custom orthotics

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

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.

INSOLE/FOOTBED/ORTHOTIC EFFECTS

The big epiphany I had about 1975, was that the foot needed to be supported in the new plastic ski boots and that it was a lack of support that was causing my difficulties skiing after switching from low cut leather boots to the new higher, rigid plastic boots.

Back then, I was an avid runner. As best I can  recall, it was an article in Runner’s World on running injuries caused by over-pronation that served as a catalyst for my conclusion that the foot needed to be supported in a ski boot. I assumed that what was being reported in running magazines was both factual and derived from science-based investigations. While I had not found anything in the literature that suggested that the foot needed to be supported in a ski boot, it seemed logical to me that if runners needed support in their shoes, the need for support in a ski boot was many times greater. But my conclusion was based on the assumption that over-pronation was a pathology and that it was a proven cause of injuries in running. Therefor, it was also a problem in skiing.

Pronation and over-pronation was a new concept to me in 1975. My running partners and I all ran in flats with no arch support. None of us had ever heard of, let alone experienced, knee pain or the myriad of other problems that were fast becoming an integral part of running and were claimed to be caused by overpronation.

Soon after I read the article on overpronation, I made an appointment with a podiatrist in Vancouver to have my feet examined. I was hopeful that he would find the defect(s) in my foot that were causing me difficulties in skiing in the new plastic boots. But after a thorough examination, he pronounced my feet healthy and normal. Undeterred, I made an appointment for my wife and I with a well known sports podiatrist in Seattle, Washington, almost 800 miles round trip to and from Whistler. We made a special trip to Seattle to have prescription orthotics made for our ski boots. But far from helping, they made both of our skiing worse, much worse. Still, I remained convinced that the foot needed support in a ski boot.

Between 1977 and about 1983, I made a lot of footbeds for ski boots. From the first pair of footbeds I made, I received positive feedback. Skiers loved them. Some skiers told me they would never ski again without the footbeds I made for them. Even today, I encounter skiers who are still using the same footbeds I made for them 40 years ago. Did this subjective feedback serve as evidence that my footbeds made skiers ski better? No.

By about 1989, I was still unable to understand why I was continuing to experience difficulty skiing even after trying numerous pair of plastic ski boots. At that time, I was struggling to invent and patent a ski boot based on sound principles of functional anatomy

I finally came to the realization that the only way to arrive at meaningful conclusions about how the human system should ideally function in skiing was to design and fabricate an open-architecture research vehicle, one that minimized any neural noise that was unavoidably caused by interference with the physiologic function of the user by structures of the conventional ski boot. It had become apparent to me that it is the level of ‘neural noise’ and interference to physiologic function caused by a tightly fitting ski boot that prevents anyone from proving how a ski boot affects a skier.

The Birdcage allowed the capture of data during actual ski maneuvers that showed how some of the world’s best skiers skied and especially what happened when specific joint actions were interfered with.

It was was also about 1989 that I was starting to question how an insole or orthotic fit to one ski boot could produce the same result in a different ski boot or with skis with different sidecuts, especially width underfoot and different lift heights of the sole of the foot above the surface of the snow. I was also starting to question how the same stock or custom insole or orthotic could produce the same effect when used in different shoes. A custom insole for a female might be used in casual shoes, flats, running shoes, walking shoes, hiking boots and even spiked, high heel shoes. And what happens to the effects produced by an insole or orthotic when the sole of the shoe it is used in wears unevenly?

It was obvious to me, and should be obvious to anyone, that it is impossible for an insole or orthotic to consistently produce the same effect in widely varying footwear that each affect the foot in a different way. 

Despite the many questions I was having about insoles and orthotics, I continued to believe that they had value in some applications. In the years following the Birdcage tests of 1991, my wife had two different pairs of prescription orthotics made, both by reputable labs, for issues with hip and back pain. Neither pair provided any perceivable benefit. Both pairs were eventually discarded.

