Mikaela Shiffrin


Since I started this blog with my first post, A CINDERELLA STORY: THE ‘MYTH’ OF THE PERFECT FIT (1.) on 2013-05-11, THE SHOCKING TRUTH ABOUT POWER STRAPS (2.) is by far the most widely viewed post. This is significant because the content of this post challenges premises that are widely embraced and cited as knowledge that is fundamental to skiing.

The greatest enemy of knowledge is not ignorance; it is the illusion of knowledge.

                                                                                    – attributed to Stephen Hawking

Widely accepted false beliefs can negate incentives to pursue the acquisition of knowledge necessary to understand complex issues that fall outside the limits of established paradigms. A prime example being the ability to balance perfectly on the outside ski.

Observing great skiers like Marc Giardelli or Ingemar and more recently, Mikaela Shiffrin, Lindsey Vonn and Marcel Hirscher balance perfectly on their outside ski suggests it is possible. But uninformed observation in itself does not impart, let alone lead to, an understanding of the associated mechanics, biomechanics and physics of perfect balance on the outside ski as it equates with neuromuscular mediated dynamic balance of triplanar torques acting across the joints of the ankle/foot complex, knee and hip. The intrinsic need of those who regarded as authorities on ski technique to provide plausible explanations for the actions of elite skiers led to the fabrication of terms such as knee angulation that served to create an illusion of knowledge of the mechanism of balance on the outside ski. Knee angulation also provided an effective mechanism with which to demonstrate the mechanics of edge hold.

To raise new questions, new possibilities, to regard old problems from a new angle, requires creative imagination and marks real advance in science.

                                                                                                                          – Albert Einstein

While knee angulation provides a plausible explanation for a mechanism with which to rotate a ski onto it’s edge, it does not explain the mechanism of perfect balance on the outside ski in accordance with Newton’s Laws and the principles of functional anatomy. Solving this mystery required raising new possibilities and creating a new paradigm; one that looked at the function of the human lower limbs from a new perspective with new possibilities.

It took me from 1980 to 1990 to discover how the mechanism of balance on the outside ski works. Trying to impart an understanding of this mechanism to others has presented significant challenges because the illusion of knowledge within the ranks of the ski industry has resulted in a hardened mental model that makes the real mechanism all but invisible. The resulting information bias causes people to seek information that supports what they believe while filtering out information that conflicts with what they believe; i.e.

I don’t need new information on how to balance perfectly on my outside ski because I have been doing this for years and I don’t need to know anything more.

But the reality is, that with rare exception, while elite skiers and even World Cup racers may think they can balance on their outside ski they have no way of recognizing the correct feeling, let alone confirming that they are actually doing what they think they are doing.

I have designed and fabricated a device with which to train skiers/racers to create a platform under their outside ski on which to stand and balance perfectly on. The device can be used to capture what I call a skier’s personal Balance Signature using technologies like CARV. More on this in my next post.

  1. https://wp.me/p3vZhu-p
  2. https://wp.me/p3vZhu-UB


Comments made by followers of my blog suggest that significant confusion exists 0n the meaning of terms and representations of mechanics, biomechanics and physics used in typical explanations of ski technique and ski mechanics. In particular, there appears to be confusion between pressure and the representation of point forces.

Pressure is a physical force applied to an object that is distributed over the surface of the object.

Center of Pressure or COP is the point center of ground reaction force opposing a corresponding center of applied force acting on a object supported on the ground or a stable surface that acts in the capacity of ground in terms of providing a source of reaction force.

Torque or Moment of Force results from an offset between the centers of opposing physical forces acting on either side of an object.  This offset results in a torque or moment arm that tends ti create rotation about a center. When one force has a greater magnitude than the other force, rotation of the object will occur around the point of rotation.

