ankle joint

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

 

 

 

SKIER BALANCE: IT’S ABOUT BALANCING OPPOSING TORQUES

The subject of my 4th post published on May 14, 2013 was the role of torques in skier balance. That this was one of my most important yet least viewed posts at 109 views suggests that the role of torques in skier balance is a concept foreign to skiers especially the authorities in the ski industry. This post is a revised version supplemented with information results from a recent study on balance control strategies.


While everyone recognizes the importance of good balance in skiing, I have yet to find an definition of what is meant by good balance, let alone a description of the neurobiomechanical conditions under which a skier is in balance during actual ski maneuvers. In order to engage in a meaningful discussion of balance, one needs to be able to describe all the forces acting on the skier, especially the opposing forces acting between the soles of the feet of the skier and the snow surface (ergo – applied and ground or snow reaction forces). Without knowing the forces involved, especially torques, any discussion of balance is pure conjecture. In 1991,  I formulated a hypothetical model that described these forces.  I designed a device with biomedical engineer to capture pressure data from the 3-dimensional forces (torques) applied by the foot and leg of the skier to the internal surfaces of the boot during actual ski maneuvers.

Test subjects ranged from Olympic and World Cup champions to novice skiers. By selectively introducing constraints that interfered with the neurobiomechanics of balance even a World Cup or Olympic champion calibre skier could be reduced to the level of a struggling beginner. Alternatively configuring the research device to accommodate the neurobiomechanical associated with skiing enabled novice skiers to use  balance processes similar to those of Olympic champions. To the best of my knowledge, no one had ever done a study of this nature before and no one has ever done a similar study since.

When analyzed, the data captured using the device called into question just about everything that is accepted as fact in skiing. This study was never published. For the first time I will present the data and describe the implications in future posts. We called the device shown in the photo the Birdcage. It was fully instrumented with 17 sensors strategically placed on a 3 dimensional grid.

Birdcage

The Birdcage instrumentation package was configured to detect coordinated neuromuscularly generated multiplane torques that oppose and maintain dynamic balance against external torques acting across the running surface of the inside edge of the outside ski in contact with the source of GRF (i.e. the snow).

  1. plantarflexion-dorsiflexion
  2. inversion-eversion
  3. external/internal vertical axial tibial rotation

Ankle torques are applied to the 3 points of the tripod arch of the foot (heel, ball of big toe, ball of little toe) and can manifest as hindfoot to rearfoot torsion or twisting wherein the forefoot rotates against the rearfoot.

A recent study (1.) on the role of torques in unperturbed (static) balance and perturbed (dynamic) balance found:

During perturbed and unperturbed balance in standing, the most prevalent control strategy was an ankle strategy, which was employed for more than 90% of the time in balance.

In both postures (unperturbed and perturbed) these strategies may be described as a single segment inverted pendulum control strategy, where the multi-segment system is controlled by torque about the most inferior joint with compensatory torques about all superior joints acting in the same direction to maintain a fixed orientation between superiorsegments.

The alignment of opposing forces shown in typical force representations in discussions of ski technique is the result of the neuromuscular system effecting dynamic balance of tri-planar torques in the ankle-hip system.

NOTE: Balance does not involve knee strategies. The knee is an intermediate joint between the ankle abd hip and is controlled by ankle/hip balance synergies.

The ankle strategy is limited by the foot’s ability to exert torque in contact with the support surface, whereas the hip strategy is limited by surface friction and the ability to produce horizontal force against the support surface.

Ankle balance strategies involve what are called joint kinematics; 3 dimensional movement in space of the joint system of the ankle complex. Contrary to the widely held belief that loading the ankle in a ski boot with the intent of immobilizing the joint system will improve skier balance, impeding the joint kinematics of the ankle will disrupt or even prevent the most prevalent control strategy which is employed for more than 90% of the time in balance. In addition, this will also disrupt or even prevent the CNS from employing multi-segment balance strategies.

Regardless of which strategy is employed by the central nervous system (CNS), motion and torque about both the ankle and hip is inevitable, as accelerations of one segment will result in accelerations imposed on other segments that must be either resisted or assisted by the appropriate musculature. Ultimately, an attempt at an ankle strategy will require compensatory hip torque acting in the same direction as ankle torque to resist the load imposed on it by the acceleration of the legs. Conversely, an attempt at a hip strategy will require complementary ankle torque acting in the opposite direction to hip torque to achieve the required anti-phase rotation of the upper and lower body.

Balance is Sensory Dependent

As a final blow to skier balance supporting the arch of the foot and loading the ankle impairs and limits the transfer of vibrations from the ski to the small nerve sensory system in the balls of the feet that are activated by pressure and skin stretch resulting in a GIGO (garbage in, garbage out) adverse effect on balance.

