human feet

THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: WINDLASS POWER

Two factors can prevent a skier from being able to develop a platform under the body of the outside ski on which to stand and balance on during a turn using the same processes used to balance on one foot on solid ground:

  1. The biomechanics of the foot and leg have been compromised by traditional footwear and,
  2. The structures of the ski boot, especially insoles, footbeds, orthotics and form fit liners, are interfering with the foot to pelvic core tensioning of the biokinetic chain that starts in the forefoot.

The torsional stiffening of the ankle and knee joints resulting from fascial tensioning of the biokinetic chain is fundamental to the ability to create a platform under the body of the outside ski by internally rotating the outside leg from the pelvis. It may sound complicated. But it is actually quite simple. Once learned, it can become as intuitive as walking.

The best method I have found to appreciate how ski boots, custom insoles and form fitting liners can affect the function of the feet and even the entire body, is do a series of exercises starting with the short foot. The short foot helps to assess the ability to harness the Windlass Power associated with the big toe. Once proper function has been acquired in the foot and leg, a skier can go through a methodical, step-by-step process to assess the effect of each component of the ski boot on the function of the feet and legs.

The latest edition of Runner’s World (1.) reports on a study done by a team at Brigham Young University that compared the size and strength of the foot’s “instrinsic” muscles in 21 female runners and 13 female gymnasts. Gymnasts train and compete in bare feet.

The researchers found:

Of the four muscles measured with ultrasound, the gymnasts were significantly bigger on average in two of them, with no difference in the other two. The gymnasts were stronger in their ability to flex their big toe, with no difference in the strength of the second, third, and fourth toes.

Although balance is important in all sports, it is especially critical in gymnastics. So it is significant that study found that the big toes of the gymnasts were stronger than the big toes of the runners.

Until recently, I found it much easier to balance on my left leg than my right leg. The big toe on my left foot was noticeably larger than the big toe on my right foot and the big toe on my left foot was aligned straight ahead whereas the big toe on my right foot was angled outward towards my small toes. This misalignment had pushed the ball of my foot towards the inside of my foot causing a bunion to form on the side, a condition known as hallux valgus. I now understand why I could balance better on my left foot than my right foot.

The muscle that presses the big toe down is called the Flexor Hallucis Longis (FHL). It is inserted into the last joint of the big toe where it exerts a pull that is linear with the big toe and ball of the foot. When the arch is maximally compressed in late stance, the Flexor Hallucis Longis is stretched and tensioned causing the big toe to press down. It’s insertion on the upper third of the fibula causes the lower leg to rotate externally (to the outside). When stretched, the FHL acts in combination with the Posterior Tibialis to support the arch. Footwear that prevents the correct alignment of the hallux weakens the arch making it more difficult to balance on one foot; the foot pronates unnaturally.

Going mostly barefoot for the past 10 years and wearing minimal type shoes for the past 6 years, made my feet stronger.  But it had minimal effect in correcting the hallux valgus in my right foot. It was only after doing the exercises in the links that follow, such as the short foot, that the big toe on my right foot became properly aligned and grew in size. It is now the same size as my left toe and I am able to balance equally well on both feet. The problem with ski boots and most footwear, is that they can force the big toe into a hallux valgus position while preventing the forefoot from splaying and spreading naturally weakening the arch and significantly impairing natural balance.

In the early 1970’s, when the then new plastic ski boots were making a presence in skiing, research on human locomotion was in its infancy. Studies of the effects of sports shoes on human performance were virtually nonexistent. The only technology available back then with which to study the biomechanics of athletes was high speed (film) movies. Ski boot design and modification was a process of trial and error. Many of the positions that predominate even today were formed back then.

