Ski boot modification posts

TRANSITIONING FROM FIT TO HIGH PERFORMANCE FUNCTION


That footwear can negatively impact the physiologic function of the user has been known for many decades. But the issue of the effect of footwear on athletic performance came into sharp focus in 1987 with the publication of the medical textbook, The Shoe in Sport (published German in 1987 as Der Schu im Sport). The Shoe in Sport brought together the collective expertise of 44 international authorities on orthopedics and biomechanics to focus their attention on the SHOE PROBLEM in the context of problems shoes can cause for athletes in terms of compromising performance and contributing to injury. The Shoe in Sport focusses on the medical orthopedic criteria in offering guidelines for the design of shoes for specific athletic activities including skiing and ice skating.

In the Introduction to the Shoe in Sport, Dr. med. B. Segesser and Prof. Dr. med. W, Pforringer state that the findings in the textbook should enable the interested reader to distinguish between hucksterism and humbug on the one side and the scientifically sound improvements in the athletic shoe on the other. The Shoe in Sport made it abundantly clear that it is not a question of if structures of footwear will affect the physiologic function of the user, it is a question of how structures of footwear will affect the physiologic function of the user and especially whether they will compromise athletic performance and/or contribute to injury.

With regard to guidelines for ski boots, the international authorities on orthopedics and biomechanics who contributed their expertise and knowledge to Part IV The Ski Boot took the position that, among a number of other things:

  • ………. the total immobilization by foam injection or compression by tight buckles are unphysiologic.
  • The ski boot and it’s shaft must be adapted to the technical skill of the skier, and the technical skills of the skier must be adapted to the preexisting biomechanical functions of the leg and the foot.
  • It (the design) should not make compromises at the expense of other joints ………
  • It (the ski boot) must represent the ideal connecting link between man and ski (steering and feedback).

The position of international authorities on orthopedics and biomechanics on the medical and biomechanical criteria for ski boots was succinct, concise and unequivocal:

…….total immobilization by foam injection (implying by any means) or compression (of the foot) by tight buckles are (both) unphysiologic.

Dr. E. Stussi,  Member of GOTS and Chief of the Biomechanical Laboratory ETH, Zurich, Switzerland made a prescient statement with implications for the future of knee injuries in skiing:

Improvements in the load acting on the ankle (implying load from improved fit) make it biomechanically very likely that the problems arising in the rather delicate knee joint will increase.

While the international authorities on orthopedics and biomechanics who contributed to The Shoe in Sport provided valuable guidelines for the design of the ski boot they did not offer a specification that would assist designers and those who work with ski boots in meeting the medical and biomechanical criteria in the guidelines. My hope and intent was that the Birdcage studies and the content of my US Patent 5,365,350 (issued on 11-30-1993, expired on 12-28-2005) would serve as a foundation on which to build a specification that would enable the structures of ski boots to be adjusted to accommodate the personal functional requirements of the skier.

The steps in my transition from Fit to High Performance Function

After the unprecedented success of my dorsal loading invention with Crazy Canuck, Steve Podborski, I used the same system with similar success in the boots of a small number of other racers. I also incorporated this system into my own and my spouses’ ski boots in conjunction with suitable liner modifications and a reduction of the ramp angle of the boot boards to just under 3 degrees which I had identified in about 1978 as the maximum angle for skier performance.

I can’t recall exactly when, but about 20 years ago I decided to move away from Lange ski boots. I purchased a pair Head World Cup 335 mm ski boots for myself and a pair of Head X-80 295 mm ski boots for my spouse. I say built because to me ski boots are raw material.

I had to completely disassemble the Head X-80s and drastically modify and reconfigure the components to adapt them to the morphology of my spouses’ feet and legs. The process took me about 35 hours. I was able to modify my Head World Cup liners to make them work without the same degree of modification. I made a dorsal loading system for my spouse similar to the one I made for Steve Podborski’s Lange ski boots.  But I was able to modify the existing Head tongue so it would adequately load the dorsum of my foot. The reason I went this route is that the shell of my Head World Cup boot is very stiff. This makes inserting my size 12 US men’s foot and a dorsal system, like I fabricated for my spouse, challenging. In the order of things the dorsal system is inserted after inserting the foot in the shell.

The photo below shows my Head liner after initial modifications.