Spectral analysis of joint kinematics during longer duration trials reveal that balance can be described as a multi-link pendulum with ankle and hip strategies viewed as ‘simultaneous coexisting excitable modes’, both always present, but one which may predominate depending upon the characteristics of the available sensory information, task or perturbation.


  1. Balance control strategies during perturbed and unperturbed balance in standing and handstand: Glen M. Blenkinsop, Matthew T. G. Pain and Michael J. Hiley – School of Sport, Exercise and Health Sciences, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK – Royal Society Open Science

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

WHY STANCE TRAINING IS ESSENTIAL

When readers click on my blog address at skimoves.me, analytics give me a hierarchy of the countries with the most views and the most popular posts in ascending order. This helps me identify which content resonates most strongly with viewers and which content draws a blank.

As I write this post, the top five countries are the US followed by Croatia, the United Kingdom, Slovakia and France.

The most viewed post today is THE SHOCKING TRUTH ABOUT POWER STRAPS; far and away the most popular post I have published to date. But the most important posts by far that I have ever written, A DEVICE TO DETERMINE OPTIMAL PERSONAL RAMP ANGLE and STANCE MUSCLE TENSIONING SEQUENCE EXERCISE barely sputtered in comparison. This strongly suggests that far from just some small gaps in the knowledge base skiing is founded on, massive craters exist.

Arguably the most important aspect of skiing is a strong stance. Any variance in the fore-aft angle of  the plane of support under the feet and the plane of the base of the ski has significant impact on stance. Yet these subjects are barely blips on the Doppler Radar of the ski industry.

Since I started the dynamic ramp angle assessment project a few weeks ago I have found that when asked to do so, it is rare for a skier of any ability to be able to assume a strong ski stance in an off the ski hill environment. Even when a skier  skis with a relatively strong stance, they seem to lack a sense of what a strong stance feels like. Because of this, they lack the ability to consciously replicate a strong stance. If asked to do so, they would be unable to coach a skier in the sequence of events that I described in my last post

In the dynamic ramp angle assessment project, I  have also observed that skiers with with a boot/binding ramp angle greater than 2.8 degrees appear to have become accustomed to the associated unstable, dysfunctional feeling and identify with it as ‘normal’. Before I can test them, I have to spend time coaching them into the correct stance because it feels unnatural to them.

When I go back and forth between a strong functional stance on a flat, hard level surface to a stance on the dynamic ramp angle device set to an angle of 4 degrees, I can get close to the same angles of ankle, knee and hip. But when I do, I feel strong tension, stiffness and even pain in my mid to lower back which is  common in some skiers and even racers.

Based on results to date with the dynamic ramp angle device, it appears as if strong skiers ski best with ramp angles close to zero. But depending on their sense of balance and athletic ability, they may have a wide range in which they sense little difference on the effect of ramp angle until they approach the upper limit of stability. While they may be able to ski well with a ramp angle close to the maximum limit of stability, ramp angles much above 1.2 to 1.5 degrees may not offer any benefits. This can only be tested on skis where balance is tested by dynamic forces which cannot be replicated in a static setting.

Issues affecting skier stance were discussed in detail in my post, THE SHOCKING TRUTH ABOUT POWER STRAPS. Here are the excerpts I posted from the chapter on The Ski Boot in the book, The Shoe in Sport (1989), published in German in 1987 as Der Schuh Im Sport– ISNB 0-8151-7814-X

“If flexion resistance stays the same over the entire range of flexion of the ski boot, the resulting flexion on the tibia will be decreased. With respect to the safety of the knee, however, this is a very poor solution. The increasing stiffness of the flexion joint of the boot decreases the ability of the ankle to compensate for the load and places the entire load on the knee”. – Biomechanical Considerations of the Ski Boot (Alpine) – Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland

“The shaft of the boot should provide the leg with good support, but not with great resistance for about two thirds of the possible arc, i.e., (14 degrees) 20 to 22 degrees. Up to that point, the normal, physiologic function of the ankle should not be impeded”.

“Previous misconceptions concerning its role in absorbing energy must be replaced by the realization that shaft pressure generates impulses affecting the motion patterns of the upper body, which in turn profoundly affect acceleration and balance.

“When the lateral stability of the shaft (the leg) is properly maintained, the forces acting in the sagittal direction should not be merely passive but should be the result of active muscle participation and tonic muscular tension. If muscular function is inhibited in the ankle area, greater loads will be placed on the knee”. – Kinematics of the Foot in the Ski Boot – Professor  Dr. M. Pfeiffer – Institute for the Athletic Science, University of Salzburg, Salzburg, Austria

It has been over 40 years since international authorities on sports science and safety raised red flags concerning the adverse effects of ski boots design and construction on skier stance, balance and the potential to cause or contribute to injury. It is time that their concerns were taken seriously and acted on. Research on stance and the effect of such things as zeppa and delta ramp angles is urgently needed.

