THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: CLOSED CHAIN OUTSIDE LEG ROTATION


A recently published study on foot pressure data acquired during skiing (1.) recognized that compressive force pressure data acquired in skiing is underestimated by 21% to 54% compared to pressure data acquired on a force platform in a controlled environment.  The underestimation varies depending on the phase of the turn, the skier’s skill level, the pitch of the slope and the skiing mode. The paper states that other studies have stated that this underestimation originates from a significant part of the force actually being transferred through the ski boot’s cuff (to the ski). As a result, the CoP trajectory also tends to be underestimated along both the anterior-posterior (A-P) and medial-lateral (M-L) axes compared to force platforms.

In conclusion, these studies have highlighted a major contribution of different factors to the nGRF applied throughout a turn, such as the foot’s position during a turn (inside vs. outside), the CoP A-P (front to back) displacement, or precise loading of different foot sole regions.  Unfortunately, these results have been studied separately.

There is a lack of continuity across the various positions in skiing and, in particular, a lack of a model with which to explain mechanisms such as balance on the outside ski and open and closed chain internal rotation of the leg and foot in conjunction with progressive inclination and G force loading on it as the skier crosses the fall line in the bottom of a turn. The associated mechanics and biomechanics represent a new paradigm requiring new thinking and new insights. Existing text-book explanations are not sufficient to explain these mechanisms.

Open Chain Whole Leg Rotation vs. Closed Chain Rotation

Rotation of an unloaded (non-weight bearing) lower limb is relatively straight forward when there is a small angle at the knee. As resistance to rotation of the foot is progressively introduced with increasing weight imposed on it, the kinetic chain begins to close. As it closes, the points at which the foot transfers torque to the walls of rigid shell footwear such as ice skates and ski boots starts to emerge as an issue as does the loading of the foot created by the weight of the body imposed on it and the position of COM in relation to the foot.

In order to tension the biokinetic chain and trigger the two-phase Second Rocker, COM must be aligned over the foot as shown in the grahic below.  This alignment requires that the leg adduct (move towards the center of the body) approximately 6.5 degrees. To bring the 3 points of the tripod of the foot into contact with the ground, the foot must evert (sole turn outward) the same amount. Eversion is accompanied by a corresponding torque coupled 6.5 degrees of internal rotation of the leg as shown in the left hand figure in the graphic below (see my post – OUTSIDE SKI BALANCE BASICS: STEP-BY-STEP). The bipedal figure on the right shows adduction, eversion and internal rotation as 0.0 – 0.0 – 0.0 for reference. The monopedal figure on the left shows the changes in adduction, eversion and internal rotation as 6.5 – 6.5 – 6.5.

 

The alignment of COM with the foot can be achieved by moving COM laterally as shown by the arrow emanating from COM in the Monopedal figure or by moving the foot medially as shown by the white arrow or through a combination of the two movements.  The act of positioning COM over the outside foot (Getting Over It), sets in motion internal rotation of the outside leg and eversion of foot into the turn. This engages an over-centre mechanism between the platform of the ski and the inside edge underfoot.

The over-centre mechanism results in an alignment of the resultant force R forming an angle with the transverse aspect of base of the ski that is slightly less than 90 degrees. In order to Get (COM) Over It (the foot), it is essential that the outside leg is not only able to adduct and rotate internally as the foot everts, but to achieve this configuration without delay in order to set up the over-center mechanism. The problem for the majority of skiers is that the objective of most boot fit systems and boot-fitting procedures is to support the foot in a neutral configuration. Eversion of the foot is a component of pronation. Impeding or preventing pronation, restricts or even prevents the required amount of eversion.

Closing the Kinetic Chain on Whole Leg Rotation

Open kinetic chain leg/foot rotation with the foot unloaded (not bearing weight) is relatively simple. But the mechanics and biomechanics begin to get complicated when resistance is progressively introduced that starts to close the kinetic chain as happens when the outside ski is rotated across the path of the skier in the fall line in the bottom of a turn.
The graphic below shows a foot supported on a platform with 2 points of resistance (FR) applied to the platform opposite the 2 points of application of the moments of force, ML (green) and MM (red). The forces tangent to the arc of the moments of rotation are shown as FT.
When the weight of the body is progressively shifted to one foot (i.e. Monopedal Stance) and the foot everts, the talus (shown in gray in the graphic above) moves inward towards the center of the body and shifts slightly rearward as evidenced by the corresponding movement of the inside ankle bone.  This is easily seen when moving from bipedal to monopedal stance on a hard, flat surface while barefoot.In order to effectively transfer torque from the foot to the platform, the forefoot and ankle and knee joints must be fascially tensioned. This requires that the big toe (Hallux) be aligned on the anatomical axis (dashed line) and the forefoot fully splayed. This stabilizes the heel and head of the 1st metatarsal (ball of the foot).  Torque from internal rotation of the leg will be transferred to two discrete points adjacent the Load Counters mounted on the resistance platform.

