THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: HEEL/FOREFOOT ROCKER


An essential mechanism to the ability to create a platform under the outside ski to stand and balance on using the same processes used to stand and balance on stable ground, is the Heel to Forefoot Rocker. A slide presentation called Clinical Biomechanics of Gait (1.) by Stephen Robinovitch, Ph.D. (Simon Fraser University – Kin 201) is a good reference for the various aspects of gait.

Slide 19 of the Gait presentation describes the ankle Inversion-Eversion-Inversion sequence of the ankle. The sequence begins with heel strike (HS), followed by forefoot loading (FF), followed by heel off (HO) followed by toe off (TO).

The normal foot is slightly inverted in the swing phase (unloaded) and at heel strike. It is everted through most of the stance phase. The ankle begins to invert in late stance. The kinetic flow of pressure is from the heel to the ball of the foot and big toe. This is what should happen in the transition phase of a turn sequence when a skier begins to transfer more weight to the inside foot and ski from the outside foot and ski. Up until the start of the transition, the skier’s center of mass is behind the inside foot with the majority of pressure under the heel on the transverse center of the foot and ski where is exerts an inversion torque that is tending to rotate the ski into contact with the surface of the snow. The skier maintains the edge angle by applying a countering eversion torque with a combination of external rotation-abduction of the inside leg.

When the skier begins to transfer more weight from the outside ski to the inside ski, the leg releases the countering eversion torque and the ski begins to invert in relation to the surface of the snow.

The presentation on the Clinical Biomechanics of Gait did not include important aspects of the stance phase that occurs in late stance. Nor, did it mention Achilles forefoot load transfer.

The Three Rockers

Slide 23 shows the Three Rockers associated with the gait cycle.

First Rocker – occurs at heel strike. It causes the ankle to plantarflex and rock the forefoot downward about the heel into contact with the ground. The rocker movement is controlled by eccentric dorsiflexor torque.

Second Rocker – shifts the center of pressure from the heel to the forefoot. Eccentric plantarflexor torque controls dorsiflexion of the ankle.

Third Rocker – occurs at heel separation from the ground that occurs in terminal phase of stance.

Slide 13 shows how the knee shifts gears and transitions from flexion in early stance to extension in late stance. In late stance, the Achilles goes into isometric traction. At this point, further dorsiflexion of the ankle passively tensions the plantar ligaments to intiate forefoot load transfer. Load transfer is accentuated when the knee shifts gears and goes into extension moving COM closer to the ball of the foot increasing the length of the lever arm.

Two Phase Second Rocker

Classic descriptions of stance and the associated rockers do not include a lateral-medial forefoot rocker component that occurs across the balls of the feet from the little toe side to the big toe side in conjunction with the heel to forefoot rocker creating what amounts to a Two Phase Second Rocker.

In his comment to my post, OUTSIDE SKI BALANCE BASICS: STEP-BY-STEP (2.), Robert Colborne said:

….… regardless of where the centre of mass is located relative to the centre of pressure in the above-described mechanism, when you go into a stable monopedal stance, as you would when you are in a turn, the ankle is dorsiflexed forward and as this occurs the tibia rotates internally several degrees.

COMMENT: The tibia rotates internally (i.e. into the turn) as a consequence of ankle dorsiflexion. It does not require conscious action by the skier.

This means that the main muscle forces acting across the ankle (the plantarflexors) are no longer acting along the long axis of the foot, but rather partly across it, medially toward the big toe.

So, the beneficial effect of that muscle force is to force the base of the big toe into the ground, and that becomes the centre of the turn (centre of pressure).

In the absence of this internal rotation movement, the center of pressure remains somewhere in the middle of the forefoot, which is some distance from the medial edge of the ski, where it is needed.

The photo below shows a skier in bipedal stance with weight distributed equally between the two feet standing on a plush carpet with foam underlay. Black hash marks show the positions in space of key aspects of the right foot and leg.