 

THE ZEPPA TIPPING POINT PROBLEM

A recent post on The Foot Collective FaceBook page titled Humans aren’t meant to walk on ramps!, highlighted the problems caused by elevating the heel above the forefoot known in the footwear industry as drop. Like the author of the post, I also wear zero drop shoes like Xero and Lems exclusively  (with NABOSO insoles) and spend all of my time indoors barefoot. Like the author, I too have experienced an immediate, unnatural and a sense of disorientation in terms of a connection with the ground, when I have worn dress shoes and winter boots with moderate drop.

While some amount of boot board ramp angle or zeppa appears to necessary for a strong, tensioned stance (what I refer to as a planted or rooted stance), the amount of zeppa is turning to be much less than I originally thought. It may be less than 1.5 degrees total (zeppa + delta). Assuming zero delta, there appears to be a very narrow range within which zeppa is optimal after which a tipping point is reached in terms of adverse effects on the motor control and balance systems.

It has also become apparent that some racers are tuning ski response by adjusting binding delta. Zeppa and delta each have a different effect on ski response especially edge control and the ability of a skier to resist the forces acting on them in the load phase of a turn. I will discuss issue this in a future post.


Humans aren’t meant to walk on ramps!

Powerful post by TFC Educator @optimize.physiotherapy
👣
Why do most shoes have a heel on them?
This really hit home the other day when I put on my winter boots (because it snows in November in Canada). Being someone who goes barefoot all day at work and at home (and wears zero drop shoes), it was a very unnatural feeling. It really threw my walking off, and I noticed the effects immediately. It changed the way I walked, stood, and made me use different muscles.
Humans are meant to have a flat base. No other animal wears mini ramps on their feet, but we do. The problem is that your body adapts to having a heel on, and it works different from a biomechanical perspective in any given movement pattern (the higher the heel, the worse the effect…but even most casual, running, and gym shoes have heels)


One thing it really does is affect your ankle/foot function. It has a huge effect on ankle ROM and tissue tension around the ankle. The problem is, when you wear a heel all day at work/at the gym/walking around, your tissues adaptively shorten and you don’t require as much ankle ROM. But then you take your shoes off and walk, go up your stairs, squat down to get things around the house etc. This is where people have issues. Not only at the foot/ankle but all the way upstream at other joints



Ankle ROM is incredibly important, and walking on a ramped surface all the time is incredibly unnatural. So do yourself a favour and spend less time in heeled footwear or get rid of it altogether


The Foot Collective is a group of Canadian physical therapists on a mission to help humans reclaim strong, functional and painfree feet through foot health education.

The Foot Collective are empowering people with the knowledge they need protect their feet from the dangers of modern footwear and the guidance to fix their own feet.

http://www.thefootcollective.com

THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: CLOSED CHAIN OUTSIDE LEG ROTATION

A recently published study on foot pressure data acquired during skiing (1.) recognized that compressive force pressure data acquired in skiing is underestimated by 21% to 54% compared to pressure data acquired on a force platform in a controlled environment.  The underestimation varies depending on the phase of the turn, the skier’s skill level, the pitch of the slope and the skiing mode. The paper states that other studies have stated that this underestimation originates from a significant part of the force actually being transferred through the ski boot’s cuff (to the ski). As a result, the CoP trajectory also tends to be underestimated along both the anterior-posterior (A-P) and medial-lateral (M-L) axes compared to force platforms.

In conclusion, these studies have highlighted a major contribution of different factors to the nGRF applied throughout a turn, such as the foot’s position during a turn (inside vs. outside), the CoP A-P (front to back) displacement, or precise loading of different foot sole regions.  Unfortunately, these results have been studied separately.

There is a lack of continuity across the various positions in skiing and, in particular, a lack of a model with which to explain mechanisms such as balance on the outside ski and open and closed chain internal rotation of the leg and foot in conjunction with progressive inclination and G force loading on it as the skier crosses the fall line in the bottom of a turn. The associated mechanics and biomechanics represent a new paradigm requiring new thinking and new insights. Existing text-book explanations are not sufficient to explain these mechanisms.