The photo below shows the Lange tricot liner I used in my spouses’ Head boots on the left with no modification other than removing the Lange flow fit pads in the side pockets. I was unable sufficiently modify the liner that came with her Head X-80 boot. The version on the right in the photo below is the same liner after modifications i made for it work with the dorsal system shown in the photo underneath. The dorsal system in itself took many hours of painstaking effort to fabricate and fine tune.

With our modified Head boots fit with my dorsal loading technology my spouse and I would easily be classified as expert skiers. As recreational skiers with skiing limited to 10-15 days a season, most skiers would have no incentive to question the adequacy of their boots or especially devote time and effort towards finding ways to reach a higher level of performance. To the contrary, I found it disturbing that the ability to ski better than the majority of skiers fostered an intoxicating sense of superiority. But I knew what I didn’t know and I knew that I still had a lot to learn. In my mind, the transition required to realise our full performance potential was not yet complete.  I knew that the potential for improvement has no boundaries.

The transition to High Performance Function continues In my next post……….

FIT VS. FUNCTION

With rare exceptions, the consistently stated objective of boot-fitting systems and modification efforts is to create a perfect fit of the foot and leg of a skier with the rigid shell of a ski boot by applying uniform force to the entire surface of the foot and the portion of the leg in the boot in what pits Fit against Function. The end objective of the Perfect Fit is to achieve a secure connection of the leg of the skier with the ski. In the name of achieving a secure connection of the foot with the ski, the function of the skiers’ foot has become unitended collateral damage.

But boot design and boot fitting effors didn’t start off with the intent of compromising the physiologic function of the foot. It just sort of happened as a consequence of the limited ability to change the shape of the rigid plastic ski boots to address issues of user discomfort when plastic boots were first introduced. The new plastic boots worked well for some skiers. But for most, myself included, my foot moved around inside the shell when I tried to ski. The feeling of insecurity created by the looseness made skiing with any semblance of balance or control impossible. The fix seemed to be a simple matter of trying to figure out where to place a pad or pads between the foot and shell to stop the foot from moving.

In 1973 when I first started tinkering with my own ski boots the craft of boot fitting barely existed. Like myself, those who were trying to solve the problem of a loose fit were doing proceeding by trial mostly with alot of errors. After what seemed like unending frustration from many failed attempts at trying to find and then solve the source of my loose fit, a consensus began to emerge within the ranks of the ski industry that the easiest and quickest solution was a process that would create a tight fit of the foot everywhere with the boot instead of wasting time trying to find the elusive right place to add pads. The Perfect Fit was born.

Injected foam fit was first off the mark as a Perfect Fit solution. But injected foam fit wasn’t tight or precise enough for my standards. So I tried to take the Perfect Fit to the next level with Crazy Canuck, Dave Murray. I started the process by carefully trimming and laminating together pieces of sheet vinyl to form a matrix of solid material that I inserted into the liners of Mur’s boots. The process took about 2 weeks of painstaking effort. Finally, I satisfied that Mur’s feet were securely locked and loaded; ready for the best turns of his life. The result? One of the world’s best racers was instantly reduced to a struggling beginner, the exact opposite of what I had expected! This experience served as a wakeup call for me; one that caused me to rethink what I thought I knew and question whether the Perfect Fit was the best approach or even the right approach.

I started looking for alternate ways to restrain the foot so it was secure in the shell of a ski boot without compromising foot function. In 1980 when I was building a pair of race boots for Crazy Canuck, Steve Podborski I literally put my finger on the solution when I pressed firmly, but not forcefully, on the instep of his foot just in front of the ankle and asked if he thought we should try holding his foot like this in his new race boots. Without the slightest hesitation he said, “That feels amazing. Let’s do it!”

It took me more several few days to fabricate a system to secure Pod’s foot in his boots by loading the area of the instep that I had pressed my finger on. The problem we faced when the system was finished was that the liner made it impossible to use the system without modifying it. So a decision was made to eliminate the liner except for the cuff portion around the sides and back of his leg which I riveted to shell. At the time I wasn’t sure the system would even work. So I made a pair of boots with fined tuned conventional fit as backup. A boot with no liner seemed like an insane idea. But Podborski was not only able to immediately dominate his competition on the most difficult downhill courses on the World Cup circuit but go on to become the first non-European to win the World Cup Downhill title. Even more remarkable is that in his first season on the new system he was able to compete and win less than 4 months after reconstructive ACL surgery.

