biomechanics

THE HIRSCHER/SHIFFRIN HAIRPIN TURN EXPLAINED

The topics of interest in recent views of my blog combined with comments on online forums on ski technique where nebulous terms such as pressure and tipping are an integral part of the narrative, have highlighted the need for a uniform frame of reference as a basis for meaningful discussions of ski technique as well as for the analysis and accurate identification of factors that explain the superior technique of racers like Marcel Hirscher and Mikaela Shiffrin. Simply trying to emulate the moves of the great skiers without re-creating the equipment factors that enable superior performance is not a productive exercise.

I touched on some of the factors that enable Marcel Hirscher and Mikaela Shiffrin to dominate their competition in my posts WHY SHIFFRIN AND HIRSCHER ARE DOMINATING (1.) and WHY HIRSCHER AND SHIFFRIN CAN CROSS THE LINE (2.). Over the coming weeks, I will post on the factors that I believe explain the ability of Hirscher and Shiffrin to make rapid, abbreviated hairpin turns even on the steep pitches of a course using what I call the problem-solving matrix jigsaw puzzle format. In contrast to the linear step-by-step progression problem-solving format, the matrix jigsaw puzzle format lays out information relevant to a situation in a grid format much like a jigsaw puzzle.  Known factors are assembled where there is a fit with the interfaces and arranged in relation to other components until a solution begins to emerge much like a coherent picture begins to emerge in a jigsaw puzzle as the pieces are correctly assembled. As the picture becomes more clear, tentative connections between the known segments are hypothesized to try and extrapolate the big picture. As the process progresses, less certain or flawed information is discarded and replaced with more certain information

A lot of critical information on the neurobiomechanics and even the mechanics and physics of skiing is either missing, misapplied or misunderstood in the narrative of ski equipment and technique.

Biomechanics of Sports Shoes

A valuable reference on neurobiomechanics and the future of sports shoes is the technical text, Biomechanics of Sports Shoes by Benno M. Nigg. Used in conjunction with the chapter on the Ski Boot in the medical text, The Shoe in Sport, valuable insights can be gleaned on the mechanics, neurobiomechanics and physics of skiing.

Nigg’s book can be ordered at NiggShoeBook@kin.ucalgary.ca. The following chapters in particular contain information relevant to skiing:

3. Functional Biomechanics of the Lower Extremities (pp 79 to 123) – contains essential information on the human ankle joint complex, tibial rotation movement coupling and foot torsion.

4. Sensory System of the Lower Extremities (pp 243 to 253) – contains essential information on the sensory system responsible for balance and precise movement, both of which are key to effective skiing.

In order to advance skiing as a science, a mutual objective must be getting the right answer as opposed to a need to be right.

The wisdom of Albert Einstein is appropriate.

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

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

We cannot solve our problems with the same thinking we used when we created them.

If you can’t explain it simply, you don’t understand it well enough.

In my next post, I will start laying out the functional principles that I currently believe explain the factors that enable the superior performance of racers like Marcel Hirscher and Mikaela Shiffrin and their ability to rapidly redirect their line and maximally accelerate by making rapid, abbreviated, hairpin turns.


  1. https://wp.me/p3vZhu-2q3
  2. https://wp.me/p3vZhu-2qo

TIGHTLY FIT SKI BOOTS COMPROMISE SKIER BALANCE AND CONTROL

In reviewing recent articles on ski boot fitting I encountered the same perfect fit of the boot with the shape of the foot and leg and ski boots must be tightly buckled for good balance and control narrative fabricated decades ago to justify the interference with the actions of the joints of the ankle and leg created by the rigid plastic shell ski boot.

When the first rigid shell plastic ski boots were introduced, the field of biomechanics, as it exists today, was in its infancy. Even until recently, the human foot was modelled as a rigid block which was consistent with the shoe last theory and the theory that the perfect fit of ski boots with the foot and leg of the user is the best option for skiing. Further support for the support and immobilize theory came from the vilification of pronation arising out of the misapplication of Root’s Neutral theory (1.)