As methodologies began to develop that enabled the study of the effect of sports shoes on users, biomechanists and medical specialists became convinced that excessive impact forces and excessive pronation were the most important issues affecting performance and causing or contributing to injury. I suspect that biomechanists and medical specialists arrived at this conclusion even though there was little evidence to support it because it seemed logical. Soon, the term, excessive pronation became a household word. The perceived solution? Arch supports, cushioned soles, motion control shoes and a global market for arch supports.  This appears to have precipitated an assumption within the ski industry that the feet of all skiers needed to be supported in ski boots and pronation, greatly restricted, or even prevented altogether. Even though no studies were ever done that I am aware of that demonstrated that pronation was a problem in skiing, support and immobilization became the defacto standard. Custom footbeds, orthotics and form fitted liners became a lucrative market.

As the support and immobilize paradigm was becoming entrenched in skiing, studies were increasingly concluding that, with rare exceptions, excessive pronation, is a non-existent condition with no pathologies associated with it and that the role of impact forces was mis-read. Today, it is increasingly being recognized that interference to natural foot splay and joint alignment of the big toe by the structures of footwear, causes weakness in the foot and lower limbs through interference with the natural processes of sequential fascial tensioning that occurs in the late stance phase. But the makers of footwear and interventions such as arch supports, have been slow to recognize and embrace these findings.

A key indicator of whether a skier has successfully developed a platform under the outside ski with which stand and balance on, is the position and alignment of the knee in relation to the foot and pelvis as the skier enters the fall line from the top of a turn. I discuss this in my post, MIKAELA SHIFFRIN AND THE SIDECUT FACTOR.

Best Surfaces for Training

A good starting point for the short foot and other exercises is Dr.Emily Splichal’s YouTube video, Best Surfaces for Training https://youtu.be/gvJjIi3h1Bs

Although it may seem logical to conclude that soft, cushioned surfaces are best for the feet, the reality is very different. The best surfaces to balance on are hard, textured surfaces. Dr. Splichal has recently introduced the world’s first surface science insoles and yoga mats using a technology she developed called NABOSO which means without shoes in Czech.

The skin on the bottom of the foot plays a critical role in balance, posture, motor control and human locomotion. All footwear – including minimal footwear – to some degree blocks the necessary stimulation of these plantar proprioceptors resulting in a delay in the response of the nervous system which can contribute to joint pain, compensations, loss of balance and inefficient movement patterns. I’ve been testing NABOSO insoles for about a month. I will discuss NABOSO insoles in a future post. In the meantime, you can read about NABOSO at https://naboso-technology.myshopify.com/products/naboso-insoles

Short Foot Activation

 

Short Foot Single Leg Progressions


  1. Here’s the Latest Research on Running Form – May 30, 2017
  2. Biomechanics of Sports Shoes – Benno M. Nigg

INTRODUCING THE FOOT COLLECTIVE

The Skier’s Manifesto places a high priority on foot function and exercises that make feet strong and healthy. (THE IMPORTANCE OF STRONG HEALTHY FEET IN SKIING).  There is a rapidly emerging camp of medical professionals and trainers aligned with this cause who offer excellent articles on this subject. One such group is TheFoot Collective – http://www.thefootcollective.com.

TheFoot Collective has kindly given me permission to repost material from their blog on the Skier’s Manifesto. The graphic below is from the home page of TheFoot Collective.

What is the Foot Collective?

The Foot Collective is a group of Canadian physical therapists giving people back control over the health of their feet through education. Most modern day humans have poorly functioning feet and our mission is to spread the truth about footwear and give people the information needed to independently restore their own feet.

The collective exists to spread awareness of the importance of foot health and to provide quality advice on restoring proper foot function.

Foot problems have reached epidemic levels and the solution is simple: Quality foot health education to help people fix their own feet.

There’s a big problem with modern footwear

The modern shoe is harming the human foot. Footwear companies are creating products to make money, not in the interest of foot health and its slowly killing our feet. We’re here to spread the truth about footwear.

Most footwear today has an elevated heel, narrow forefoot and a slab of foot numbing cushioning between your foot and the ground below you.