What I discovered set me off in a whole new direction. Pressing on the instep of Podborski’s foot activated what I later found out is called the Longitudinal Arch Auto-Stiffening Mechanism of the Foot. This system is normally activated as the mid stance (support) phase of walking approaches late mid stance where the foot is transformed into a rigid structure so it can apply the forces required for propulsion. As I learned about the processes that transform the foot into a rigid lever I began to understand how interfering with the function of the foot can compromise or even prevent the Longitudinal Arch Auto-Stiffening Mechanism from activating and, in doing so, cause the structures of the foot to remain ‘loose’ regardless of any efforts made to secure it.  A rigid foot is necessary to effectively apply force to a ski.

The graphic below shows a sketch on the left from Kevin Kirby, DPM’s 2017 paper, Longitudinal Arch Load-Sharing System of the Foot (1.) Figure 44 A on the right is from my 1993 US Patent 5,265,350.

The above graphics clarify the details of the arch loading system I first disclosed in my US Patent 4,534,122. This system challenges the current Perfect Fit paradigm in which the physiologic function of the foot is compromised in an effort to try and achieve a secure connection of a skier’s foot with the ski.

Figure 44A above shows the principle components of the arch loading system which is comprised of a number of complimentary elements. I will discuss these elements in my next post which will focus on solutions.


  1.  Kirby KA. Longitudinal arch load-sharing system of the foot. Rev Esp Podol. 2017 – http://dx.doi.org/10.1016/j.repod.2017.03.003

 

THE 2018 SOELDEN GS: A LITMUS TEST OF DYNAMIC STABILITY

Challenging  course conditions, especially in GS, are the litmus test of dynamic stability. The 2018 World Cup GS at Soelden had challenging conditions in spades.

The ability to rapidly achieve dynamic stability across the inside edge of the outside ski is key to moving the Center of Force forward to the point where the biokinetic chain of the outside leg attains sufficient tension to enable the stretch reflex. The stretch reflex (SR) can then modulate pertubations due to asperities in snow surface and terrain with ankle strategies. The principle muscle in ankle balance synergies is the soleus. Dynamic stability enables a racer to float between turns, accelerate under gravity then land on line and load the outside ski. A racer with good dynamic stability is on and off the edges in milliseconds and back into the float phase. Like a skilled gymnast elite skiers and racers can choose their line and stick their landing. Tessa Worely excelled at this in the 2018 Soelden GS.

Tell Tale Signs of Dynamic Stability

Key indicators of dynamic stability are a quiet upper body and the speed at which a racer achieves their line and crosses over into the new turn with their upper body. It’s like watching a flat rock thrown low skipping off water; fly-skip-fly-skip.

In my post, WHY YOUNG TALENTED SKI RACERS FAIL AND EVENTUALLY QUIT RACING (1.), I discuss the 3 levels of balance:

  1. The first reaction is the myotatic stretch reflex, which appears in response to changes in the position of the ankle joints, and is recorded in the triceps surae muscles. This is the earliest mechanism, which increases the activity of the muscles surrounding a joint that is subject to destabilization. Spinal  reflex triggered by the myotatic stretch reflex response causes the muscle to contract resulting in the stiffening of the surrounding joints as a response to the stimulus that has disturbed the balance. For example, changes in the angle of the joints of the lower limbs are followed by a reflexive (fascial) tensioning of adjacent muscles. The subsequent release of the reaction prevents excessive mobility of the joints and stabilises the posture once again.
  2. The next reflex in the process of balancing is the balance-correcting response, which is evoked in response to a strongly destabilising stimulus. This reactive response has a multi-muscle range, and occurs almost simultaneously in the muscles of the lower limbs, torso and neck, while the mechanisms that initiate the reaction are centrally coordinated.
  3. The last of the three types of muscular reaction is the balance-stabilising response. In a situation of a sudden loss of balance, a myotatic stretch reflex first occurs and is then is followed by a balance correcting response, which prevents or attempts to prevent a fall.

I call these balance responses Green (postural reaction 1), Orange (postural reaction 2) and Red (postural reaction 3).

If a racer is no able to use the myotatic reflex (Green = Normal) balance response, the CNS shifts to Level 2 (Orange = Caution) or even Level 3 (Red = DANGER).

Level 1 balance is characterized by a stable, well-controlled upper body (aka quiet upper body) with well controlled and directed positions of the arms.