By the time the authoritative medical text, The Shoe in Sport, was published in 1987, the knowledge of the biomechanics of the human foot had progressed to the point where tight-fitting ski boots and loading the ankle joint were recognized as unphysiologic.

Few forms of athletics place as high demands on the footwear used in their performance as alpine skiing. It (the ski boot) functions as a connecting link between the binding and the body and performs a series of difficult complex tasks. (2.)

Investigations by Pfeiffer have shown that the foot maintains some spontaneous mobility in the ski boot. Thus the total immobilization by foam injection or compression by tight buckles are unphysiologic.(2.)

Many alpine skiers have insufficient mobility in their knees and ankle. The range of motion, particularly in the ankles, is much too small.(2.)

From a technical (skiing) point of view, the ski boot must represent an interface between the human body and the ski. This implies first of all an exchange of steering function, i.e., the skier must be able to steer as well as possible, but must also have a direct (neural) feedback from the ski and from the ground (snow). In this way, the skier can adapt to the requirements of the skiing surface and snow conditions. These conditions can be met if the height, stiffness, angle and functions (rotational axes, ankle joint (AJ)/shaft) of the shaft are adapted, as well as possible to the individual skier. (3.)

The articles on ski boots in the Shoe in Sport identified the objectives I was seeking in my efforts to design a ski boot based on principles of what is now referred to as neurobiomechanics. By the time I had formulated my hypothetical model of the mechanics, biomechanics and physics of skiing in 1991 I understood the need to restrain the foot in contact with the base of a ski boot and maintain the position of the foot’s key mechanical points in relation to the ski while accommodating the aspects of neurobiomechanical function of the foot and leg required for skiing. This was the underlying theme of the US patent that I wrote in February of 1992.

Existing footwear does not provide for the dynamic nature of the architecture of the foot by providing a fit system with dynamic and predictable qualities to substantially match those of the foot and lower leg. – US patent No. 5,265,350: MacPhail

On June 2, 2013 I published the post TIGHT FEET, LOOSE BOOTS – LOOSE FEET, TIGHT BOOTS (4.) in which I describe how attempts to secure the foot to a ski in a manner that interferes with the physiologic mechanisms that fascially tension and stiffen the structures of the foot that render it dynamically rigid actually reduce the integrity of the joint system of the lower limbs and hips resulting in a looser connection with the ski.

Studies done in recent years confirm the role of the active state of the architecture and physiology of the foot to postural control and balance.

These findings show that rather than serving as a rigid base of support, the foot is compliant, in an active state, and sensitive to minute deformations. In conclusion, the architecture and physiology of the foot appear to contribute to the task of bipedal postural control with great sensitivity. (5.)

The science of neurobiomechanics and the understanding of the mechanisms of balance and the role of the sensory system in human movement is accelerating. The time is long overdue for skiing to abandon it’s outdated concepts and align it’s thinking with the current state of knowledge.


  1. IS ‘SUBTALAR NEUTRAL’ SKIINGS’ HOUSE OF CARDS? – https://wp.me/p3vZhu-2mn
  2. Ski-Specific Injuries and Overload Problems – Orthopedic Design of the Ski Boot –  Dr. med. H.W. Bar, Orthopedics-Sportsmedicine, member of GOTS, Murnau, West Germany
  3. Biomechanical Considerations of the Ski Boot (Alpine) – Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland
  4. https://wp.me/p3vZhu-2K
  5. Foot anatomy specialization for postural sensation and control

WHAT’S YOUR PQ? [PERFORMANCE QUOTIENT]

After my disastrous experience in 1977 with the mythical Perfect Fit with Crazy Canuck, Dave Murray (.1); one that transformed Mur from a World Cup racer to a struggling beginner, my work on ski boots became focussed on removing instead of adding material and making room to allow a skier’s foot to assume its natural configuration in the shell of the ski boot. As I improved the accommodation of a skiers’ neurobiomechanical functional requirements in the ski boot, skier performance improved in lockstep. I was merely reducing the structures of the boot that interfered with performance to enable a skier/racer to use the performance they already had.