Why typical balance explanations of skier balance are wrong

Balance in skiing is often depicted as a simple alignment of opposing point forces, usually a resultant force R acting in opposition to a snow reaction force S. The mechanics that make the edges of a ski grip are often shown as a simple alignment of opposing forces acting a single point on the edge. Explanations of this nature are physically impossible. What the authorities in skiing seem to conveniently be ignoring is the fact that pressure is applied by the snow along the entire running surface of the edge in contact with the the snow while an opposing area of pressure applied by the weight of the skier is acting on the body of the ski with an offset between the two centers of pressure. The authorities in skiing also seem to conveniently ignore what is arguably the key even in establishing a platform under the outside ski for the skier to stand and balance on, edge change.

Mikaela Shiffrin’s Get Over It drill on the Burke Mountain YouTube site makes a good segue to an explanation of the Mechanics of Edge change in the my next post – https://youtu.be/Bh7KF49GzOc

Bridget Currier is the model every skier should aspire to. She perfectly executes what I call the skimove. The skimove engages the external forces at ski-flat/edge-change to drive multi-plane torques acting about her outside ski into the turn while setting up a solid platform under her outside foot for her to stand on. Magnificent! This video should have at least a million views.

My comment from 2 years ago

Note carefully Currier’s stance in balance on her new outside ski, in particular, the angle of her torso with the snow. This is key to loading the ball of her outside foot.

Note carefully Shiffrin’s comment to move forward onto her new ski and how she used to think the movement was a lateral (sideways) move.

Most important of all – Patient Initiation. The reason? Shiffrin and Currier, don’t tip their outside ski on edge. They rock it on edge with a rocker impulse loading mechanism. The sequence is Rock, Roll n’ Rotate then Rotate the outside leg.


This is a revised version of a post I published on February 28, 2017.

It was my intent to discuss the key move in the First Step to Balance on the Outside Ski; Impulse Loading of the Forefoot. However, it has become apparent that it is necessary to preface this subject with a discussion on the source of ground in relation to the outside foot in order to impart an appreciation of why a mechanism is required to extend ground from the running edges of the ski in order to create a platform for a skier to stand and balance on when the outside ski is on its inside edge.

In typical discussions of ski technique and the mechanics, biomechanics and physics of skiing, the prevailing mental model assumes that a skier is in balance (see REVISION TO FEATURE POST: CLARIFICATION OF DEFINITION OF SKIER BALANCE) if they are able to stand upright and exercise a degree of control over their skis. In balance studies performed in gait labs, ground reaction force in the form of stable surface under the entire area of the foot or feet for subjects to stand balance on is assumed. There is no basis to assume this is the case when a ski is on edge because the source of ground is on the wrong side of the platform underfoot.

Mental Models

Mental models are a form of cognitive blindness. Once people assume they know something, they not only don’t question what they believe, they filter out information that conflicts with their mental model. And they typically fail to see the real issue even when it is in plain sight.

A man should look for what is, and not for what he thinks should be.

                                                                                                                 –  Albert Einstein

The Skier Balance Paradox

Even though I quickly became a competent skier soon after I started skiing,  I struggled to hold an edge on firm pistes and especially glare ice. It was disconcerting to see elite skiers hold an edge on ice with minimal effort while making controlled turns. When I sought the advice of the experts, they claimed that holding an edge on ice was matter of sharp edges and/or driving the knees into the hill. When I protested that after trying both and found it harder to hold an edge, the experts claimed that the ability of some skiers to do what I couldn’t was due to superior technique. They were just better skiers. No further explanation was needed.

The inability of experts to explain why a small number of skiers seemed able to balance on their outside ski and hold an edge even on ice provided me with the impetus to look critically at this issue with the objective of formulating an explanation based on principles of applied science.

The only plausible explanation for the ability of a skier to be able to stand and balance on their outside ski when it is on its inside edge is that some source ground (reaction force) must be present under ski that they are using to stand and balance on. Hence, the question, Where is the Ground?

On very hard pistes, ground as a source of reaction force, is limited to the running portion of the inside edge of the outside ski and the small portion of the base adjacent the edge with the edge and base supported on a small shelf cut into the surface of the snow/

In Figure 2.11 on page 26 of his book, Ultimate Skiing, LeMaster explains how the sidecut of a ski creates a smaller radius turn as the edge angle increases.