When the myotatic (stretch) reflex is compromised by restriction of the ankle flexion range required to tension the soleus the balance system will shift to level 2 or level 3 depending on the degree of interference. As the degree of interference with required range of ankle flexion increases the degree of reflexive balance will progress from small, rapid, reactive arm movements to gross reactive arm movements that eventually include gross movements of the torso.

The authors of the Polish skier balance study cited in my post state that ski boots exclude the ankle joint complex from the process of maintaining the stability of the body. However, I don’t believe this is the case with all skiers and especially all racers as evidenced by Soelden video of Tessa Worley, Federica Brignone and Michaela Shiffrin. In my next post I will discuss what I look for in analyzing that suggests dynamic stability and especially a lack of dynamic stability and the indications of compromise and the potential cause.

In the meantime, here’s something to think about.

Early in my boot modification career I came to the conclusion that some skiers, especially racers, were born with the right shape of feet and legs (2.) and this explained why they could ski in ski boots right out of the box with minimal or no modifications better than the majority of skiers even after extensive boot modifications. In a recent series of posts I discussed the results of the 2012 skate study that I modified hockey skates for; the NS (New Skates – Blue bars in the graphics below). The modifications I made were based on ski boot modifications that had resulted in dramatic improvement in performance and race results. Although I optimistically predicted improvements in performance metrics of at least 10% (110%) based on my experience with World Cup skiers, I knew that there was the possibility of a wild card competitive skater who was already close to their maximum performance in their OS (Own Skates – Red bars in the graphics below). If this were the case the skater would realize minimal improvement from the New Skates.

My previous posts only included the results for four competitive skaters. There were actually five competitive skaters in the study. Skater number 1 was the wild card. Look what happened to the results when the wild card skater was added.Look carefully at the graph of the Impulse Force below. Compare Skater number one’s Impulse Force results with the Peak Force results in the preceding graph.This raises the question: Do Tessa Worely, Federica Brignone, Mikaela Shiffrin and other top World Cup racers have the right shape of feet and legs or do they have the right modifications made to their ski boots.


  1. (https://skimoves.me/2017/02/15/why-young-talented-ski-racers-fail-and-eventually-quit-racing/)
  2. THE IDEAL SKIER’S FOOT AND LEG – https://wp.me/p3vZhu-qf

 

 

 

IN THE BEGINNING: HOW I GOT STARTED IN SKI BOOT MODIFICATIONS

I originally published this post on May 12, 2013. This is a revised and edited version.


Before I started ‘tinkering’ with ski boots in 1973, I didn’t just read everything I could find on the subject of fitting boots, I devoured every bit of information I could find on the subject. The assumption I made at that time was that the experts in the field not only knew what they were talking about, but that they also had the requisite knowledge and understanding of the underlying principles to back up their positions with applied science and/or research. Based on this assumption, I started modifying ski boots by doing all the things the experts recommended such as padding the ankle to ‘support’ and ‘stabilize’ it in the boot shell and cuff and adding cants between the soles of the boots and the skis to make the skis sit flat on the snow. But the big breakthrough for me came when I started making footbeds to support the foot.

Within a year I had gained expertise in my craft to the point that skiers from all over Canada were starting to seek out my services. In  response, I started a company called Anatomic Concepts. Soon, I was spending most of my free time working on ski boots. But while I was helping a lot of skiers ski better, none of what I was learning or doing was helping my own skiing. I was still struggling after switching from low-cut leather boots to the new stiff, all plastic boots.

The (Un)Holy Grail

Despite the inability to solve my own problems, my thinking remained aligned with conventional thinking right up until my experience with Mur and the ‘Holy Grail’ of ski boots; the perfect fit of the boot with the foot and leg of the skier.

In 1977, Roger McCarthy (head of the Whistler Ski Patrol), whose boots I had worked, on introduced me to Nancy Greene Raine in the Roundhouse on top of Whistler Mountain. The timing was perfect. Racers on our National Ski Team were having boot problems. They needed help. It was a classic case of me being in the right place at the right time. Nancy recruited me, flew me to Calgary at her expense and introduced me to the National Team and Dave Murray. She set up a working arrangement with the team, one in which I was completely independent. Nancy also introduced me to Glen Wurtele, head coach of the BC Ski Team. At Wurtele’s request, I began working on the boots of members of the team.