Fit: The Antithesis of Human Function

Fit, by it’s definition of joining or causing to join together two or more elements so as to form a whole, is the antithesis (def: the direct opposite) of enabling the function of the human foot and lower limbs as one of the most dynamic organs in the human body. Fitting a ski boot to the foot and leg of a skier, especially a racer, equates with imposing a disability on them (2.). Although I didn’t realize it until I read The Shoe in Sport and learned of the barefoot studies done at the Human Performance Laboratory at the University of Calgary, my work on ski boots had transitioned from Fitting (disabling), by adding materials to liners to fill voids between the foot and leg and shell wall, to UnFitting (abling) by removing materials from liners and expanding and grinding boot shells so as to accommodate the neurobiomechanical functional requirements of the foot and leg of a skier.

But the big breakthrough for me came when Steve Podborksi won the 1981-81 World Cup Downhill title using the dorsal constraint system (Dorthotic) I developed and later patented. The Lange boot shells the device was used in had the least constraint of any ski boot I had ever worked with. The instantaneous quantum leap in Steve’s performance compared to the same shell using a conventional liner raised the question of how could a skier’s maximum performance be achieved and was there a way to compare to a skier’s performance in different ski boot/liner configurations to an optimal reference standard?

A reliable indicator that my un-fitting was trending in the right direction was that skiers consistently found that skiing became easier. For racers, coaches would typically report that the racer was skiing better. Improved race results served as further confirmation of my efforts. But these indicators were subjective. I wanted a way to not just measure performance with quantifiable metrics generated from data specific to the activity, I wanted to be able to compare the same metrics to a reference or baseline standard that represented the optimal performance of a skier or racer at a given moment in time. Without a way to measure and compare performance there is no way of knowing how a ski boot is affecting a skier or racer and especially no way of knowing how close they are to skiing at their maximum level of performance. I wanted to develop a skier Performance Quotient or PQ.

Definition of Quotient

  • Mathematics: – a result obtained by dividing one quantity by another.
  • a degree or amount of a specified quality or characteristic.

A skier Performance Quotient would capture baseline metrics from a skier’s performance in a ski boot that provides the optimal functional environment for the foot and lower limbs to the skier’s peformance in different ski boots including a skier’s current ski boot. The ski boot that provides the optimal functional environment for the foot and lower limbs would be designated as 100%. If the same metrics captured in a different ski boot were 78% of the reference standard, the skier’s PQ in the ski boot would represent a PQ of 78% against a possible 100% or 78/100.

Raising the bar of skier/racer function with body work and/or conditioning will raise the PQ. But it cannot close the PQ gap created by the performance limitations of the interference with neurobiomechanical function caused by their ski boot. Nor can trying harder or training more intensely overcome the limitations of a ski boot. Assuming 2 ski racers of equal athletic ability and mental strength, the racer with the ski boot that enables a higher PQ will dominate in competition. The only way to improve a skier’s PQ when it is less than 100% is to improve the functional environment of the ski boot.

In current ski boot design process, manufacturing and aesthetic considerations override skier functional requirements. An innovative approach to the design of the ski boot is needed. This is the subject of my next post.


  1. IN THE BEGINNING: HOW I GOT STARTED IN SKI BOOT MODIFICATIONS, May 12, 2013 – https://wp.me/p3vZhu-y
  2. LESS REALLY IS MORE, May 13, 2013 – https://wp.me/p3vZhu-N

 

(NEURAL) BIO (MECHANICAL) ENGINEERING: DOES IT WORK?

The proof of a performance concept lies in the data. If neural bio mechanical engineering can improve human performance in a specific application such as skiing, skating or cycling then it should be possible to demonstrate meaningful performance improvement with quantifiable metrics generated from data captured from the actual activity. In the case of cycling, meaningful improvement would be shown by an increase in metrics such as peak force transferred to the pedal spindle that cannot be explained by other factors. In order to attribute any change in performance, whether positive or negative, to neural bio mechanical engineering the effect must be immediate, reversible and reproducible. In the case of the improvement seen with Podborski’s performance using ski boots fit with the dorsal fit system, reverting to identical boot shells fit with a conventional liner reversed the improvement in performance.