In Figure 2.12 on the following page, LeMaster shows misaligned applied (green arrow) and ground reaction (purple arrow) forces creating an unbalanced moment of force (yellow counter-clockwise rotation arrow) that  rotates the ski down hill (out of the turn). LeMaster goes on to state that as the skier edges the ski more, the ski bites better. But he fails to offer an explanation as to how the skier can edge a ski more against an unbalanced moment of force acting to reduce the edge angle.

The mechanism that generates a moment of force that opposes the moment force shown by LeMaster in Figure 2.12 and has the effect of extending ground (reaction force) acting along the running length of the edge of the ski  is the subject of this series of posts.

Edge Angle Sidecut FXs

A simple way to acquire an appreciation for the location of ground relative to the outside ski on edge is to make a simple model out of flexible piece of sheet plastic material a few mm thick.

The photo below shows a model I made from a piece of sheet plastic about 8 inches long. The upper portion of the plastic piece has a shorter sidecut with less depth than the sidecut in lower portion of the piece of plastic piece. Both the model and sketches that follow are for illustrative purposes to demonstrate the effects of sidecut geometry on edge angle and a source of ground. Although the basic principles are the same, it is not intended that they be viewed as an accurate representation of actual ski geometries  The symmetrical geometry is for the benefit of the simplifying what is already a complex issue.


There is a relationship between the depth and length of a sidecut in that the greater the ratio of the depth to the length of a sidecut, the lower will be the edge angle it forms with the surface in relation to the camber radius. In the sketch below, the upper rectangular ski shape will maintain a vertical relationship with a surface regardless of the camber radius.

There is also a relationship between the edge angle a ski with sidecut will form with a uniform surface and the radius of the camber with the edge angle formed with a uniform surface. The edge angle will increase (become more vertical) with a decrease in the radius of the camber. This explains why GS skis that are longer and have less sidecut depth than SL skis can attain much higher edge angles.


The photo below shows how the aspect of the model I made with the smallest sidecut ratio forms a steep angle with a uniform surface when bent to sufficient camber radius to allow the sidecut to become compliant with a uniform surface.


When viewed from the rear of the model, the location of ground in relation to the structure of a ski with sidecut and camber should become readily apparent.

The graphic below shows what a photo taken at a low enough vantage point to the snow would capture looking straight on at a ski carving a turn with its edge compliant with the surface of the snow. This may seem foreign, even extreme to some. But when the edge of a ski is compliant with a uniform surface, the curve of the sidecut becomes linear.

The left image below depicts the schematic model of the ski shown in the second graphic with the camber angle sufficient to make the edge in contact with the uniform surface compliant with it. The angled line represents the surface of the snow. The schematic model of the ski represents the proximate end profile associated with a high load GS turn. A photograph in Figure 1.18 on page 17 of the Skier’s Edge  shows a similar profile in Hermann Maier’s outside (left) ski which is at a very high edge angle.

The graphic on the right shows some penetration of the running surface of the edge of the ski in conjunction with the forces commonly shown in the prevailing mental model that are used to explain how forces acting on the outside are balanced.



The reality is of the applied forces acting on the ski are shown in the vertical profiles in the graphic below as captured by digitized force plate data. Once the foot is loaded on a surface there is what is called a Center of Pressure as shown by the peaks in all 3 graphs. But when the foot is in compliance with a uniform surface, some pressure is expressed by the entire contact surface of the foot. So, the point application of applied force in opposition to a point application of GRF as depicted in the right hand graphic above is a physical impossibility.

Screen Shot 2015-12-20 at 9.59.06 PM

Viewing a transverse vertical profile of a ski on edge from the perspective of ground as a source of GRF for a skier to stand and balance on puts the issue of skier balance in a whole new, albeit unfamiliar, perspective. But it is a reality that must be dealt with in order to engage in realistic narratives on the subject. Overly simplistic explanations of skier balance attributed to a basic alignment of opposing forces do not serve to advance the sport of skiing as a credible science.

I concur with LeMaster’s position that the platform angle a ski forms with resultant and GR forces must be at 90 degrees or slightly less in order for the edges to grip. In my next post, I’ll start to introduce mechanical principles that explain how this can be accomplished.