I started working on the boots of NAST (National Alpine Ski Team) racers with Dave Murray; ‘Mur’ as he was affectionately known. My thinking at that time vis-a-vis the need to immobilize the foot and achieve a ‘perfect fit’ of the boot with the foot was aligned with the approach of the  ‘experts’ in the  field. Mur didn’t live far from me. When I was working on his boots, he seemed to spend more time at our home than his. Because of my ready access to Mur, I saw an opportunity to achieve the Holy Grail of skiing with a fit of the boot with the foot so perfect that the foot was for all intents and purposes rendered rigid and immobile and united with the structures of the ski boot.

To achieve this lofty goal I spent the better part of 2 weeks working for hours every night carefully crafting a matrix of heat formable 1 mm thick vinyl around Mur’s foot and leg and the shells of his boots with my inserts inside the liners of the boot. When Mur finally confirmed he was ‘loaded, locked and ready’ he went skiing to test the results. I waited for the inevitable confirmation of success and certain celebration that would follow. But after what seemed like an eternity, instead of the expected good news, Mur called to tell me that he could barely ski with my perfect fit. He had little or no balance or control. The Holy Grail had reduced a world class skier to a struggling beginner. I didn’t need to be a rocket scientist to know that the industry had to be way off track especially in view of the recent publication of Professor Verne T. Inman’s seminal book, The Joints of the Ankle.

After this experience I knew that there was way more going on than I understood. I started learning about human physiology, in particular, about the mechanics, neuralbiomechanics and physics of skiing. I started asking hard questions that no one in the industry seemed to have answers for. And I started going off in a very different direction from the one the industry was acquiring increasing momentum in. If the perfect fit could impose what amounts to a severe disability on one of the world’s best skiers I could only imagine what such indiscriminate constraint was doing to the average recreational skier. It could not be good. For me it certainly wasn’t.

A major turning point came for me in 1988 when a husband and wife radiology team who had heard about my efforts to try and develop a ski boot based on anatomical principles presented me with a copy of a medical text called The Shoe in Sport published in German in 1987. This seminal work contains an entire chapter dedicated to The Ski Boot. I discuss the issues raised about the design and fabrication of ski boots by international experts in the articles in chapter on The Ski Boot in my most viewed post to date; THE SHOCKING TRUTH ABOUT POWER STRAPS (1.)

The Root of Misinformation

Unfortunately for skiing, the relevance and significance of the knowledge contained in The Shoe in Sport was overshadowed by the publication in 1971 of the book, the Biomechanical Examination of the Foot, Volume 1 by Drs. Merton Root, William Orien, John Weed and Robert Hughes. The book lists what the authors call their “Eight Biophysical Criteria for Normalcy”. These criteria, which have since been challenged and shown to be largely invalid,  were claimed to represent the “ideal physical relationship of the boney segments of the foot and leg for the production of maximum efficiency during static stance or locomotion”.

A key component of the biophysical criteria was that a bisection  of the lower third of the leg be perpendicular to the ground and the subtalar joint rest in neutral. Root described neutral as occuring when the subtalar joint was neither supinated or pronated.

In order to be considered normal, a foot had to meet all eight biophysical criteria. The effect of this criteria, which was arbitrary, was to render the majority of the feet of the world’s population abnormal and candidates for corrective interventions. Although Root never stated, implied or suggested it, his neutral sub-talar theory appears to have been misinterpretated in the ski industry to mean that the foot functions best in static ski stance when its joints are immobilized in neutral (sub talar).

In recent years, Root’s Sub-Talar Neutral Theory has come under increasing challenge with calls to discontinue its use (2.).

Conclusions
Taken as part of a wider body of evidence, the results of this study have profound implications for clinical foot health practice. We believe that the assessment protocol advocated by the Root model is no longer a suitable basis for professional practice. We recommend that clinicians stop using sub-talar neutral position during clinical assessments and stop assessing the non-weight bearing range of ankle dorsiflexion, first ray position and forefoot alignments and movement as a means of defining the associated foot deformities. The results question the relevance of the Root assessments in the prescription of foot orthoses.

The results of the wider body of evidence have the potential to have profound implications for skiing in terms of the application of Root’s Subtalar Neutral Theory as putting the foot in the most functional position for skiing by supporting and immobilizing it in neutral (subtalar).


  1. https://wp.me/p3vZhu-UB
  2. https://jfootankleres.biomedcentral.com/articles/10.1186/s13047-017-0189-2

SKI BOOT ASSESSMENT PROTOCOL

Step 1 of the synergy 5 Step performance Program described in my last post is a Footbed Check using the Novel Pedar insole pressure analysis system.