Where possible, standard protocols should be used and testing performed by experts in the field. In the case of the cycling shoe Tekscan F Scan data comparison, my only involvement was to analyze the human lower limb requirements for cycling and generate the design and specification for the device that produced the neural bio mechanical engineering effect. I played no role in designing the test protocol or conducting the tests. I was not even an observer.

So how did the neural bio mechanical engineering system work in the application to cycling?

Performance Metrics

There are numerous metrics that can be used to assess and compare performance. The subject test was limited to pressure analysis using the Tekscan F Scan system.

In the graphic below the upper image outlined in red is the F Scan pressure image of an elite cyclist captured at 3 o’clock in the crank cycle at moderate to high load using their own conventional cycling shoe. The F Scan pressure image outlined in green below the first image is of the same cyclist using the device that bio engineered the foot and lower limb.

Results

There are a number of significant differences between the forces applied with the same cyclist with the conventional cycling shoe and the bio-engineering. In the 2 F Scan images, the contact area of the application of force across the heads of the metatarsals over the pedal spindle with bio-engineering is much greater and the force much higher than the force applied to the heads of the metatarsals in the conventional shoe.

The two significant, quantifiable (measurable) metrics that relate directly to cycling performance are: Peak Force and Anterior-Posterior (forward-backward) Excursion of the Center of Force.

Peak Force

The peak (maximum) force with the device that bio-engineered the foot and lower limb was 140% of the peak force applied with the conventional cycling shoe.

Anterior-Posterior (fore-aft) Force Excursion

This is the range of forward-backward movement of center of force through the crank cycle.

The graphic below shows the tracking of center of force forward and backward in the pedal stroke. Notice how straight the force tracks in the lower image with bio engineering compared to the upper image captured from the conventional cycling shoe.

The ability to move the center of force forward and backward, not just down and, more important, substantially aligned with the crank rotation is both more consistent and efficient than the excursion tracking with the conventional shoe. The bio engineering device improved excursion by 175% in rearward tracking force (long bars) and 200% in forward tracking force (over the top) as shown by the short bars below the images.

In my next post I will discuss how I designed a ski boot from the snow up using principles of neural bio-mechanical engineering.

INTRODUCTION TO (NEURAL) BIO (MECHANICAL) ENGINEERING

Neural biomechanical engineering is an external, non-invasive, reversible process that alters the neuralbiomechanics of the human feet and lower limbs in a manner that makes their function specific to an activity such as skiing, skating or cycling. In some cases, the process can potentiate muscle power and/or the processes of balance. Neural biomechanical engineering is human function centric.

The graphic below is the pressure image of the right foot of the same elite cyclist in the last post except that the foot has undergone neural biomechanical engineering.Red is highest force. Dark blue is the lowest force. Forces were recorded with a Tekscan F Scan system fit to the shoe.

The highest primary forces are now across the heads of all five metatarsals with secondary forces under the big toe and, to a lesser extent, the second and third toes. The primary force is now applied to the pedal spindle. The applied force is also more focussed under the heel.

There are many other more subtle but important changes. But the central issue is whether metrics show a significant quantifiable improvement in performance.

What, if any, improvement in performance resulted from the application of neural biomechanical engineering? Any guesses?

 

 

WHAT SHOULD A SKI BOOT DO?

After Steve Podborski won the 1981-82 World Cup Downhill title using a revolutionary dorsal fit technology I developed for his ski boots in June of 1980, he proposed that we become partners in a venture to develop a new ski boot that would do for every skier what the dorsal fit system had done for him. In exchange for my creative efforts, Podborski would fund the venture up to a point after which we would try to raise funds from investors for the project.

If I accepted Podborski’s proposal (which I eventually did), I knew the we faced significant hurdles. After giving the proposal a lot of thought, I accepted Steve’s offer. Steve and I became partners in a company called MACPOD Enterprises Ltd. While I had identified some of the pieces of the puzzle, I didn’t yet know the answer to the question what a ski boot should do. But I knew that when the time came to raise money I would need to provide investors with convincing evidence that I knew the answer to this question.