It is the ability of racers like Mikaela Shiffrin to stand and balance on their outside that enables them to consistently dominate World Cup competition.


In the next series of posts, I am going to focus on the single most important, but least understood, aspect of skiing; skier balance, in particular, the ability to balance perfectly on the outside ski. Given its univerally recognized importance in the ski culture, it is both perplexing and disconcerting that little attention appears to be given to the study and analysis of the mechanics associated with balance on the outside ski.

For decades, the worlds greatest skiers, including Patrick Russell and Marc Giardelli, have stressed the importance of standing on the downhill (outside) ski. Giardelli said that once you can balance perfectly on the outside ski, everything else follows. The ability to stand on the outside ski and balance perfectly on it, implies the same mechanics of balance we engage in when we balance perfectly on one leg when we take a step to move forward in locomotion. Balancing perfectly on one leg requires a stable surface under the entire plantar aspect of the foot to provide a source of GRF. The reason why the ankle-foot complex has a triplanar joint system is so the tripod-like structure of the foot can seek stable ground. This is the classic text book definition of one-footed or monopedal balance and the standard for studies on balance performed on one foot.

The problem is that there is no ground or any form of stable GRF under the outside foot of a turn when the ski is on its inside edge other than the GRF acting along the portion of the edge in contact with the snow surface and a small portion of the base of the ski adjacent the edge. If elite skiers such as Russell and Giardelli really can stand on their outside ski and balance perfectly on it the question is where is the source of GRF coming from that acts to support weight of the body expressed on the plantar foot?

By 1990, I had an explanation in a hypothesis I had articulated. According to my hypothesis, elite skiers extend GRF acting along the portion of the inside edge of their outside ski from the snow to the base of the ski by rotating their outside leg and foot into the turn. This action causes the base of the ski on the outboard side of the inside edge to pivot upward about the portion of inside edge underfoot with sufficient force to support the weight of the body. The Birdcage studies done in 1991 were designed to find out if my hypothesis were right.

Balance on the outside ski is a Two-Step process

Having seen great skiers like Nancy Greene Raine and Toni Sailor ski with ease on pistes that would be difficult, if not impossible, for most skiers to hold an edge on, I was convinced that some skiers really could balance perfectly on their outside ski when it was on its inside edge, the same way that every skier could easily balance on one ski when the base of the ski was fully supported on a firm, stable surface.

I set out to try and figure out how this was possible. It took me about 10 years between 1980 and 1990, to formulate a hypothesis that explained the mechanics. Once I had an explanation, I understood why no one else had been able to figure it out.

Balancing on the outside ski does not adhere to the text book descriptions of single leg balance where a stable source of GRF under the plantar foot is assumed. The ability to stand on the outside ski when on its inside edge and balance perfectly on it, is a Two-Step Process. The key is that the Second Step is dependent on the First Step.  The First Step makes the Second Step possible. Without getting the First Step right within a very short window of opportunity, the Second Step is not possible.

Since my hypothesis predicted that sequence and timing is the critical, it was quite simple to prove my hypothesis with strategically placed strain gauges mounted in the Birdcage on discrete force plates positioned opposite the predicted force transfer points of the foot. The critical nature of the sequence was easily confirmed by preventing the First Step from occurring.

In my next post, I will discuss the Two Steps of the balance process and provide examples using screen shots and video clips from recent World Cup races showing the sequence in a turn where racers such as Mikaela Shiffrin make the two steps to balance on the outside ski.




A follower of skimoves posed the following;

“I’m trying to determine my optimal boot shaft angle and ramp angle given my physiology – i.e. what works best for me. I’ve done some of this work on my own by adjusting binding ramp angle (last season). What is interesting is the shaft angle of my newer Head vs. Lange boots”.