Step 3 of the program is the Ski Boot Assessment detailed below. As with the 5 Step performance Program, the Ski Boot Assessment protocol and report were intended to serve as a template to base future programs on. The assessment report was intended to provide clients with information on the effects of their ski boots on their performance and/or as a work order for them to take to a boot-fitter to have any necessary issues identified in the report addressed.  Synergy Sports Performance Consultants Ltd. did not sell products or perform boot modifications.

 



My next post will be called FOOTBEDS: THE GOOD, BAD AND THE UGLY.

 

 

 

 

THE SKI BOOT FLEX INDEX INSTABILITY PROBLEM

It has been known for decades that an unbalanced moment of force or torque will be present on the outside ski when the center of pressure of the load applied to the ski by a skier is acting along the center of the transverse axis of the ski where it is offset from GRF acting along the inside edge. Ron LeMaster acknowledges the existence of an unbalanced moment of force on the ouside ski in both The Skier’s Edge and Ultimate Skiing (Edging the skis). LeMaster states in Ultimate Skiing;

The force on the snow is offset from the center of the skier’s and creates a torque on it that tries to flatten the ski.

Ron didn’t get the mechanics right. But he correctly shows the unbalanced torque acting on the ankle joint. LeMaster tries to rationalize that ice skates are easy to cut clean arcs into ice with because the blade is located under the center of the ankle. While this is correct, ice skaters and especially hockey players employ the Two Stage Heel-Forefoot Rocker to impulse load the skate for acceleration. Hockey players refer to this as kick.

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

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

The load or weight of COM is transferred to distal tibia that forms the ankle joint. This is the lower aspect of the central load-bearing axis that transfers the load W from COM to the foot. What happens after that depends on the biomechanics. But the force will tend to be applied on the proximate center of the stance foot. This is a significant problem in skiing, (one that LeMaster doesn’t offer a solution for) when the ski is on edge and there is air under the body of the ski. The unbalanced torques will move up the vertical column where they will manifest at the knee against a well stabilized femur.

But this unbalanced torque creates another problem, one that is described in a paper published in 2005 by two Italian engineers (1.) that describes how this load deforms the base of the boot shell.

The Italian study found large amounts of deformation at mean loads of up to 164% body weight were measured on the outer ski during turning. The paper suggests that the ski boot flex index is really a distortion index for the boot shell. The lower the flex index, the greater the distortion potential.

For the ski-boot – sole joint the main problem is not material failure, but large amounts of local deformation that can affect the efficiency of the locking system and the stiffness of the overall system.

Values of drift angle of some degree (>2-3°) cannot be accepted, even for a small period of time, because it results in a direct decrease of the incidence of the ski with the ground.

My post GS AND KNEE INJURIES – CONNECTING THE DOTS (2.) cites studies that found that knee injuries are highest in GS in the shortest radius turns where peak transient forces are highest.

As shown in Figure 2a FR (sum of centrifugal and weight forces) and F GROUND (ground reaction force) are not acting on the same axis thus generating a moment MGR that causes a deformation of the ski-boot-sole system (Figure 2b) leading to a rotation of the ground reaction force direction. The final effect is to reduce the centripetal reaction force of the ground, causing the skier to drift to the outside of the turn (R decreases, causing the drift event).

An imperfect condition of the ski slope will emphasize this problem, leading to difficulties maintaining constant turning radius and optimal trajectory. The use of SGS ski-boot in competitions requires a particular focus on this aspect due to the larger loads that can be produced during races.

I have added a sketch showing that the moment arm M R created by the offset between the F Ground and F R is in the plane of the base of the ski where it results in an Inversion-lateral rotation torque.

The importance of sole stiffness is demonstrated with a simplified skier model…..…ski boot torsional stiffness with respect to ski longitudinal axis in particular is very important as it deeply influences the performance of the skier during turning…. A passage over a bump or a hollow may generate a sudden change in ground reaction force that may lead to a rapid change in the drift angle delta. The ski boot must be as stiff as possible going from the lower part of the boot to the ski (i.e. lower shell-joint-sole system)

As explained in the method section using the simplified model, values of some degree cannot be accepted, even for a small period of time, because the skier stability and equilibrium could be seriously compromised especially when the radius of curvature is small. A non perfect condition of the ski slope will emphasize the problem, leading to big difficulties for maintaining constant turning radius and optimal trajectory.