The photo below shows the same skier in monopedal stance with all the weight on the right foot. Forefoot loading from the Two Phase Second Rocker has pushed the toes down into the carpet by compressing the underlay.

The video below shows the dynamic action of the Two Phase Second Rocker.

The Two Phase Second Rocker results in a heel to ball of foot diagonal rocker action acting towards the centerline of the body; i.e. diagonally across the long axis of the ski with the load acting inside the shovel.

A primary objective of the Birdcage studies was to validate my hypothetical model of the Two Stage Diagonal (heel – forefoot) Second Rocker in creating a balance platform under the outside ski for a skier to stand and balance on.

The graphic below shows the alignment of the Two Stage Diagonal (heel – forefoot) Second Rocker.

In my next post, I will discuss the Two Stage Diagonal (heel – forefoot) Second Rocker Turntable Effect.


  1. http://www.sfu.ca/~stever/kin201/lecture_outlines/lecture_17_clinical_biomechanics_of_gait.pdf
  2. http://wp.me/p3vZhu-29n

7 comments

  1. Mr. Wolter seems to have no concept of proprioception, muscle synergy or reflex postural responses. He ignores the balance point of CoP with the foot touching the ground as a determinant for effective and efficient skiing.

  2. Hi David – I read with interest the comments of JWolter7. The image they conjure up is of a young child making it’s first faltering steps. The brain soon figures out that life is better working from the feet up. Of all of your articles this is at the top of my list. It accurately describes the elements that are fundamental to my inline skate technique and should be required reading for anyone who is serious about improvement. I think you will enjoy this sequence of Stefano Belingheri putting into practice the rocker sequence you describe – he has zero chance of staying on his feet without it. https://youtu.be/q51L1sz9msc

  3. The whole original post here shows and proves the whole problem with ski equipment and getting skiers to change their ‘position’ on what is in reality best for their skiing. Without arguing with ‘scientific’ (as scientists have or haven’t proved but say is a fact:) proof I will say that I bought into the status quo of ski equipment fitting because one assumes (often wrongly) that the ‘experts’ working in a certain field know best.

    Even though I had proof; a knee injury from my knee being bent sideways; I spent years ‘angulating’ my knees sideways to try to edge better. It worked better with knees bent than straight so does that prove the knee bends sideways enough to edge skis? No, I just didn’t realize, care, or pay attention that all the factors I have learned on “The Skier’s Manifesto” allows one to ‘get on the inside edge’ even though the knee still isn’t bending sideways (thank heavens for that!) Turns out the ‘status quo’ boot fitting had everything below the knees so locked in the wrong place I couldn’t ‘get to the inside edge’ without funky rest-of-the body-contortions and had to think about many things, hopefully quick enough before maxing out the open spaces and hitting an immovable object (tree:( Unfortunately I learned from meeting with a ski company trying to sell a fore/aft balance adjuster which mounts between the ski and binding, that current ski equipment works quite well for a certain body type (that of the equipment designers) and they have no clue that it doesn’t and can’t work for every one that doesn’t fit that body type!!

    Freeing up the feet (in a scientific way by measuring each aspect of the ski boot to see if it allows one to balance correctly or not) allows me to ‘get to the edge’ the second I shift from one foot to the other as if I was walking. The problem with JWolter7 and so many others is that when the ski equipment has one so blocked in the wrong positions for ones whole life of skiing one can’t fathom that there is a better way because one has never been close to being functional in ski boots. And there are so many places they are wrong that for most skiers correcting one factor may bring you from 5% optimum alignment to 15% optimum alignment which still sucks.

    Foot note; “The Athletic Skier” pub. 1993 has a chapter “80/20; 20/80” in which the authors are stating: the first 80% alignment gives you 20% performance and the last 20% alignment gives you 80% of the performance. Unfortunately what’s taken me over 25 years to figure out is that the biggest benefit from the last 80% alignment appears to be in the final 5% alignment!!