As discussed in recent posts, the importance of the cumulative effect of boot board ramp (zeppa) and binding ramp (delta) angles on stance is becoming increasingly recognized. Although binding ramp angle (delta) typically varies widely from one binding to another in recreational bindings, boot board ramp angle seems to be coming into line with functional reality in race boots. Reliable sources in Europe tell me that the boot boot board ramp angle in World Cup boots is in the order of 2.6 degrees. After I eliminated the arch profile in boot boards for a 23.5 Head race boot, I calculated the ramp angle at 2.35 degrees, a far cry from the 5 degrees claimed for the boot boards. I calculated the boot board ramp angle of an Atomic race boot of a local ski pro at a little over 2 degrees. I have also been told that shim kits are available for all race bindings that allow the delta angle to be zeroed.

The default barefoot ramp angle for humans is zero. It has been unequivocally established that anything more than a small amount of ‘drop’ (heel higher than forefoot) in footwear will have a detrimental effect on stance, balance and movement patterns. This especially true for balance on one foot, something that is fundamental to sound ski technique.

Elevating the heel relative to the forefoot will cause the muscles in the back of the lower leg to contract. Over time, these muscles will become chronically shortened. The key muscles affected are the calf muscles; the gastrocnemius and soleus. But the small muscles that stabilize the knee and pelvis are also adversely affected, not a good thing.

If I want to find the optimal boot shaft angle and compare the shaft angle of two or more boots, I start by making the boot boards perfectly flat with the transverse aspect horizontal with the base of the ski. I set the boot board ramp angles for both boots at 2.5 or 2.6 degrees. Since it can take a long time for the body to adapt to even small changes in ramp angle underfoot, the angle is not hypercritical.   I have settled on 2.5 to 2.6 degrees of total ramp (zeppa + delta) as an arbitrary starting point. Although there appears to be a positive effect of a small delta binding angle in SL and GS, I prefer to work with a zero delta angle initially since a positive or negative delta affects the shaft angle of a ski boot.

When moving from one boot model to a different model or to another boot brand, the first thing I do is remove the boot boards and calculate the ramp angles with the top surface monplanar. If the boot boards are not flat, I plane or grind them flat. If a new boot is to be be compared to a current boot with a boot board angle of 2.5 to 2.6 degrees, I modify the boot board of the new boot so it has the same angle as the current boot.

Next, I compare the shells and the angles of the spine at the back of the shaft of each boot. Even if the angles of the spines of the boot shells appear similar, there is no guarantee that what I call the static preload shank angle (more on this in a future post) will be the same.

A quick check of how the structure of the shell of the new boot is affecting the functional configuration of the foot and leg compared to the current boot, is to put the current boot on one foot then put the new boot shell with the liner from the current boot on the other foot. If a significant difference is perceived, the source is the new shell.

At this point, it may be apparent that there is a difference in the shank angles of the left and right legs when comparing the current boot to the new boot. But whether one boot is better than the other or even if one boot eanables the optimal static preload shank angle would not be known. I will explain how I identify this angle in my next post. For now, study this recent video of Lindsey Vonn starting off by skiing in what appears to be a strange ski stance. In fact, the exercise Vonn is doing is a familiar routine to me, one that I do before I start skiing – https://www.facebook.com/LindseyVonnUSA/videos/10154672700589728/

Why is Vonn skiing this way? What is she trying to do?

Also, check out this screen shot of Anna Fenninger. Note her compact, forward in the hips stance.


Finally, watch this video in which Brandon Dyksterhouse compares Shiffrin and Fenninger – Shiffrin GS Analysis – https://youtu.be/phchHWwDhdY

What do Vonn and Fenninger have in common? Why?



The Balance Axis

The two University of Ottawa pressure studies that used elite ski instructors found that, without exception, every ski instructor started their turns with pressure under the head of the first metatarsal (ball of the foot) and that in some, but not all turns, the pressure moved back to the heel in the last part of the turn. What the studies did not make clear is that when the Center of Pressure (COP) is under either the heel or the head of the first metatarsal, there will always be a secondary center of pressure under some other aspect of the foot. When COP is under the heel, there will be a diffused, but weak, area of pressure in the forefoot across the heads of metatarsals 1-2-3. But when COP is under the head of the first metatarsal, there will always be a well-defined, localized secondary area of pressure under the heel. The relationship of the two COPs (heel or head of the first metatarsal), affects the relationship of the balance axis of the foot and the balance axis of the ski.

