Centre of Pressure


July 1991: Birdcage Research Vehicle – Cost approximately $140,000

Secret  Toshiba Prototype Portable Computer used for Birdcage studies – Value? Priceless!

Birdcage Co-Designer and Team Science Leader, Alex Sochaniwskyj, P. Eng.

After interviewing a number of candidates in the spring of 1991 for the science component of the MACPOD project to develop a ski boot based on anatomical principles, I chose Alex Sochaniwskyj, P. Eng. as the most qualified candidate and one of the most intelligent and creative persons I have ever had the privilege of meeting.

Alex provided the CV that follows in his letter in support of my nomination for the Gold Medal in the categories of Applied Science and Engineering in the 1995 British Columbia Science & Engineering Awards.

Alex Sochaniwskyj, P. Eng.

Alex is a 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 Physiology 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 numerous publications in refereed medical and engineering journals and has produced several video productions regarding biomedical and rehabilitation engineering.

              – March 24, 1995

Team Birdcage

2000 – Novel Pedar In-Shoe pressure technology used by Synergy Sports Performance Consultants – Cost, approximately $60,000 with 2 Sony VAIO laptop computers


2017 – CARV: Cost? Approximately $300 US – See footnote re special price


Birdcage to CARV: “Where have you been? I’ve been waiting 26 years for you. Welcome! The future of skiing has arrived.”

CARV is currently taking pre-orders at $249 at 


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 studies of balance performed in gait labs, ground reaction force in the form of stable surface for subjects to balance on is assumed.

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.


I started this blog with the objective of stimulating thinking and discussion among serious skiers, racers, ski pros and coaches about the mechanics, biomechanics and physics of skiing. Ingrained and accepted beliefs in skiing, that have little or no basis in science, have created resistance to ideas that question these beliefs .

The response to a post serves as a barometer of the type of audience my blog is connecting with. Since my post on excessive ramp angle as a stance killer stimulated interest in my post on the meaning of high COP pressures reported in University of Ottawa studies, I have decided to delay my post on my vision of a standard specification for boot boards and instead provide links to two papers below that are related to the Ottawa studies.

In the first paper (1), the authors correctly identify the moment of force created on the outside foot and ski of a turn by an offset in the alignment of the opposing force applied to the ski by the weight of a skier Fr with the ground reaction force F Ground as shown in Figure 2 below from the paper. The resulting drift angle creates medial compression of the associated knee joint.


In the second paper (2), the authors also correctly identify the moment of force created on the outside foot and ski of a turn by an offset in the alignment of the force applied by the weight of the skier with the ground reaction force, GRF as shown in Figure 1 below from the paper. Sketches a) and b) show the effect of greater width of the ski underfoot in increasing the length of the moment arm.

Waist 2

The authors of both of the papers recognize the problem; an offset in the alignment of opposing applied and ground reaction forces sets up a moment arm that causes the outside ski to rotate out of the turn. Each of the papers attempts to resolve the problem in a different manner.

Do either of the approaches presented in the 2  papers cited above address the problem?

…… to be continued.

1. An innovative ski-boot: Design, numerical simulations and testing



2. The Waist Width of Skis Influences the Kinematics of the Knee Joint in Alpine Skiing 



The science of the study of human balance is well established. Studies of balance use two key metrics; COM (Centre of Mass) and COP (Centre of Pressure). The following text is excerpted from Human balance and posture control during standing and walking – D A Winter PhD, P. Eng. – Gait & Posture: 1995; Vol 3: 193-214, December. (1)

Centre of Pressure (COP) is the point location of the vertical ground reaction force vector. It presents a weighted average of all pressures over the surface of the foot that is in contact with the ground. It is totally independent of COM. If one foot is on the ground, the net COP lies within that foot. If both feet are in contact with the ground, net COP lies somewhere between the two feet depending on the relative weight taken by each foot.

The location of COP under each foot is a direct reflection of the neural control of the ankle muscles (my emphasis added).