This excellent paper by the two Italian engineers concludes with the following statements:

Authors pushed forward the integration of experiments and modeling on ski-boots that will lead to a design environment in which the optimal compromise between stiffness and comfort can be reached.

The possibility of measuring accurately the skier kinematics on the ski slope, not addressed in the presented study, could represent a further step in the understanding of skiing dynamics and thus could provide even more insightful ideas for the ski-boot design process.

I first recognized the shell deformation, boot board instability issue in 1980, at which time I started integrating rigid structural boot boots into the bases of boot shells I prepared for racers. The improvement in ski control and balance was significant. The instability of  boot boards associated with shell/sole deformation with 2 to 3 degrees of drift at modest loads of up to 164% body weight has significant implications for footbeds.


  1. AN INNOVATIVE SKI-BOOT: DESIGN, NUMERICAL SIMULATIONS AND TESTING – Stefano Corazza 􀀍 and Claudio Cobelli Department of Information Engineering – University of Padova, Italy – Published (online): 01 September 2005 – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3887325/
  2. http://wp.me/p3vZhu-zx

BOOT-FITTING 101: THE ESSENTIALS – SHELL FIT

In this post, I am going to discuss the process I follow to assess what I call the essential foot to shell clearances. This is a 2-step process.

Step 1 – Establish the clearances between the structures of the foot and the inner wall of the boot shell required for the foot to function.

Step 2 – Establish the physical connections between discrete restraint force transfer areas of the foot and the inner walls of the boot shell required for the effective force transfer to the ski, for containment of the foot required to support the processes of balance and for the coupling of the foot to specific mechanical references in the boot shell related to the running surface of the ski.

As a prelude to discussing shell fit, it is necessary to point out that a major shift is occuring in the area of focus on the human foot.

Until recently, most discussions on the human foot have focussed almost exclusively on the rearfoot; the ankle complex, the tibial-talar and sub-talar joints, ankle dorsiflexion and plantarflexion, ankle mobility, inversion, eversion, etc. This limited focus has been at the expense of an appreciation and understanding of the role of the forefoot and the complex lever mechanism that enables the first MTP joint to apply large forces to the ground. A study (1) published in 2004 commented:

The plantar aponeurosis (plantar fascia) is known to be a major contributor to arch support, but its role in transferring Achilles tendon loads to the forefoot remains poorly understood.

 Fascia is a sheet or band of fibrous tissue such as lies deep to the skin or invests muscles or various body organs.

The most plausible reason why the role of the  plantar aponeurosis in transferring Achilles tendon loads to the forefoot is poorly understood is that it has not been given much attention until recently.  

The above cited study concluded:

Plantar aponeurosis forces gradually increased during stance and peaked in late stance.

The almost exclusive focus of attention on the rearfoot has led to assumptions about the function of the foot as a system which are only now being called into question and found to be erroneous or invalid. One result is the erroneous assumption that the arch of the human foot is weak and collapses under the weight of the body. This has spawned a lucrative market for custom made arch supports intended to provide what is perceived as needed support for the arch of the foot.

In boot-fitting, the process of fascial tensioning, in which the height of the arch decreases and the forefoot splays, has been misinterpreted as an indication of a collapsing (implied failure) of the arch due to its inability to support the weight of the superincumbent body during skiing maneuvers. This has led to an almost universal perception and acceptance in skiing of custom arch supports as essential foundations for the foot and the most important part of a ski boot.

The Fascial Tension/SR Stance Connection

Plantar aponeurosis forces peak in late stance in the process of fascial tensioning where they act to maximally stiffen the foot in preparation for the application of propulsive force to the ground. When fascial tensioning of the plantar aponeurosis peaks, forward rotation of the shank is arrested by isometric contraction of the Achilles tendon. This is the shank angle associated with the SR Stance.

Immobilize – Support – Stabilize

Discussions of foot function in the context of the foot to shell clearances necessary for foot function and especially fascial tensioning, tend to be obscured by a consistent, persistent narrative in the ski industry spanning decades that the foot should be supported, stabilized and immobilized in a ski boot. Foot splay, associated with fascial arch tensioning, is viewed as a bad thing. Efforts are made to prevent foot splay with arch supports and custom formed liners in order to the fit the foot in the smallest possible boot size in the name of optimizing support.