The best skiing experience today for my wife and I is with perfectly flat insoles and boot boards that provide no perceivable interference with the dynamics of the arches of our feet. Even the slightest impingement is immediately perceived. All of the shoes I wear have either flat insoles or no insoles. If I purchase a shoe with an insole with arch support, I modify it to remove the support.

 

 

WHAT DO INSOLES AND ORTHOTICS DO?

According to Benno Nigg, no one knows for sure. From 1981 until he retired recently, Nigg founded and was the Director of the Human Performance Laboratory (HPL) at the University of Calgary in Calgary, Alberta, Canada. The Human Performance Laboratory is a multi-disciplinary research centre concentrating on the study of the human body and its locomotion. From 1971 until 1981, Nigg was the Director of the Biomechanics Laboratory at ETH Zurich (Swiss Federal Institute of Technology).

For more than 30 years, Nigg studied the effects of insoles and orthotics on the lower limbs. What he found was that most of the time they didn’t do what was claimed. Often, the effect of the same insole or orthotic varied greatly from one subject to another even though they had the same condition. In some cases, Nigg found that orthotics had a large effect on muscles and joints, increasing muscle activity by as much as 50% for the same movement while increasing stress on joints by the same amount as the body fought to overcome the effect of the orthotic. Nigg also found that “corrective” orthotics do not correct so much as lead to a reduction in muscle strength. He details his findings in his book, Biomechanics of Sports Shoes. The book can be ordered from NiggShoeBook@kin.ucalgary.ca

If no one knows what insoles and orthotics in footwear affect the user, how is it possible for anyone to know insoles and orthotics in ski boots affect skiers? I am not taking about claims made for insoles and orthotics made for ski boots. I am talking about how they affect the skier during ski maneuvers as confirmed by on-snow studies. The pivotal issue is how the CNS manages, or isn’t able to manage, the forces across the inside edge of the outside ski in a turn. This is what any claims should focus on. But I have yet to find evidence that any studies to this effect have been done.

You’ve been to a ski boot-fitting shop or perhaps a foot professional and had custom insoles or orthotics made for your ski boots. You may have been told that these interventions will create a specific alignment of your knees with some aspect of your feet.  You may have also been told that your feet pronate or over-pronate and that insoles or orthotics will correct these issues. In addition, you may have been told that you will ski better with the insoles or orthotics or an expectation was created that you would. This expectation may have been reinforced by the fact that you probably felt very different standing in your boots with the insoles or orthotics fit to them than you did without them.

Out on the ski hill with your boots and skis on you probably also felt different than you did without your new insoles or orthotics. But are you skiing better? You might think you are, especially after paying several hundred dollars or more. But how do you know for sure? You don’t. Unless the person who made your custom insoles or orthotics instrumented your ski boots and captured data during actual skiing both before and after the insoles or orthotics were installed and then compared the data sets to peer reviewed, independent studies that provided compelling evidence that the data captured during skiing conclusively demonstrated a positive effect of the insoles or orthotics on your skiing, any claims made were speculative and any conclusions, subjective. More important, claims tend to be biased because a product is associated with them.

You are probably thinking that none of this matters because there is an abundance of science in support of custom insoles and orthotics. But in a  New York Times article, Close Look at Orthotics Raises a Welter of Doubts – January 17, 2011 (http://www.nytimes.com/2011/01/18/health/nutrition/18best.html?pagewanted=all), Benno Nigg looked critically at insoles and orthotics. His overall conclusion? Shoe inserts or orthotics may be helpful as a short-term solution, preventing injuries in some athletes. But it is not clear how to make inserts that work. The idea that they are supposed to correct mechanical-alignment problems does not hold up.”