Increasing plantarflexion activity moves COP posteriorly (ergo, toward the back of the foot). Increasing inverter activity moves COP laterally (ergo, towards the outside of the foot). COP is often mistakenly equated with COG (Centre of Gravity). COP is calculated with software from pressure data obtained from a force plate or in-shoe pressure insole. (my emphasis added)

Because it is calculated COP can reside in the arch of the foot even though it may not be in contact with the ground.  – my comment

“Centre of Mass (COM) is a point equivalent of the total body mass in the global reference system (GRS). It is the weighted average of the COM of each body segment in 3-dimensional space. It is a passive variable controlled by the balance control system. The vertical projection of COM onto the ground is often called the Centre of Gravity (COG).

“Balance is a generic term describing the dynamics of body posture to prevent falling. It is related to the inertial forces acting on the body and the inertial characteristics of body segments.  The CNS is totally aware of the problems of controlling a multisegment system and interlimb coupling that can facilitate balance control.

“In the literature there is a major misuse of the COP when it is referred to as ‘sway’, thereby inferring that it is the same as the COG. Unfortunately some researchers even refer to the COP directly as the COG.”

In the mechanism of balance control, COP is the equivalent of the Balance Police. It keeps COM from breaching the limits of there base of support by outpacing COM in the race to the limits of the base of support within the foot or feet. In quiet standing, the force of gravity disturbs equilibrium by pulling COM forward. This causes the ankle to dorsiflex. As COM moves forward, it starts to overtake COP. In order to prevent a forward fall, the CNS signals muscles that plantarflex the ankle to increase their contraction. This increases the force of COP and pushes COM rearward. As COP shifts rearward, the CNS reduces the contractive force of plantarflexion so that COP passes COM in the race to the rear of the foot.

A similar process is employed by the CNS to prevent a sideways fall. Here, the force of gravity disturbs equilibrium towards inner or medial aspect of the foot. This causes the foot to pronate. To oppose the disturbing force, the CNS signals muscles to contract that invert the foot.

It is important to recognize that it is the external forces that disturb equilibrium  that cause the foot to pronate.

The same process is at play in skiing. However, since the sideways balance strategy involves inverter muscles, it is only possible to establish a balance platform (DOT 4: PLATFORM) on the outside foot of a turn and only then under specific conditions. In the skier/ski equipment system, COP is the point where the Resultant Force acting on a skier at ski flat that pulls COM downward towards the snow is opposed by muscles that the CNS recruits to oppose the pending collapse of the skeletal system and prevent a fall.

COP is calculated from pressure data obtained from a force plate or in-shoe pressure insole such as  the Novel Pedar system or Tekscan. Since COP reflects neural control of ankle muscles when a foot (the whole foot) is in contact with the ground or a stable source of (ground) reaction force, the use of the term COP is not technically correct in a situation where a ski is on edge unless a platform exists as described in DOT 4: PLATFORM. Until the ski lies flat on the snow between edge changes and there is full foot contact ground reaction force the appropriate term to describe the force applied by the foot to snow through the stack of ski equipment is centre of force or COF.

In a turn, COP is a good COP when it is on the right side of the law: ergo, when COP lies under the head of the 1st metatarsal and R is aligned between the inside edge underfoot and the limits of sidecut. The sketches below show the progression of COF at ski flat that moves COP to the head of the 1st metatarsal. If COP arrives at the head of the 1st metatarsal before the outside ski has attained a significant edge angle and COP remains in this position through the turn COP is a good COP.

Sketch 1 below shows the 2 key mechanical points in skiing (red cross)

Centres of key pts

Sketch 2 below shows the Centre of Force (COF) under the heel of the inside foot at the start of the transition between turns. The red dashed line shows the approximate trajectory of COF as it moves forward and becomes COP at ski (foot) flat between turns as the external forces cause the foot to pronate.



Sketch 3 below shows the forward progression of COP towards the head of the 1st metatarsal.