Podborski’s success lent credibility to the project. But his credibility was based on his subjective assessment supported by his race results. To be credible, a ski boot design based on principles of science would need to be supported with data from actual skiing maneuvers that could generate meaningful, quantifiable metrics for such things as balance and ski control. When the metrics were compared to the same metrics from data captured from the same skiers using conventional ski boots, they would need to unequivocally demonstrate superior performance of the MACPOD ski boot. I had to come up with a format that would satisfy potential investors that the new ski boot MACPOD would develop would be at least as good, if not better, than the system Podborski used to win the 1981-81 World Cup Downhill title. Whatever format I came up with had to be capable of allowing investors who skied to ski in it.

In 1992, MACPOD raised money from investors to fund the first phase of the venture. The pressure was on.

The single variable assessment protocol

The factor that convinced Podborski of the merits of my dorsal fit system was the comparison test he did against identical Lange boot shells fit with conventional Lange liners.

After rupturing his ACL testing skis at the end of July in 1980 Steve went to France 2 weeks before the opening downhill race of the 1980-81 World Cup season at Val d’isere to be with the team to support them. He had not planned on skiing, let alone racing, because he had been told by his doctors he was out of commission for the 1980-81 World Cup Downhill season. But Podborski had brought 2 pair of identical Lange boot shells to France with him just in case. One pair had the untested dorsal fit system with only the upper cuff of a Lange liner mounted on the boot shaft. The other pair had conventional Lange liners.  The only difference between the boots was the fit system; the classic single variable assessment protocol.

The graphic below from my US Patent shows a conventional tongue format (20) in FIG 3 (prior art) compared to my dorsal fit system (30) in FIG 5. The shin component (31) is like a conventional tongue.

On a whim, Podborski decided to see if he could ski in the boots with the dorsal fit system. He was amazed to find that he could ski well with little pain in his partially healed, reconstructed ACL. But when he tried to ski in the boots with the conventional liner he could barely ski.  I found this interesting because the impetus for the new fit system was my hypothesis that dorsal loading of the bones of the midfoot might reduce strain on the knee by dampening decompression of the arches resulting from perturbations in ground reaction force due to asperities and undulating terrain. A conventional liner could not be used because it would have interfered with the interface of the lower shell overlap closure on the upper surface of the dorsal fit system required to apply force to it. Fig 9 below from the patent shows how the overlap of the shell applies force to the upper surface of the dorsal fit system. The buckle closures allow the force, which should be minimal, to be regulated.

The ability to compare the dorsal fit system against a conventional liner system on the same day and in same conditions made the superiority of the dorsal fit system apparent. The unprecedented improvement in performance with no run-in period or special training program strongly suggested that the improvement resulted from reducing factors in conventional ski boots that limit or degrade human performance. This experience caused me to undertake a critical analysis of the functional requirements of the human system for skiing. This exercise opened the door to the possibility of technologies that would integrate external appendages such as skis and skate blades with the human system, what I later came to term Bio-Integration.

Bio-Engineering

If structures of ski boots, ice skates and cycling shoes can limit or degrade the human performance of the user it also became apparent to me that it might be possible to modify the function of the feet and lower limbs that would make it specific to activities such as skiing, skating or cycling and even potentiate neuromuscular function. I termed this concept Bio-Engineering. I didn’t realize until 1991 that the dorsal fit system used principles of Bio-Engineering.

The graphic below is the pressure image of the right foot of an elite cyclist showing the forces applied by the foot to the sole of the shoe on the pedal spindle at 3 o’clock in the stroke sequence at a low cadence with a moderate to high load on the crank. The cyclist is wearing a conventional rigid sole cycling shoe with no arch supports, wedges or other accessories.

Red is highest force. Dark blue is the lowest force. Forces were recorded with a Tekscan F Scan system fit to the shoe.