Your feet are magically designed body parts with the primary purpose of sending your brainsignals about the ground below you. When they get compressed and are prevented from sensing the ground because of cushioning, they lose their ability to function and create nasty upstream effects for our bodies.


The kind of shoes you wear daily, especially the type of shoe you train in, affects how your body functions in skiing. Cushioning and cushioned insoles are especially bad. This is a recent post on the TheFoot Collective.

THE DANGER OF HEELED FOOTWEAR
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wearing a shoe with an elevated heel might seem harmless but it has real effects on your posture upstream. These postural changes change how your body moves by making certain muscles more dominant (quads especially) and others weak (glutes)
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Over time, heeled footwear is a big culprit for knee problems and tight ankles so avoid them whenever you can. Finding a zero drop flat shoe can be quite difficult but taking the time to find one makes a massive difference in your joint health and movement patterns
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Most modern day running shoes and dress shoes have this nasty heel lift so beware of the consequences and transition to zero drop barefoot footwear. Your body will thank you
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I have been testing different brands of minimal shoes; zero drop, thin flexiable, low resilency soles, for the past few months and will posting on this issue soon. For reasons I will explain in future posts, it appears as if a small amount of positive toe down ramp (aka drop) – approximately 2.5 degrees, is important to a strong stance in skiing. But my regular footwear is all minimal, zero drop.

STANCE HACK: TUNE UP YOUR FEET

Biohacking Your Body with Barefoot Science

“…… hacking” or finding a way to more efficiently manipulate human biology.  This can include areas of sleep, nutrition, mental health, strength, recovery. (1)
– Dr. Emily Splichal – Evidence Based Fitness Academy

 

Last ski season, I developed some simple cues or hacks to help skiers and racers quickly find the body position and joint angles required to create the pressure under the outside foot with which to impulse load the outside ski and establish a platform on which to stand and balance on through the turn phase –  THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: IMPULSE LOADING

The primary source of information that helped me develop these cues are the exercises developed by Dr. Emily Splichal. Her exercises also helped me to appreciate the extent to which traditional supportive footwear with raised heels and cushioned soles has damaged my feet and deadened the small nerves responsible for maintaining upright balance and the ability to initiate precise movement. Since implementing Dr. Splichal’s evidence based science, I am not only skiing at a level beyond what I considered possible, I am starting to walk naturally for the first time in my life.

The information contained in Dr. Splichal’s videos will challenge everything you know or thought you knew about what we have been conditioned to believe about our feet and the footwear we encase them in. Contrary to what we have been told, cushioning under the feet does not reduce impact forces on the lower limbs and protect them. Instead, it actually increases impact forces while slowing what Dr. Splichal refers to as the time to stabilization; the time required to stabilize, stiffen and maximally protect the joints of lower limb from impact damage – THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: TIMING OF EDGE CHANGE

The Best Surfaces to Train On

A good place to start is to learn which surfaces are best to train on. Again, while it may seem logical and intuitive that surfaces with cushioning are best because they will protect the body from shocks, studies show the exact opposite to be true. Over time, support and cushioning in shoes can diminish the sensitivity of the rich small nerve matrix in the feet that acts as a neural mapping system for balance and movement. In her YouTube video, Best Surfaces to Train On (https://youtu.be/gvJjIi3h1Bs), Dr. Splichal discusses the effects of different surfaces on plantar small nerve proprioception and explains how barefoot training is a form of small nerve proprioceptive training designed to activate the plantar foot. Balance training is best done barefoot.

The Power of Plantar Proprioceptors

Watching Dr, Splichal’s webinar presentation Understanding Surface Science: The Power of Plantar Proprioceptors – https://youtu.be/t5AU-noqMFg will further your appreciation of the power of plantar proprioception.