Removing the resistance force FR from the inner (big toe) aspect of the platform provides insights to what I refer to as the Turntable Effect that is associated with internal rotation of the leg and eversion of the foot that creates an over-center mechanism. The turntable rotation is shown in light yellow. The effect will vary for different structures of the foot depending on the location of the center of rotation of the platform under the foot.

The location of the blade of an ice skate on the anatomical center of the foot has been used to explain why it is easier to cut into a hard ice surface with a skate compared to the edges of a ski. But the real reason it is easier is because ice skaters use the Second Rocker, Over-Center, Turn Table Mechanisms as shown in the graphic below. The skate is positioned under COM. It can be readily seen that the skater is not using the inner aspect of the shaft of the skate to hold the skate on edge.

In my next post, I will discuss the progress of emerging CARV and NABOSO technologies after which I will continue with my discussion of the Mechanics of Balance on the Outside Ski.


  1. Influence of slope steepness, foot position and turn phase on plantar pressure distribution during giant slalom alpine ski racing: Published: May 4, 2017  – Thomas Falda-Buscaiot, Frédérique Hintzy, Patrice Rougier, Patrick Lacouture, Nicolas Coulmy
  2. http://wp.me/p3vZhu-29n

11 comments

  1. Interesting article – but HUGE problems with the mechanics of skiing as presented here.

    First of all – there is no torque applied by the leg or foot to the inside of the boot. Attempting to do so is a major error – probably the worst error a skier can make. Unfortunately the most common source of this error is from national ski instruction organisations.

    The torque comes from the ski – through the boot – against the leg in the opposite direction. You should feel “external” pressure on the inside of the heel and the outside of the forefoot.

    The Centre of Mass is not lined up over the inside of the ski or anything else – it is intentionally displaced (initially) towards the centre of the turn (the opposite direction of your graphic). This generates dynamics (as defined by Newton’s second law) and this functions along with the ski design to both generate stability and directional change. There is no “balance” involved. We are working with organised accelerations – disequilibrium.

    Your “balance” terminology – with lining up the centre of mass etc. is based on a faulty understanding of mechanics. So called forces such as “centrifugal force” are actually fictional “d’Alembert” forces – a mathematical construct substituted for actual accelerations in calculations only. They do not represent reality in terms of forces.

    1. Hi Ian, are you new to my blog? If so, I suggest you read ‘About Me’ on the main page menu, especially 1995 Science Award documents and letters in support of my nomination as well as my posts on the Birdcage studies. Meantime, here is what the biomedical engineer who worked on our research team said about me:

      “During 1991 and 1992, I had the opportunity of working with David MacPhail in the realization and testing of conceptually innovative sports footwear.

        Design of this type requires, knowledge, understanding and experience in a combination of disciplines including anatomy, physiology, biomechanics, sports dynamics, physical mechanics and design. David MacPhail exhibited this unique combination throughout all aspects of the project, and continues to research and explore developments in: influences of footwear on the kinematics and kinetics of human movement

      ; the design of athletic footwear; and the etiology, occurence, frequency and prevention of athletic injuries.

      My relationship with David MacPhail began in 1991. As member of the Industrial Design team hired to refine, build and test prototype ski boots based on David’s original ideas, I was responsible for the development of an “on-hill” testing program. The objective of this program was to record and evaluate the biomechanical impact of the new boot design in a real dynamic environment, and then compare the results to the theoretical model. Unlike other (ski boot) designs, David’s theoretical described skiers’ biomechanics in a realistic, uncompromising, dynamic situation.

      Rigid prototype boots which embodied all of the desired characteristics were constructed and instrumented with force sensors mounted at key boot-foot interface locations. A variety of skiers including men and women, with skills ranging from novice to World Cup racers, skiied while wearing these prototype boots. Data from the boots were recorded during these trials. Analysis of the results validated the original hypothetical model and provided valuable insight which enhanced the next generation of design.……these design concepts are currently referred to as a “dynamic coupling interface”.

      The design and development strategies used by David MacPhail are very holistic in nature, placing the human system as the central and most critical component in the biomechanical system. His intent is to maximize human performance and efficiency, while foremost preserving the well-being and safety of users and minimizing biomechanical processes.”

      – Alex Sochaniwskyj, P. Eng. – March 24, 1995

      Here is Alex CV as of that date.

      Professional engineer with 12 years of biomedical and rehabilitation engineering research experience in the Human Movement and Motor Functions Research Programmes at the Hugh MacMillan Rehabilitation Centre in Toronto, Ontario, Canada. The principle aim of these labs is to provide detailed information and objective analysis of movement, dynamics and motor function of persons with various physical disabilities. The information is used to objectively assess the effects of a variety of therapeutic and surgical interventions.