The highest force is applied under the ball of the great toe and the great toe and to a lesser extent, the second, third and fourth toes. The dashed line shows the approximate location of the pedal spindle which is the source of resistance/reaction  force. This pressure pattern is typical of elite cyclists. Ideally, the highest force should be applied across the width of the pedal spindle by the heads of all five metatarsals. Note that aside from the high pressure patterns on the ball of the foot and toes 1 through 4 the pattern is diffuse across the heads of metatarsals 2 through 5 and under the heel.


In my next post, I will show a pressure pattern of the same foot in the same position with a technology that Bio Engineers the foot and lower limbs and discuss the significant differences.

IS ‘SUBTALAR NEUTRAL’ SKIINGS’ HOUSE OF CARDS?

If you purchased custom footbeds for your ski boots or had your ski boots custom fit you may have been told that your foot was placed in subtalar neutral and that this created the strongest position of the bones of the foot and leg for skiing. Neutral in this context refers to a neutral configuration of the subtalar joint of the ankle/foot complex.

As best I can recall, the term subtalar neutral began to emerge in the ski industry about 1978. The authoritarian manner in which it was presented and promoted suggested that it was science-based and supported with evidence that conclusively demonstrated superior performance. But I never saw or heard any explanation as to how subtalar neutral could create the strongest position for skiing of the bones of the foot and leg and I have still not seen such an explanation.

Back in 1978, I didn’t even know what the subtalar joint was. I couldn’t envision how the bones of the foot and leg could be maintained in a specific configuration while foam was injected into a liner around the foot and leg or through some other custom fit system. But in spite of the lack of even a theory to support the premise of subtalar neutral as creating ideal biomechanical alignment of the bones of the foot and leg for skiing the premise seemed to be readily accepted as fact and quickly became mainstream. By the time The Shoe in Sport (which questioned the principles on which the plastic ski boot is based) was published in 1989 (1987 in German), neutral subtalar was firmly entrenched in the narrative of skiing.

In my US Patent 4,534,122 (filed on December 1, 2013) for a dorsal support system that I called the Dorthotic, I had unkowingly tried to fix the subtalar joint in a static position as evidenced by the excerpt below from the patent:

The system of the invention applies significant pressure to the dorsal (upper) surface of the foot over the instep, including the medial and lateral aspects thereof, and hence to the bones of the mid-foot to substantially prevent these bones from moving relative to each other.

Note: The prior art refers to the current paradigm in existence.

The objective of the dorsal support system was to immobilize the joints of the bones below the ankle in conjunction with the joints of the bones of the midfoot while allowing unrestricted dorsi-plantarflexion of the ankle joint within it’s normal range of motion. But the significant medial (inner) pressure applied by the  system to the bones of Podborski’s foot below his ankle made it difficult for him to stand and balance on one foot with the system in a ski boot shell even on the concrete floor of my workshop. Removing the offending structure from the dorsal support system quickly resolved the issue by allowing his foot to pronate. This made me aware that structures that impede supination did not appear to create issues. This insight raised the possibility of a fit system based on selective constraint applied to specific aspects of the foot and leg as opposed to what I termed indiscriminate (general) constraint.

Even though at the time that I wrote my US Patent No, 5,265,350 in February of 1992 I still did not comprehend the mechanism behind the claimed superior performance associated subtalar neutral, I knew enough to know that attempting to fix the subtalar joint in any configuration in a ski boot would interfere with, or even prevent, a skier from balancing on one foot.

Here is what I said in the patent:

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 prior art which teaches, in an indirect manner, that the ideal function for skiing will result from fixing the architecture of the foot in a position closely resembling that of bipedal function, thus preventing monopedal function (balance on one foot on the outside ski).

I later discovered that the above statement came close to the truth.

I also discussed the issue of subtalar neutral in my post NO NEUTRAL GROUND (2.) published on September 1, 2014. But I did not learn about the origins of subtatar neutral and especially the intense controversy surrounding it in professional circles until recently when I came across a discussion on Root and his subtalar neutral theory in an online podiatry forum.