The ski has two balance axes. These are the theoretical points where a ski would balance on the head of a pin if the mass of the ski were perfectly distributed. The graphic below shows the two balance axes of a ski. A section below the top view of the ski shows the minimum profile width or waist of the ski in black. The limits of the sidecut are shown in grey. A triangle indicates the Balance Axis.

Balance Axis

(click on graphics to enlarge them)

Heel COP

When COP is under the heel, the load or pressure, will lie on the transverse Balance Axis of the ski where it will be distributed outward from its center as shown in the graphic below. Because of the weak, diffused secondary area of pressure across metatarsals 1-2-3, pressure under the heel will extend more forward, towards the shovel, than rearward, towards the tail. The load on the heel lies on the anatomical center axis of the foot that runs through center of the heel and the head of the second metatarsal. As with the Balance Axis of the ski, the anatomical center axis of the foot is the theoretical transverse balance point in bipedal stance. This is the reason why skate blades are mounted on this axis. LeMaster and others have used the skate example to explain why a ski is hard to hold on edge compared to an ice skate. But the balanced load across the transverse aspect of the ski will keep the ski level on flat terrain and resist forces that attempt to tip it onto one of its edges. Heel COP

What has not been recognised in skiing, is that the transverse balance axis of foot becomes re-aligned in monopedal stance so that it runs through the center of the heel and the center of the head of the first metatarsal as shown in the graphic below. By placing the highest load on the ski under the head of the first metatarsal, the re-alignment of the anatomic balance axis that occurs in monopedal stance, unbalances the load on the outside ski causing it to tip on edge. This unbalanced load causes the limits of the ski at the shovel and tail to displace away from the center of the load (COP) on the portion of the inside edge under the head of the first metatarsal. The diagonal load axis across the Balance Axis of the ski sets up components of force that are perpendicular and parallel to the Balance Axis of the ski.

Screen Shot 2016-01-23 at 1.46.42 PM

The re-alignment of the balance axis of the foot that occurs in monopedal stance can be clearly seen in images below of speed skaters and Mikaela Shiffrin balancing on a slack line.

Speed skaters


Shiffrin Slack line 1

The re-alignment of the anatomical balance axis of the foot is the end result of the transition to the inside ski while it is still on its current edge and the creation of pressure from extending the leg. This mechanism enables racers such as Shiffrin to Get Over it at ski flat and initiate edge and turning effects simply by balancing on the new outside ski and managing the heel/first metatarsal pressure differential.

The actual mechanics are more complicated than what I have described in this post. But as the saying goes, “One step at a time.”


The most important event in a turn is whole leg internal rotation (Event 7) following ski flat (Event 3). But the mechanism by which whole leg internal rotation is applied to the ski is as important, if not more important, than the actual whole leg rotation.

As the outside ski changes to its new inside edge, the racer rotates the whole leg internally using top down rotation from the pelvis. The purpose of ski flat at the conclusion of the transition (Event 1) phase, is to neutralize torsion across the pelvis so it is square to the trajectory of the racer. In order to use whole leg internal rotation, the COM of the racer must be positioned on the new outside foot at ski flat in what I call monopedal stance. Monopedal stance (aka monopedal function) is a physiologic state wherein balance is achieved with the weight of the body borne on the medial plantar aspect of a fully pronated foot.

The graphic below is Figure 23 from my US Patent No. 5,265,350.

Bi-MonopedalFigure 23 A depicts bipedal stance. The points of the central load-bearing axis are stacked vertically over top of each other in the frontal plane. The load W from COM is centred between the feet with each foot carrying half the load (W2).

Figure 23 A depicts monopedal stance. In monopedal stance, the load W from COM is aligned over the proximate centre of the head of the first metatarsal in both the frontal plane (across the racer) and saggital plane (front to back). Monopedal stance at ski flat is eloquently demonstrated by Bridget Currier  in the Burke Mountain Academy YouTube video, Get Over It with commentary by Mikaela Shiffrin – (http://youtu.be/Bh7KF49GzOc).