In the new paradigm that exists today, the foot is increasingly viewed in the context of a deeply-rooted structure. In the design and fabrication of footwear, attention is now being directed to the accommodation of the  fascial architecture  and the importance of fascial tensioning as it pertains to the science of the human lever mechanism of the foot.

Fascial Tensioning and the Human Foot Lever

Fascial tensioning is critical to the stiffening of the foot for effective force transmission and to foot to core sequencing.

The body perceives impact forces that tend to disturb equilibrium as vibrations. It damps vibration by creating fascial tension in the arches of the foot and the lower limb. Supporting the structures of the foot, especially the arch, diminishes both the degree and speed of fascial tensioning to the detriment of the processes of balance and the ability to protect the tissues of the lower limbs through the process of damping of impact forces.

Dr. Emily Splichal has an excellent webinar on The Science of the Human Lever – Internal Fascial Architecture of the Foot as it pertains to foot to core sequencing – https://youtu.be/_35cQCoXp9U

The DIN Standard is Not a Foot Standard

A major problem for the human foot in a ski boot is the DIN standard toe shape. DIN stands for ‘Deutsches Institut für Normung’ which means ‘German Institute of Standardization’.

The DIN toe shape creates a standard interface for bindings. In a strong, healthy foot, the big toe or hallux should be aligned straight ahead on the center axis of the boot/ski. But as an interface for the human foot, the DIN standard toe shape of a ski boot is the equivalent of a round hole for a wedge-shaped peg.

The graphic below shows a photograph of a foot overlaid over a photograph of the ski boot for the same foot. The outline of the wall of the boot is shown in red. Even though the length of the boot shell is greater than the length of the foot, the big toe will be bent inward by the wall of the shell using the one finger space behind the heel shell length check.

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The Importance of Foot Splay

The progressive fascial tensioning that occurs as CoM advances over the foot transforms foot into a rigid lever that enables the plantar foot to apply force the ground or to a structure underneath the plantar foot such as a ski or skate blade. Forefoot splay is important to the stiffening of the forefoot required for effective plantar to ground force transfer.

Ski boot performance is typically equated with shell last width. Performance boots are classified as narrow. Such boots typically have lasts ranging from 96 mm to 99 mm. Narrow boots are claimed to provide superior sensitivity and quick response, implying superior control of the ski.

The outside bone-to-bone width shown in the photo below is not quite 109 mm. The boot shell has been expanded. The 2 red arrows show the 5th and 1st toe joints (metatarsophalangeal joint or MTP joint). A prime hot spot in less than adequate shell width in the forefoot, is the 5th MTP joint. Even a minimal liner will narrow the boot shell width by 3 to 4 mm.

mt-width

Shell Check: Start Point 

I start with a skier standing in both boot shells with the insole in place from the liner then have them claw each foot forward in the shells using their toes until they can just feel the wall of the shell with the outside (medial) aspect of the big toe when they wiggle the toe up and down. If there is a finger space behind the heel, the shell is in the ball park.

A second check is made with the skier standing on one foot. Some allowance for the correct alignment of the big toe  can be made by grinding the inside of the shell where it is forcing the big toe inward. When fully weighted, a fascially tensioned forefoot will splay approximately 3 mm for a female and 5 mm for a male.  The ball shaped protrusion of the 5th MTP joint is typically almost directly below the toe buckle of a 4 – buckle boot.

Once a skier can stand on one foot in each shell with adequate space for normal foot splay, the rear foot can be checked for clearance. The usual sources of problems are the inside ankle bone (medial malleolus) and the navicular and/or the medial tarsal bone. A good way to locate the prime areas of contact is to apply a thick face cream or even toothpaste to the inside ankle bones then carefully insert the foot into the boot shell, stand on it to make contact with the shell, then carefully remove the foot. The cream will leave tell tale smears on the boot shell which can then be marked with a felt pen.

Getting Step 1 successfully completed can involve alternating back and forth between forefoot and rearfoot clearance. Until, both areas are right, full normal foot splay may not occur. Step 2 is done in conjunction with liner modifications which can be a process in itself and is often the most problematic aspect of creating an environment in a ski boot that accommodates and supports foot function especially fascial tensioning.


  1. Dynamic loading of the plantar aponeurosis in walking – Erdemir A1, Hamel AJ, Fauth AR, Piazza SJ, Sharkey NA  – J Bone Joint Surg Am. 2004 Mar;86-A(3):546-52.