First Stance Hack – Plantar Foot Release for Optimal Foot Function

Dr. Splichal’s 6 Minute Plantar Foot Release for Optimal Foot Function – https://youtu.be/zyrKgFwsppI will dramatically improve foot function.
Dr Splichal explains how to use RAD rollers (golf ball or other firm balls will also work) to optimize foot function by releasing tissues in the plantar foot by applying pressure to the 6 areas shown in the graphic below.
Dr. Splichal advises to focus on using a pin and hold technique  (not rolling the foot on the balls) to apply pressure to these 6 spots on each foot holding for about 20 seconds on each spot with each of the three different sized rounds for a total time of about 6 minutes. The foot release should be done 2 times and day and prior to each training session.
In my next post I will talk about the second Stance Hack: Pressing Down on the Big Toe to Impulse Load the Ski and Power the Turn

1.  https://barefootstrongblog.com/2017/04/28/biohacking-your-body-with-barefoot-training/

REVISIONS TO CORE CONCEPTS

Due to recent interest in CORE CONCEPTS on the HOME page menu (above), I have started to revise it. The current version appears below. Links to a number of pertinent papers and videos by Dr. Emily Splichal (Evidence Based Fitness Academy – EBFA ) have been appended to CORE CONCEPTS.


As bipeds, we propel our bodies forward by moving from one fascially tensioned base of support with foot to core sequencing on one foot to another fascially tensioned base of support with foot to core sequencing.

Skiing uses the same basic pattern. In skiing, we need to establish a fascially tensioned base of support with foot to core sequencing on one foot in order to be able to move with precision to another fascially tensioned base of support with foot to core sequencing on another foot. As far back as the 70’s, the famous French ski technician, Patrick Russell, said that the key to effective skiing is to ‘move from ski to ski’. What Russell was really alluding to is the process of alternating single limb support.

Ever since alpine skiing became formally established, it has been known that the best skiers move from the outside ski of one turn to the outside ski of the next turn. Although this may sound simple enough, the key to being able to effectively move from ski to ski (foot to foot) is the ability to establish a fascially tensioned base of support with foot to core sequencing one foot and then use it to move the body or Centre of Mass to the new outside foot (current uphill ski) of the next turn. Good skiers do this so seamlessly that turns seem to have no beginning or end. The turns just flow together. When viewed in the context of stance and swing phases, the resembles to walking becomes apparent,

How to make skiing as intuitive as walking is what this blog is about. I devoted an entire series of patent to this subject commencing with US Patent No. 5,265,350 and associated international patents on the elements of a minimal ski boot necessary to accommodate the process of establishing a fascially tensioned base of support with foot to core sequencing on one foot and transitioning seamlessly back and forth between bipedal and monopedal stances.

The ability to balance multi-plane torques on the outside leg of a turn is, and continues to be, the secret of the worlds’ best skiers including Toni Sailor, Nancy Greene Raine, Pirmin Zubriggen and, today, Mikaela Shiffrin, Lindsey Vonn and Ted Ligety to name but a few.


A REVIEW OF GAIT CYCLE AND ITS PARAMETERS – Ashutosh Kharb1, Vipin Saini2 , Y.K Jain3, Surender Dhiman4 – https://ijcem.org/papers72011/72011_14.pdf

Dynamic loading of the plantar aponeurosis in walking – Erdemir A1, Hamel AJFauth ARPiazza SJSharkey NA. – https://www.ncbi.nlm.nih.gov/pubmed/14996881

Active regulation of longitudinal arch compression and recoil during walking and running – Luke A. KellyGlen Lichtwark, and Andrew G. Cresswell – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4277100/

The Foots Arch and the Energetics of Human Locomotion – Sarah M. Stearne, Kirsty A. McDonald, Jacqueline A. Alderson, Ian North, Charles E. Oxnard & Jonas Rubenson – http://www.nature.com/articles/srep19403

Shoes alter the spring-like function of the human foot during running – Kelly LA1, Lichtwark GA2, Farris DJ2, Cresswell A2. – J R Soc Interface. 2016 Jun;13(119). pii: 20160174. doi: 10.1098/rsif.2016.0174. – https://www.ncbi.nlm.nih.gov/pubmed/27307512


The Science of the Human Lever: Internal Fascial Architecture of the Forefoot with Dr. Emily Splichal – https://www.youtube.com/watch?v=_35cQCoXp9U

SKIING: FEET FIRST

The processes of balance that configure the joints of the lower limbs originate in the feet in what is referred to as a bottom-up process.