Open Chain Whole Leg Rotation vs. Closed Chain Rotation

Rotation of an unloaded (non-weight bearing) lower limb is relatively straight forward when there is a small angle at the knee. As resistance to rotation of the foot is progressively introduced with increasing weight imposed on it, the kinetic chain begins to close. As it closes, the points at which the foot transfers torque to the walls of rigid shell footwear such as ice skates and ski boots starts to emerge as an issue as does the loading of the foot created by the weight of the body imposed on it and the position of COM in relation to the foot.

In order to tension the biokinetic chain and trigger the two-phase Second Rocker, COM must be aligned over the foot as shown in the grahic below.  This alignment requires that the leg adduct (move towards the center of the body) approximately 6.5 degrees. To bring the 3 points of the tripod of the foot into contact with the ground, the foot must evert (sole turn outward) the same amount. Eversion is accompanied by a corresponding torque coupled 6.5 degrees of internal rotation of the leg as shown in the left hand figure in the graphic below (see my post – OUTSIDE SKI BALANCE BASICS: STEP-BY-STEP). The bipedal figure on the right shows adduction, eversion and internal rotation as 0.0 – 0.0 – 0.0 for reference. The monopedal figure on the left shows the changes in adduction, eversion and internal rotation as 6.5 – 6.5 – 6.5.

 

The alignment of COM with the foot can be achieved by moving COM laterally as shown by the arrow emanating from COM in the Monopedal figure or by moving the foot medially as shown by the white arrow or through a combination of the two movements.  The act of positioning COM over the outside foot (Getting Over It), sets in motion internal rotation of the outside leg and eversion of foot into the turn. This engages an over-centre mechanism between the platform of the ski and the inside edge underfoot.

The over-centre mechanism results in an alignment of the resultant force R forming an angle with the transverse aspect of base of the ski that is slightly less than 90 degrees. In order to Get (COM) Over It (the foot), it is essential that the outside leg is not only able to adduct and rotate internally as the foot everts, but to achieve this configuration without delay in order to set up the over-center mechanism. The problem for the majority of skiers is that the objective of most boot fit systems and boot-fitting procedures is to support the foot in a neutral configuration. Eversion of the foot is a component of pronation. Impeding or preventing pronation, restricts or even prevents the required amount of eversion.

Closing the Kinetic Chain on Whole Leg Rotation

Open kinetic chain leg/foot rotation with the foot unloaded (not bearing weight) is relatively simple. But the mechanics and biomechanics begin to get complicated when resistance is progressively introduced that starts to close the kinetic chain as happens when the outside ski is rotated across the path of the skier in the fall line in the bottom of a turn.
The graphic below shows a foot supported on a platform with 2 points of resistance (FR) applied to the platform opposite the 2 points of application of the moments of force, ML (green) and MM (red). The forces tangent to the arc of the moments of rotation are shown as FT.
When the weight of the body is progressively shifted to one foot (i.e. Monopedal Stance) and the foot everts, the talus (shown in gray in the graphic above) moves inward towards the center of the body and shifts slightly rearward as evidenced by the corresponding movement of the inside ankle bone.  This is easily seen when moving from bipedal to monopedal stance on a hard, flat surface while barefoot. In order to effectively transfer torque from the foot to the platform, the forefoot and ankle and knee joints must be fascially tensioned. This requires that the big toe (Hallux) be aligned on the anatomical axis (dashed line) and the forefoot fully splayed. This stabilizes the heel and head of the 1st metatarsal (ball of the foot).  Torque from internal rotation of the leg will be transferred to two discrete points adjacent the Load Counters mounted on the resistance platform.

Removing the resistance force FR from the inner (big toe) aspect of the platform provides insights to what I refer to as the Turntable Effect that is associated with internal rotation of the leg and eversion of the foot that creates an over-center mechanism. The turntable rotation is shown in light yellow. The effect will vary for different structures of the foot depending on the location of the center of rotation of the platform under the foot.

The location of the blade of an ice skate on the anatomical center of the foot has been used to explain why it is easier to cut into a hard ice surface with a skate compared to the edges of a ski. But the real reason it is easier is because ice skaters use the Second Rocker, Over-Center, Turn Table Mechanisms as shown in the graphic below. The skate is positioned under COM. It can be readily seen that the skater is not using the inner aspect of the shaft of the skate to hold the skate on edge.

In my next post, I will discuss the progress of emerging CARV and NABOSO technologies after which I will continue with my discussion of the Mechanics of Balance on the Outside Ski.


  1. Influence of slope steepness, foot position and turn phase on plantar pressure distribution during giant slalom alpine ski racing: Published: May 4, 2017  – Thomas Falda-Buscaiot, Frédérique Hintzy, Patrice Rougier, Patrick Lacouture, Nicolas Coulmy
  2. http://wp.me/p3vZhu-29n

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.

screen-shot-2017-02-09-at-3-55-38-pm

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.