In the same NY Times article, Scott D. Cummings, president of the American Academy of Orthotists and Prosthetists, acknowledged that the trade is only now moving toward becoming a science and that when it comes to science and rigorous studies, “comparatively, there isn’t a whole lot of evidence out there.” Dr. Nigg would agree. The proof that orthotics provide benefit? Some people feel better using them than not using them. So any evidence is in the form of highly individualized, subjective feel. What about skiing? Is claiming that the foot needs to be supported and/or especially that the foot functions best in skiing when its joints are immobilized in neutral, sufficient to claim a benefit or implied need for insoles or orthotics in skiing? Hardly.

The first thing to consider is that unless the load W from the central load-bearing axis is transferred to the inside turn aspect of the inside edge of the outside ski it is impossible for the foot to pronate. In addition, in this configuration, the outside foot cannot be ‘supported’ because there is no support in the form of a contiguous source of snow reaction force under the base of the outside ski.

When Lange introduced the world’s first all plastic ski boot in in 1962, biomechanical research on human locomotion was in its infancy. Biomechanical studies of sports shoes, including ski boots, were nonexistent. The first edition of Inman’s seminal work, The Joints of the Ankle, wasn’t published until 1976. What did it take for the new rigid plastic ski boot to be universally accepted? A few trips to the podium.

When running and jogging took off in the early 1970s, insoles and orthotics and were widely promoted in response to injuries that were erroneously assumed to be caused by excessive (over) pronation. Were there any studies to support this conclusion? No. Nor, was there any evidence that I am aware  to support the position of the proponents of insoles and orthotics that the foot needed or would benefit from support in ski boots. As far as I have been able to determine, the need to support the foot in a ski boot was and still is based on a widely accepted assumption. If pronation was a problem in running, then it had to be a problem in skiing. That made sense. Except that it didn’t. In the late 1980s and early 1990s, studies were showing that there was only minimal correlation between high pronation and high impact loading and typical running injuries. Nigg and other researchers suggested that no evidence was found because there was no evidence. Researcher had been trying to prove pronation was the cause of running injuries instead of trying to find the cause.

Two recent studies question the validity of the premise of supporting the longitudinal arch of the foot, especially in ski boots.

_____________________________

Dynamic loading of the plantar aponeurosis in walking http://www.ncbi.nlm.nih.gov/pubmed/14996881

BACKGROUND: The plantar aponeurosis is known to be a major contributor to arch support, but its role in transferring Achilles tendon loads to the forefoot remains poorly understood. The goal of this study was to increase our understanding of the function of the plantar aponeurosis during gait. We specifically examined the plantar aponeurosis force pattern and its relationship to Achilles tendon forces during simulations of the stance phase of gait in a cadaver model.
RESULTS:  Plantar aponeurosis forces gradually increased during stance and peaked in late stance. Maximum tension averaged 96% +/- 36% of body weight. There was a good correlation between plantar aponeurosis tension and Achilles tendon force (r = 0.76).

CONCLUSIONS: The plantar aponeurosis transmits large forces between the hindfoot and forefoot during the stance phase of gait. The varying pattern of plantar aponeurosis force and its relationship to Achilles tendon force demonstrates the importance of analyzing the function of the plantar aponeurosis throughout the stance phase of the gait cycle rather than in a static standing position.

_____________________________

For years, experts have claimed that skiing is done in the mid phase of stance in what is called the gait cycle. What the preceding study clearly shows is that the strongest stance in skiing in terms of the ability to transfer force to the head of the first metatarsal and functional stability of the structures of the foot occurs in the late phase of stance, not the mid phase. The graphic below provides a simulated representation of the sequence by which Achilles tendon force tensions the plantar aponeurosis and transfers large forces to the forefoot, especially to the head of the first metatarsal.

Foot Dynamcs 3

New studies are questioning the premise of supporting the arch of the foot with anything  because neural activity in the arch of the foot appears to  be potentiated by tension in the plantar aponeurosis and surrounding soft tissue. Rather than being a passive static entity in its role as a support structure for the superincumbent body, the arch is a dynamic, neurally charged system whose height changes in response to changes in perturbations in GRF that challenge the balance system.