The following text is excerpted from my US Patent No. 5,265,350 which was published in the major developed countries of the world on or about February 3, 1993. I have added emphasis with bold text to highlight key statements

The foot articulates in order to facilitate muscle function. Muscles respond in opposition to loads imposed upon the foot. A process ensues wherein the chain of articulations, initiated at the foot, are continuously mobilized so as to maintain a state of dynamic balance across the plantar-ground interface.

COMMENT: Structures of a ski boot that interfere with or otherwise impede the articulation of a joint or joints will reduce or even prevent a muscle or muscles from effectively contracting and shortening.

Bipedal function or bipedal stance, in the context of the invention disclosed, is defined as being a weight-bearing state wherein the feet are neither supinated (rolled outward) or pronated (rolled inward). This is described as a “neutral” state of the foot. It is usually associated with weight-bearing on two feet wherein each foot bears an equal proportion of the weight of the body.

Monopedal function or monopedal stance is defined as being the state achieved at the conclusion of a progressive weight transfer from two feet to the medial aspect of the plantar surface of one foot. As the weight transfer occurs, the foot to which the weight is being transferred undergoes pronation until a physiologic state of balance is achieved on one foot. Monopedal function distinguishes itself from other possible states of balance on one foot in that the ability to mobilize the joints of the body required to re-orient the centre of mass relative to the foot is possible while simultaneously maintaining a state of balance in relation to the forces acting on the user.

As an example, one can bear weight on one foot without having that foot pronate and, thus, not assume the position required for monopedal function, thus there is no significant inward movement of the ankle bone. This is done by shifting the weight to bear on the lateral aspect of the foot, and using the extrinsic leg and intrinsic foot musculature to support its medial arch. However, this form of balance produces a relatively static position in terms of ability to re-orient the centre of mass of the body relative to the supporting limb. This static quality is typical of states of balance on one foot achieved by other than monopedal function. With monopedal function, medial movement of the inside ankle bone is involved.

Monopedal function is a physiologic state wherein balance is achieved with the weight of the body borne on the medial plantar aspect of one foot. It has been recognized that the ability to balance on one foot (usually the one to the outside during a skiing or skating turn) is superior, in terms of balance and control, to balance on two feet, in sports such as skiing and skating wherein an instrument such as a ski or ice blade is affixed to the sole of the footwear. Monopedal function is extremely relevant in such applications for the following reasons:

(i) Balance on one foot, achieved through pronation, provides superior control of the articulations over balance on two feet. This translates to superior control of the ski or skate blade. It also translates into superior dynamic or kinetic balance. The mechanics of monopedal function permit the centre of mass of the body to be accurately placed and its relative position maintained, if necessary, with regard to the ski or skate blade affixed to the sole of the footwear.

(ii) A dominant position on the outside foot in the arc of a turn affords more efficient and precise control of the instrument since the inner limb, being relatively passive, is utilized primarily for the purpose of assisting balance.

(iii) The most important source of rotational power with which to apply torque to the footwear is the adductor/rotator muscle groups of the hip joint. In order to optimally link this capability to the footwear, there must be a mechanically stable and competent connection originating in the plantar processes of the foot and extending to the hip joint. Further, the balanced position of the skier’s centre of mass, relative to the ski edge, must be maintained during the application of both turning and edging forces applied to the ski. Monopedal function accommodates both these processes.

(iv) In skiing, the mechanics of monopedal function provide a down force acting predominantly through the ball of the foot (which is normally almost centred directly over the ski edge). In concert with transverse torque (pronation) arising from weight bearing on the medial aspect of the foot which torque is stabilized by the obligatory internal rotation of the tibia, the combination of these forces results in control of the edge angle of the ski purely as a result of achieving a position of monopedal stance on the outside foot of the turn.