Sketch 4 below shows the successful transition of COP to the head of the 1st metatarsal where it lies over top of the inside edge of a ski of appropriate width.



Sketch 5 below shows axis on which COP and R must align in order to engage the external force R to drive edging and turning mechanics.


Sketch 6 below shows R on the same axis as COP.  In this configuration the alignment of R described under DOT 4: PLATFORM will enable multiplane torques generated by pronation to be directed into the turn.


In sketch 7 below COP has failed to make a transition to the head of the 1st metatarsal. When COP fails to make the transition to the head of the 1st metatarsal at ski flat between edge change before the new outside ski attains a significant edge angle, a moment arm will be setup between the inside edge and COP that will create an inversion moment of force or torque with an associated external vertical axial rotation of the whole leg.



In sketch 8 below COP has reversed direction. Once an inversion moment arm has been set up on the outside ski there is no way to undo it. The odds are great that COP will revert to its default position under the heel because it is under the mechanical line of the lower limb.


When this happens COP becomes a bad COP.

1. You can obtain a copy of David Winter’s paper at the following link:


It is hard to find a movement sequence  in video footage that shows what I call the Ski Move from the optimal angle. In order to clearly see how a skier’s centre of mass rotates about the inside (uphill) edge of the inside ski and changes its position in relation to the inside ski when Shiffrin or Ligety start to step on it, the camera has to be looking at the racer head on. Since this sequence is almost impossible to find, I am going to try and create it in Poser.

The marked up sequence in the photo below shows how Shiffrin’s center of mass starts to rotate about the inside edge of her inside ski and into her new turn as she progressively steps on her new ski during the transition phase. As she does this, a point will be reached where her inside ski flattens on the snow. But it does not stop rotating at this point. Shiffrin’s momentum and the pressure she is applying to the ski cause it to change edges and begin to rotate into the new turn. As this is happening, Shiffrin is extending and moving forward in the hips. This drives the centre of pressure to the ball of her foot. The foot pronates and reinforces the rotation of the ski into the new turn. The circled images show where this happens. As Shiffrin’s foot pronates and the forces she is applying rotate her left ski into the new turn, she applies another layer of in-phase rotation with her hip rotators. These synchronized actions produce an over-centre mechanism that rotates her ski in multiple planes into the new turn.

There is a common perception that racers let their outside ski create all the turning effects. But the pivoting Shiffrin and Ligety apply at this key moment does much more than simply rotate the ski into the turn. It creates powerful forces that engage the external forces to drive the outside ski into the turn while creating a platform to balance on.




The controversy that surfaced in 2011 over the FIS decision to increase turn radius on GS skis revealed a lot about what the various authorities in skiing knew and, especially, what they didn’t know, about the mechanics, biomechanics and physics of skiing. Some critics of the ruling took the position that the reduced sidecuts would actually increase the risk of injury. An article in Ski Racing called, Black Diamond: The Deaf Ears Of The FIS, reviewed the various positions on the matter. And while some critics of the FIS ruling had very strong opinions, no one seemed able to put forth a position based on sound principles of science. In what had to be the height of irony, Guenter Hujara, director of the men’s World Cup was reported to have said, The facts are the facts. If you want safety this is a step you have to take.

Since 1977, I have been stressing the importance of the feet in skiing as the transmission path for forces transferred from the skier’s centre of mass to the snow. Knowledge of the forces acting between the soles of the feet and the snow surface is the arbiter of knowledge as a whole in skiing.  At last, a World Cup official was finally talking about taking a step. But my elation was short-lived. Hujara was talking about new regulations for GS skis, not my long hoped for new regulations for ski boots.