      Alex holds a Bachelor of Science degree from the University of Toronto in Human Phusiology and a Bachelor of Applied Science from the University of Toronto. Most recently, Alex has worked with several companies including ADCOM ELectronics Limited in Toronto, where he was responsible for the design and development of video conferencing and multi-media communication systems, and the Arnott Design Group, where he focused on physiological human factors in product system design, prototyping and testing.

      Currently, as a principal at designfarm inc., he consults to design and manufacturing firms on the development of programs to evaluate human physiological, biomechanical, ergonomic and environmental response for product and interface design, and the planning of comprehensive technology implementation strategies for the integration of computing, telecommunication and telepresence technologies. Alex is also a Certified Alias Instructor in the Information Technology Design Centre in the School of Architecture and Landscape Architecture at the University of Toronto, where he teaches courses in computer literacy, three dimensional design, modelling, simulation and animation.

      Alex is a member of the Association of Professional Engineers of Ontario (APEO), the Institute of Electrical and Electronics Engineers (IEEE), the Association of Computing Machinery (ACM) and the University of Toronto, Department of Rehabilitation Medicine Ethics Review Committee. He is co-author of numeroius publications in refereed medical and engineering journals and has produced several video productions regarding biomedical and rehabilitation engineering.

      After you have invested some time reading my posts, I will respond to your comments.

      1. Dear Ian,

        So much of life seems to be confusing when we don’t check for the meaning of the intention of the author. Many disputes are not disputes at all. Disputes likely arise from lack of understanding what the speaker wanted known. Therefore it is important to check one’s understanding with the speaker before proceeding.

        I was struck by this statement of yours referring to David’s Monpedal diagram, “The Centre of Mass is not lined up over the inside of the ski or anything else – it is intentionally displaced (initially) towards the centre of the turn (the opposite direction of your graphic).” As I understand Dave’s diagram he is demonstrating gait function as it relates to a specific point in the transition phase of a ski turn (David might want to clarify this {hearing} interpretation of is words). This just one aspect of what the body does biomechanically to shift wait from the old outside ski to new outside ski.

        My guess is if you were to sit side by side with David and view of picture of a racer in the apex of a turn your statement would likely agree with David’s biomechanical assessment at that point of the turn. Sometimes it takes this sort of intellectual back and forth to hear and understand one another with precision. Those of us who have read or participated in the blog are all too familiar with these phenomena.

        This blog with the many contributions from people all over the world has been years in the making. Many ideas have been expressed and developed.

        So welcome aboard this ship of the curious. It’s been my observation that those people who curiously check meaning of what they are hearing increase understanding and knowledge. Because if one understands the meaning of what each person is actually wanting to be known it is more likely that listener can then more effectively contribute to the sport we all love, thomas

      2. Further to Thomas’s comments, the objective of any intelligent cooperative dialogue should be to arrive at the right answer not who is right.

        The purpose of the monopedal diagram is to illustrate that balance in monopedal stance in accordance with Newton’s Third Law can only be attained when the position of COM is within the anterior/posterior and medial/lateral limits of the center of pressure of applied and ground reaction forces aligned in 2 bisecting planes in opposition with each other. If the position of COM exceeds the anterior/posterior and medial/lateral limits of applied and ground reaction forces, a fall will result.

        What is either being ignored or misunderstood with the ranks of skiing is that the center of pressure of applied and ground reaction forces cannot be physically aligned. So the conditions of Newton’s Third Law cannot be met unless GRF acting along the running surface of the inside edge of the outside ski can be cantilivered out under the body of the ski.

        “It’s been my observation that those people who curiously check meaning of what they are hearing increase understanding and knowledge. Because if one understands the meaning of what each person is actually wanting to be known it is more likely that listener can then more effectively contribute to the sport we all love, thomas”

        In order to solve a problem in the holistic perspective of a complete resolution, one must first realize that a problem exists. The next step is the most crucial: – obtain and thorough and complete understanding of the problem. Only then can a problem be completely resolved. Opinion based on uninformed observation does equal truth even if opinion results in consensus.

  2. Dear David,
    can you clarify how much heel movement the ski boot should allow or where on the heel the boot should grip?
    As I look at my feet pronating there is a rolling of the calcaneum medially until your turn table effect kicks in to produce some lateral re-centering movement at the back of the calcaneum.
    thanks

    1. Hi Mike,
      This is an issue that the Birdcage project was designed to study. Although I understand the nuances muhc better now, my hypothesis that the Birdcage studies was designed to test was that elite skiers create an eversion angle on the outside ski with 2nd rocker impulse loading at ski flat that can best be described as pitch down with on ball of the foot when pressure under it is greater than the pressure under the heel and pitch up when the pressures are reversed. Level flight would be equal pressure. I hope that this is what Pitch means in the CARV system.

