. First off, it takes very good quality at the right camera angles and many hours to properly analyze a racers’ movement pattern. Here’s a few excerpts from Analysis of Sport Motion published in 1983. I will speak to the issues you raise when time permits. For now it is important to understand the physics and why racers like Hirscher often retract their skis so they are off the snow as they approach the rise line above a gate. Think of turning the skis across the line of inertia in the float phase as braking. Acceleration is maximal in the fall or rise line. In many cases a racer will go faster in the rise line with their skis very light or in the air.

        Analysis of Sport Motion

        It is desirable that a thorough understanding of anatomic kinesiology precede the study of biomechanics. An understanding of anatomic planes of motion, the motions possible at major joints of the body, ranges of motion possible at each body joint, and comprehension of the myologic internal force components for controlling the extent of motion with and against gravity at each major joint is of utmost importance. There are positive and reciprocal relationships between anatomic kinesiology and biomechanics. An understanding of anatomic kinesiology assists the learner in comprehending aspects of biomechanics and vice-versa.

        The utilization of biomechanic subject matter is both common and diversified in the area of sport.

        The timed, sequential joint motions of the performer executing a skill are of prime concern.

        The performer, through his or her joint motions, is one major source for generating and summating forces necessary to meet the skill objectives.
        Therefore, detailed analyses of these joint motions must be made qualitatively and quantitatively in order to determine skill effectiveness.

  2. I’d like to check my understanding of this. Hitherto, I thought you were advocating creating/accessing forces that were sort of diagonal in direction – necessarily around the axis of the leg because of pronation, but also down towards the snow as the skier inclines, the skier’s initial inclination originating at least in part from that pronation. In the “perfect fit” diagram, wouldn’t the axis be less horizontal, given that the skier can only minimally pronate the foot and has to use the leg as a lever against the shaft of the boot by inclining the shin towards the apex of the turn? In short, I get that there will be rotational directions, but I’m struggling with the idea of rotation around a horizontal axis if we’re talking about high edge angle carving. My understanding of physics isn’t even rudimentary though, hence the question.

    1. “……… wouldn’t the axis be less horizontal, given that the skier can only minimally pronate the foot and has to use the leg as a lever against the shaft of the boot by inclining the shin towards the apex of the turn?”

      I’ll expand on the issues you raise in my next few posts. For now, the consistently stated objective of the perfect fit is to align the foot in neutral so it is neither pronated or supinated using a number of means including footbeds, shims, boot bottom planing, laser alignment…….. and then lock the joints of the foot so they cannot articulate. The reason proponents of the perfect fit speak only of pressure control as one of 3 separate skills, which must be consciously blended, is that a skier has no control of where force applied. If he or she has even a tiny amount of control the perfect fit is not perfect.

      Memo to Marcel Hirscher: Can you please tell me at one point in your turn you consciously blend steering with the separate skills of edge and pressure control and the number of milliseconds between each blend of a skill?

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