Two statements pertaining to injury mechanisms and ski safety were telling; Scientists at the University of Salzburg determined through a subjective study of 63 experts that the main risk factor was the “system ski, binding, plate, boot,” and By their own (FIS) admission, boots are too complex, and plates are, too. I say, ‘wait a moment’. The common denominator in the ski system/skier interface with the potential to cause injury, especially knee injury, is moments of force (torques). To be more specific, an unbalanced inversion moment of force present across the inside edge of the outside ski and the associated joints of the ankle-complex. By association, an unbalanced external (out of the turn) vertical axial moment of force acting on the tibia that tends to rotate it out of the turn against a well-stabilized femur or, worse, a femur that is being rotated into the turn by the powerful hip rotators. Between the tibia and femur lies the knee; a fragile joint with only ligaments holding the two bones in proximity to each other.

Mechanisms of Anterior Cruciate Ligament Injury in World Cup Alpine Skiing  (The American Journal of Sports Medicine, Vol. XX, No. X DOI: 10.1177/0363546511405147), states, under Background,

There is limited insight into the mechanisms of anterior cruciate ligament injuries in alpine skiing, particularly among professional ski racers.

My US Patent No.  5,459,949 published on or about November 29, 1994, goes into great detail about the importance of positioning the foot within in the ski boot and especially positioning the ball of the foot in relation to the inside edge of the outside ski of a turn so as to facilitate the setting up of moments of force (torques) into the turn with which to oppose externally generated torques out of the turn and the avoidance of mechanical relationships that result in unbalanced torques, It can be debated whether the presence of an unbalanced external vertical axial moment of force causes or contributes to an injury. But there is no debate that an unbalanced external vertical axial moment of force is a predisposing factor to injury.


Here are some excerpts from the subject patent that discuss moments of force acting about the inside edge of the ski with my notes and emphasis (bold) added. Due to the relatively short moment arm, aligning applied and ground (snow) reaction forces in opposition to each other or even creating an alignment where the applied force is on the inside turn aspect of the inside edge of the outside ski is not, in itself, sufficient to engage the external forces that drive a ski into a turn. It is merely a prerequisite. The factors that multiply moments of force once an over-centre mechanism is initiated are much complex than a simple misalignment of opposing applied and snow reaction forces.


While the adjustment of medial forefoot counter 2201 enables the foot 2001 of the user to be correctly aligned on rigid base 2100 yet another problem has arisen. The alignment of the head of the first metatarsal of the foot 2001 of the user has been altered in relation to the appliance affixed to the sole of the footwear, in this instance, a snow ski, in comparison with the alignment of the appliance in relation to the head of the first metatarsal as shown in FIG. 63.

Alignment of the center of the head of the first metatarsal is an important factor influencing physiological mechanisms which balance pronation/supination moments acting transversely across inside edge of appliances such as snow skis. The contact point of such an appliance with the surface on which it is acting can act as a fulcrum and, in so acting, establish a moment arm pivot in situations where the ground reaction force and the force applied by the user are not acting linearly in opposition to each other. In monopedal stance (pronated) the weight of the body acts substantially through the center of the head of the first metatarsal.

It is important, in activities such as snow skiing, that means be provided to allow the center of the head of the first metatarsal to be positioned so that the force applied by the user can be aligned in opposition to the ground reaction force when the snow ski is placed on its inside edge. If opposing ground reaction and applied forces can not be aligned, a moment arm will be created with the effect that the force applied by the user will tend to rotate the foot in the direction of either supination or pronation.

The location of the inside edge of (the outside ski) a snow ski tends to favour a supination moment arm since the ski edge generally lies medial of the center of the head of the second metatarsal. If the force applied by the user is sufficient in the presence of a moment arm to rotate the foot in the direction of either supination or pronation, the long axis of the tibia will also be caused to rotate through an intrinsic mechanism within the tarsus of the foot.

The means to adjust the transverse position of the foot in relation to the inside edge of a snow ski while maintaining the means to independently adjust the position of the foot on the longitudinal axis of the sole of the footwear is important and advantageous to the user and is thus an object of the present invention. FIG. 70 shows substantially the same view as FIG. 69 except that the ground reaction force FR and the force applied by the user F are shown substantially as they would be when the user is in monopedal stance (pronated) with the foot correctly positioned in relation to the inside edge of a snow ski affixed to sole 2101.