      When the foot pronates the medial aspect of the head of the 1st MT should make contact with the medial wall of the boot shell and the poster-lateral aspect of the heel should should make contact with the corresponding aspect of the boot shell. The Birdcage had rigid vertical counters for this purpose that acted in the capacity of vertical extensions of the sidewalls of the outside ski. Apply internal rotational force to the counters in a turntable action has the net effect of locking the transvers plane of the base of the ski with vertical aspect of the leg. That is why inclination results in edge grip. It’s more involved than this. But please let me know if I have not answered your question.

      1. Hi David
        Thanks for your answer. I was really considering if a tight grip on the back of the heal limits pronation? If it does how much movement is acceptable and where on the back of the heal is the best place to pad to acheive this?
        Your comments re pitch lead me to think about the turntable effect on a slope rather than for an ice-scater. Does the turntable effect produce a rotation that when combined with the change of pitch of the slope being skied across ( Le Master talks about a virtual hump- in pure traverse the slope could be considered flat, ) causes ski edge to track the slope profile and allow the tip to engage , thereby creating grip and allowing the ski to flex?
        This tipping of the front of the ski into the steeper part of the slope generates more resistance to the front of the ski than the back of the ski keeping the ground reaction force at the front of the ski. (tangents to the front section of the ski and the back part point in different directions, exagerated further by ski flexion).
        Apologies if this is obvious from your previous posts or indeed if my interpretation of the physics misunderstands your previous posts or physics!

      2. I was really considering if a tight grip on the back of the heal limits pronation?
        > No. I the issue is posteromedial constraint.Counters on this aspect have to serve as a pronation gate for articulation of the calcaneus associated with pronation. Posterior constraint should be applied to the boney surface not at the fat pad. I will discuss this in a post soon.

        Does the turntable effect produce a rotation that when combined with the change of pitch of the slope being skied across?
        > The context of pitch applied to the second foot rocker is in reference to the heel/1st MTP pressure differential. Fifty-fifty pressure = level flight; pressure bias to the heel = nose up, pressure bias to the 1st MTP – nose down. The second rocker/over center mechanism presses the inside edge under foot down in relation to the shovel and tail.

        Slope pitch is related. But it is another whole subject.

  3. Might be too early (may appear in a future post) to ask these questions:
    1) Am I correct in understanding that you’re talking about the pivot point as being under where the leg attaches to ankle?
    2) Isn’t the point of having pressure on the ball of the big toe (when skiing) that then one can inwardly rotate the foot under load?
    3) Been playing with big toe orientation because my left big toe is pointed more towards the other toes than my right. When I insert toe spreaders many good things happen. From your sharing of Dr.Splichal’s work I’ve been trying to ‘find’ the muscle that pulls the big toe into alignment with the first ray. Rather awkward and can sort of do it when my foot inwardly rotates which leads to this question: if the feet are healthy should we be walking a bit ‘pigeon toed’ (pointed inward) rather than ‘duck waddling’ (pointed out)? I’ll probably play with this concept on stairs and let you know if I form an opinion:)

    1. 1) Am I correct in understanding that you’re talking about the pivot point as being under where the leg attaches to ankle?
      > I prefer the term rotation instead of pivot, but, yes, the leg is rotated at pelvis and the lower aspect of tibia rotates the foot in an open kinetic chain and attempts to rotate the foot against resistance in a closed kinetic about a pivot axis at the inside edge below the foot. In World Cup, the pivot can be as much as 93 mm below the foot. This is what sets up the over centre mechanism that enables what I call the turntable effect. This is the real effect and benefit of elevating the foot above the base of the ski.

      2) Isn’t the point of having pressure on the ball of the big toe (when skiing) that then one can inwardly rotate the foot under load?
      > The two phase 2nd rocker transfers COP to the ball of the foot causing the ski to rotate into the turn about the inside edge underfoot. It also starts the leg rotating into the turn. Layering activation glute internal rotation of the leg quickly becomes a reflex action.

      3) Been playing with big toe orientation because my left big toe is pointed more towards the other toes than my right. When I insert toe spreaders many good things happen. From your sharing of Dr.Splichal’s work I’ve been trying to ‘find’ the muscle that pulls the big toe into alignment with the first ray. Rather awkward and can sort of do it when my foot inwardly rotates which leads to this question: if the feet are healthy should we be walking a bit ‘pigeon toed’ (pointed inward) rather than ‘duck waddling’ (pointed out)? I’ll probably play with this concept on stairs and let you know if I form an opinion:)
      > No. Biomechanically, it is not efficient.

Comments are closed.