The opening graphics advise the racer to Get Over It and Stay Over It, meaning maintain the alignment of W from COM, over the proximate centre of the head of the first metatarsal in the frontal and saggital planes throughout the entire turn. But few racers can Get Over It, let alone Stay Over It, because the structures of their ski boots prevent them from assuming monopedal stance. This is especially true of racers whose boots are closely formed to the shape of their foot and leg in what amounts to perfect envelopment.

The graphic below is a re-creation of the stick person in Figure 23 above. The notations have been revised to reflect the terminology used in blog posts. The left stick person is depicted in bipedal stance. The centre stick person is depicted in monopedal stance. The right stick person is depicted in fixed neutral stance. When the foot is fixed in neutral, pronation is not possible and the foot is prevented from everting (the sole turns outward). In order for W emanating from COM to be positioned over the proximate centre of the head of the first metatarsal, the foot must evert approximately 7 to 8 degrees as depicted in the centre stick person.

The graphic below shows the effect of fixing the foot in neutral. When a racer attempts to balance on the new outside limb at ski flat, the inability to align W with GRF at the inside edge of the outside ski will cause the racer to fall into the new turn or consciously move away from the outside ski. .

Falls into turnPreventing the foot from pronating within a ski boot causes other problems. When the leg is rotated internally relative to the foot by the hip rotators, a torsional load is applied to the foot. Conventional ski boots do not provide surfaces for the foot to transfer biomechanically generated forces such as torque to. In addition, the structures of conventional ski boots present sources of resistance which interfere with the movements necessary to establish force transfer connections of discrete aspects of the foot with the boot shell.

Figures 22 A through 22 D below are from US350. Figures 22 A and 22 B depict the architecture of a foot in bipedal stance. Figures 22 C and 22 D depict the architecture of a foot in monopedal stance. Changes in the length of the foot in bipedal and monopedal stances are annotated as  L1 (bipedal) and L2 (monopedal). Changes in the angle of dorsiflexion of the ankle joint in bipedal and monopedal stances are annotated as  A1 (bipedal) and A2 (monopedal). Changes in the height of the arch in bipedal and monopedal stances are annotated as H1 (bipedal) and H2 (monopedal). Internal rotation of the leg in monopedal stance is annotated at 6. Changes in the length of the foot in bipedal and monopedal stances are annotated as  L1 (bipedal) and L2 (monopedal).  Changes in the position of the head of the first metatarsal in bipedal and monopedal stances are annotated as  2. Changes in the position of the medial tarsal bone in bipedal and monopedal stances are annotated as  3. Changes in the width across the heads of the metatarsals in bipedal and monopedal stances are annotated as  4. Shear forces, which will be the subject of a future post, are shown in Figure 22 D.

Screen Shot 2015-01-08 at 2.12.34 PMIn order to apply top down internal rotation, the racer has to move the load W to the ball of the foot as shown in the graphic below.

IdealThe short video clip below shows how the foot must be able to pronate within the confines of the ski boot without interference in order to set up the force couple required to transfer whole leg internal rotation to the new outside ski. The typical most significant source of interference is the structures of the ski boot in front of the ankle joint on the inner aspect of the boot.


The red bars in the BIPEDAL foot define common sources of interference created by structures of the ski boot that prevent the foot from pronating and establishing force transfer connections with the shell as shown in the MONOPEDAL foot. While the connection of the two transfer points suggests that the centre of rotation lies within the confines of the foot its true centre is not intuitive. This will be the subject of the next post.


Normal medial STJ movement of the talus is followed by a mandatory normal 1:1 coupling of the tibia to encourage normal internal leg rotation and normal dorsiflexion of the ankle. This normal coupling mechanism produces a synergistic postural response enhancing internal rotation of the entire leg.  Pelvic counter ensures hip capsule tightening which stabilizes the hip joint during the turn.

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Dr.Kim Hewson is an Orthopaedic Surgeon and former Director of Orthopaedic Sports Medicine  at the University of Arizona.  He is currently a veteran Telluride Ski School Alpine Instructor and Staff Trainer in the Biomechanics of Alpine Skiing.