FIG. 71 shows substantially the same view as FIG. 70 except that the snow ski shown affixed to sole 2101 is wider on its medial aspect in comparison to the snow ski affixed to sole 2101 as shown in FIG. 70. The position of the inside edge of the snow ski in relation to force F applied by the user is such that the ground reaction force FR and the force F applied by the user are not acting linearly in opposition to each other. The transverse offset between the ground reaction force FR and the force F applied by the user creates a moment arm MA which acts lateral of the ski edge with the result that force F applied by the user acting on the moment arm MA will tend to rotate the foot in the direction of supination when the ski is placed on its inside edge.

Fig 70-72

FIG. 72 shows substantially the same view as FIG. 71 except that sole 2101 has been shifted laterally in relation to rigid base 2100 so that the ground reaction force FR and the force F applied by the user are now acting linearly in opposition to each other with the result that the moment arm MA as shown in FIG. 71 facilitates a countering muscularly generated torque from internal rotation of the leg at the pelvis.

The link to US Patent No.  5,459,949 is:

http://patft.uspto.gov/netacgi/nph-Parser Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=5,459,949.PN.&OS=PN/5,459,949&RS=PN/5,459,949


In order to be able to develop a dynamically stable base of support on the outside ski of a turn, one that supports the processes that use external forces to drive the ski into the turn, the ski must have a waist that is close to the centre-to-centre dimension ‘X’ between the balls of the great and 2nd toes. In addition, the anatomical centreline of skier’s foot must be aligned with the running centre of the ski. The graphic below shows what it looks like when the foot is correctly positioned on a ski with the appropriate width underfoot. In reality, there is a stack of equipment between the sole of the skier’s foot and the snow surface. F.I.S. rules allow up to 100 mm of stack height between the sole of a racer’s foot and the snow surface. So the graphic below does not reflect reality. The reason I am starting with the skier’s foot directly on the ski top plate is to demonstrate that an applied vertical force acting against an opposing snow reaction force (SRF) is insufficient to explain the edging mechanics that skiers like Ligety and Shiffrin are able to develop. There are other factors at play that I will introduce in future posts.

L foot on ski

US Patent No 5,265,350 – MacPhail: November 30, 1993 – “The prior art refers to the importance of a “neutral sub-talar joint”. The sub-talar joint is a joint with rotational capability which underlies and supports the ankle joint. The sub-talar joint is substantially “neutral” in bipedal function. That is to say that the foot is neither rolled inward or rolled outward. If the foot can be substantially maintained in a neutral position with the arch supported and with a broad area of the inner aspect of the foot well padded, there will exist a good degree of comfort. Such a state of comfort exists because the foot is not able to roll inward (pronate) to a degree where significant mechanical forces can be set up which would allow it to bear against the inner surface of the boot shell. In effect, this means that initiation of the transition from a state of bipedal to a state of monopedal function, is prevented. This transition would normally be precipitated by an attempt to balance on one foot. If the foot is contained in a neutral position, traditional supportive footbeds (arch supports) are quite compatible with the mechanisms and philosophies of the prior art.”

Here is what the inside and outside feet of a skier in a turn look like when  the feet are in neutral.

Neut edge

Since there are offsets or moment arm between CoP and SRF on each foot, the sole of the inside foot of the turn will tend to roll away from the centreline between the feet (ergo, it will tend to evert) while the sole of the outside foot of the turn will tend to roll towards the centreline between the feet (ergo, it will tend to invert).  Under specific conditions the external forces acting on the skier will tend to make the outside foot of the turn rotate into the turn (ergo, it will tend to evert). But, for reasons that will be provided in a future post it is not possible to create conditions under which the external forces acting on the skier will tend to make the inside foot of the turn rotate into the turn. For this reason the force applied to the snow by the skier must be directed to the inside edge of the outside ski of the turn. The inside foot and leg are used to help direct the force to the outside ski. This what Ligety and Shiffrin do so well.