The Origin of Subtalar Neutral

Merton’s Root’s subtalar joint neutral theory was first described in the textbooks, Biomechanical Examination of the Foot, Volume 1. – 1971 (Root, Orien, Weed and Hughes) and Normal and Abnormal Function of the Foot – 1977 (Root, Orien, Weed). The basic premise of Root’s subtalar neutral theory is that a neutral position of the subtalar joint (which Root defined as existing when the foot was neither supinated or pronated), is the ideal position of function in static (two-footed bipedal, erect) stance and in gait where the subtalar neutral theory posited that the foot was pronated in the first half of the stance phase then transitioned through neutral in mid stance to become supinated in the latter half of the stance phase.

Root’s paradigm proposes that the human foot functions ideally around the subtalar joint’s neutral position and that deviations from this ideal position are deformities.

What Root really said

Root and his associates never stated that the joints of the foot should be immobilized in subtalar neutral. The reference to static in subtalar neutral as the ideal position of function in static stance pertained to a subject standing in place in an erect bipedal stance on a flat, level, stable surface with the weight apportioned between the two feet. In this static stance the Root subtalar neutral theory posited that the subtalar joint should rest in neutral. Root and his associates never stated, implied or suggested that the joints of the foot should be configured and immobilized in subtalar neutral. Further, Root and his associates made no reference, of which I am aware, to the application of subtalar neutral to activities other than static stance and gait. Critrics have asserted that a subtalar neutral position in static stance is neither normal or ideal. In defining subtalar joint neutral as normal, Root’s theory implied the existence of abnormal pathologies in the feet of the majority of the world’s population.

The lack of evidence

Critics of Root and his associates “Eight Biophysical Criteria for Normalcy” claim the criteria was nothing more than hunches, that these conjectures were accepted as fact, when, in reality, there was no experimental data or research to support them and that the eight criteria were neither normal or ideal.

 The STJ neutral position problem

One of the early critics of Root and his associates was Kevin Kirby, DPM. He is an Adjunct Associate Professor within the Department of Applied Biomechanics at the California School of Podiatric Medicine at Samuel Merritt College in Oakland, Ca.

Kirby observed a large error range in determining STJ neutral position on the same foot from one examiner to another. In unpublished studies done during his Biomechanics Fellowship at the California College of Podiatric Medicine, Kirby found that the Biomechanics Professors were +/- 2 degrees (a 4 degree spread) and the podiatry students were +/- 5 degrees (a 10 degree spread)  in determining STJ neutral position.

Subtalar neutral appears to be what amounts to a knife edge between pronation and supination where neutral is the border or transition point between the two states. Unless the subtalar neutral position can be precisely and consistently identified, it is impossible to know whether the subtalar joint is pronated or supinated.

The future of subtalar neutral in skiing

Too many times theories of how the human foot functions and therefore how mechanically inducted foot problems are treated have been presented as if they were facts. The dogmatic adherence that sometimes ensues from such an approach has frequently stifled the evolution of foot mechanics. This has been particularly apparent in the field of podiatry which has been dominated by the Root paradigm. (4.)

The long standing controversy and growing challenges mounted against the credibily of Root’s subtalar neutral theory has significant implications for the continued promotion of subtalar neutral in skiing as providing the strongest position of the bones of the foot and leg.

It may eventually be shown to be unfortunate that Root’s influential textbooks were published at a time when the ski industry was attempting to come to terms with the skier/boot interface issues associated with the new paradigm created by the rigid shell plastic ski boot.

In my next post, I will discuss what a ski boot should do for the user or perhaps, more a case of what a ski boot shouldn’t do.


  1. Root ML, Orien WP, Weed JH, RJ Hughes: Biomechanical Examination of the Foot, Volume 1. Clinical Biomechanics Corporation, Los Angeles, 1971
  2. https://wp.me/p3vZhu-Bv
  3. Are Root Biomechanics Dying: Podiatry Today, March 27, 2009
  4. Foot biomechanics- emerging paradigms: Stephen F Albert, 4th Congress of the International Foot and Ankle Biomechanics (i-FAB) Community Busan, Korea. 8-11 April 2014