    JWolter7; skiing is believing so give some of this an honest try and your skiing will become more joyful and you’ll be able to ski longer and with less body stress that ever before;)

  4. So your whole argument is based on your “fact” that when the ankle is in a dosiflexion position, then the tibia must rotates medially. According to you it happens all by itself. Can you prove this? The shape of the distal head of the Talus and the shape of the proximal head of the navicular do not support your claim. It takes a downward force to get the tibal to rotate. When the foot is unweighted in a dosiflexed position it is technically in the neutral position. It’s not until the foot is weighted (a downward force is applied) that the foot/ankle starts to pronate. That is the very principle basis of unweighted casting of orthotics.

    You show photos of a person while weighted moving from a two footed stance to a one foot stance and you don’t see any forces coming from above the ankle to create those medial pressure movements?
    Are you using facts to come to your conclusions or are you searching for anything that might prove your hypothesis correct?
    In your photos you show and state that when a person goes from balancing on two legs to balancing on one leg everything starts from below. One question: what happens to the center of mass of the whole body which is located in the hips when you change your balance below? Does it change above first? Does the center of mass of the whole body not effect what happens below? Then the leg and ankle adapt to that change in COM? Look at the angle of the leg in your one footed stance it’s centering itself to the new COM.
    The COM when standing on two feet is different then when standing on just one foot.
    That’s why the body has to them adapt. The COM position in the hips changes first.
    SOB!

    The other thing you never take into account in your “findings” is the effects of gravity and centrifugal forces on the skier during a turn. Those are forces that come from the center of mass and move out to the snow.
    As you have already shown in other posts the transition phase between turns is a time of a change in pressure. We go from heavily weight positions to very light weight/pressure on our skis as we move them from one edge to the other side of the ski.
    We position our body to create forces onto the snow. If we waited for feedback from our feet before starting our turn we would be late in every turn.
    It’s a learned action to position the body according to what is coming next. If it’s a big patch of ice in one turn we use more angles and more pressure because we have learned that is what is needed. We don’t wait until our feet feel the patch of ice, we inspect the course to find the snow conditions we will need to be prepared for and then we anticipate the extra force needed based on our training.

    1. The main value in your comments is that they serve to illustrate the extreme disconnect of some of those in skiing with reality. To give but one example you states;

      “You show photos of a person while weighted moving from a two footed stance to a one foot stance and you don’t see any forces coming from above the ankle to create those medial pressure movements?”

      “A person while weighted” implies weight from COM is acting on two feet and then one. Forces are implied by the word stance.

      “We position our body to create forces onto the snow. If we waited for feedback from our feet before starting our turn we would be late in every turn.”

      This statement is laughable because it implies the use executive control which is incredibly slow. By the time a skier thinks about positioning their body they have traveled 50 feet even at modest speeds. We move most effectively by the process of automaticity.

      I suggest you read my last post until you grasp one of the fundamental problems in skiing which is deformation-instability of the base of the boot. But the most important issue is creating a platform to stand and balance on by balancing multi-plane torques across the edges of the ski and, by association, across the joints of the ankle foot complex, knee and pelvis.

      1. I watched the shadow of the right leg during the bifootal to the monofootal move and it appears that the shadow lowers, automatically, as the COM is in the neurological sequencing for balance in the stationary planes of movement stance on the carpet. Do the pix again on a descending carpet or concrete surface…..maybe.
        Deanski Tonkin

      2. Due to the over-centre mechanism associated with edge change The ball of the foot lowers as the ski rolls onto it’s new inside edge while the ball of the small toe raises in relation to the ball of the big toe. In addition, the shift of the support limb into a varus angle causes COM to drop.

        The bipedal-monopedal exercise on carpet lacks the dynamics of edge change/roll over associated with a ski turn.
        I will discuss this aspect in future post.

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