(v) The edge angle can be either increased or decreased in monopedal function by increasing or decreasing the pressure made to bear on the medial aspect of the foot through the main contact points at the heel and ball of the foot via the mechanism of pronation. As medial pressure increases, horizontal torque (relative to the ski) increases through an obligatory increase in the intensity of internal rotation of the tibia. Thus, increasing medial pressure on the plantar aspect of the foot tends to render the edge-set more stable. The ski edge-set will not be lost until either the state of balance is broken or the skier relinquishes the state of monopedal function on the outside ski.

PROBLEMATIC FEET AND LEGS

In my post, THE IDEAL SKIER’S FOOT AND LEG, I described the characteristics that I observed over the years that were consistently associated with the feet and legs of the best skiers and racers. When I first started to see this pattern I didn’t understand why these characteristics were associated with superior technical ability. What I did come to understand very quickly was that skiers with feet and leg shapes that were less than ideal had difficulty skiing without major modifications to their ski boots. The images below compare the ideal foot and leg shape to foot and leg shapes that are increasingly problematic. The dashed line indicates the top of the sides of the cuff of the ski boot shell. The vertical hash marks compare the width of the cross-sectional area of the ideal leg at the top of the sides of the cuff to foot and leg shapes that progressively less than ideal.

Foot and leg types

 

As the cross-sectional area of the legs becomes increasingly larger, it becomes increasingly difficult to accommodate the leg within the confines of a boot cuff. In some cases, leg shapes make it difficult to even close the cuff buckles without extending the bales or re-locating the buckles. These types of fleshy legs are mostly associated with females although some males have the extreme shape depicted in the righthand image.  Females with wide hips tend to have tibias that are either straight or angle inward as shown in the sketch below. This can be a big problem if the cant angle of a boot cuff cannot be adjusted sufficiently to obtain a neutral cuff alignment with legs. Boots with no cuff adjustment, like the old Lange XLR, had cuffs that were canted outward 3 degrees. This meant that female racers with tibias that were straight or angled inward would be hard on their inside edges in events like downhill if they tried to relax and let their skis glide.

Female with wide hipsSome females have tibias that angle inward and fleshy legs. Since the rear spoiler of the boot cuff determines the angle of (dorsi) flexion of the ankle joint, skiers with large calves and tibias that are straight or angle inward have too much forward lean and the wrong cuff cant. When I worked with female racers in the late ’70s and early ’80s it typically took a lot of ingenuity and a lot of work to come up with a solution.

The problem with ski boots is that the shape of the lower part of the shell and the shape of the cuff are usually designed to interface with each other in a specific configuration. This limits the ability to align the cuff in a different position with the lower shell. It was Alan Trimble, the boot tech for Lange USA, who taught me how to make cuts in the shell bottom where it interfaced with cuff, position the cuff in the desired orientation then rivet the two pieces together. Lange was one of the few boots that allowed for this kind of modification.

When I worked with Langes I had a supply of boot parts with no holes drilled in the cuffs. This made it easier to assemble boots in non-stock configurations. The soft Lange fabric liners with fit pockets made it easy to remove padding that was interfering with ankle-leg movements. A common complaint was pressure on the inside ankle bone and even along the inner aspect of the foot below and in front of the ankle. I got very good at stretching the shell wall in this area. I even had special tools made for this purpose. From feedback from racers, I came to know that it was important to not have any pressure on the inner aspect of the ankle and the area around it. But it took me years to understand why. Here is a short video clip that shows the movement of the ankle and leg that is fundamental to the technique racers such as Ligety and Shiffrin use. In a future post I will explain why and how this works.

 

 

REVELATIONS FROM THE FIS GS SKI RULING

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

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

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

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

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

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

 

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

SOLE ADJUSTMENT

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

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

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

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

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

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

Fig 70-72

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


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

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