_____________________________

Foot anatomy specialization for postural sensation and control http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3311689/

These findings show that rather than serving as a rigid base of support, the foot is compliant, in an active state, and sensitive to minute deformations. In conclusion, the architecture and physiology of the foot appear to contribute to the task of bipedal postural control with great sensitivity. Here, we show that the foot, rather than serving as rigid base of support, is in an active, flexible state and is sensitive to minute perturbations even if the entire hind and midfoot is stably supported and the ankle joint is unperturbed.

However, support of the body weight in the erect posture involves not only the counterbalancing of the gravitational load, but also equilibrium maintenance, which is dynamic in nature. Accordingly, somatosensory information on local foot deformations can be provided from numerous receptors in the foot arch ligaments, joint capsules, intrinsic foot muscles, and cutaneous mechanoreceptors on the plantar soles (Fallon et al. 2005; Gimmon et al. 2011; Kavounoudias et al. 1998; Magnusson et al. 1990; Meyer et al. 2004; Schieppati et al. 1995).

During standing, the foot arch probe and shin sway revealed a significant correlation, which shows that as the tibia tilts forward, the foot arch flattens and vice versa.

It is worth stressing that the foot represents an important receptive field, formed by numerous skin, joint, tendon, and muscular receptors (including intrinsic foot muscles), and it has long been recognized that damage to the foot, be it either by sensorineural loss or physical damage to the muscles, bones, or supporting tissues, changes posture and gait stability.

A number of cutaneous and load-related reflexes may participate in the fine control of posture or foot positioning during walking.

_____________________________

Almost any structure that provides even minimal support for the arches of the foot will prevent the arch from lowering and transferring force to the MTs and will  interfere with the function of the arch as an active, dynamic neuro-sensory mechanism.

Claims made for insoles and orthotics create a reasonable expectation in the consumer that what is experienced in an off hill controlled environment will also happen on the ski slopes. Terms of disclosure require that any claims  be qualified with statements like, “These claims have not been confirmed during actual ski maneuvers”.

 

 

 

PRONATION – WHAT SHOULD IT FEEL LIKE?

Given the widespread confusion and misunderstanding surrounding pronation and it’s role in skiing that seems to exist I am going to provide some drills that will teach you how to assume a functionally pronated position.

Functional pronation is specific to monopedal (one-foot) stance especially as it relates to the ability to assume and move from one dynamically tensioned base of support to another. Once you have a feel for the functionally pronated monopedal position you can go through series of drills standing in your bare feet on a hard flat surface. Next you can stand in your ski boots starting in the boot shell after which you can add the liner with no insole followed by the liner with an insole or custom footbed. By using the feeling associated with standing in your bare feet on a hard, level, flat surface and then comparing the sensations to standing in your ski boots on a hard, level, flat surface you can experience for yourself how the various elements around and beneath the sole of your foot affect your ability to assume a functionally pronated position or even stand properly on two feet. As a prelude to providing drills on how to assume a functionally pronated monopedal stance, I will provide a brief history of the events that contributed to my current position on pronation, footbeds and insoles in general in skiing.

In my initial years of modifying ski boots I was a big proponent of footbeds. In those days, my work on ski boots was very much aligned with conventional views of immobilizing and supporting the foot and leg. But my disastrous experiences with Dave Murray got me rethinking this. By the time I began working with Steve Podborski, I was moving in a direction away from conventional thinking. In 1980, I had a huge breakthrough with a in-boot technology for which I was later awarded a patent. This was the turning point at which  severed any association I had with conventional thinking in ski boots and started fresh with a clean sheet of paper; one that did not include any premises on which existing ski boots are based.

By 1991, when Steve Podborski and I  initiated a research program to test my hypothesis on the mechanics, biomechanics and physics of skiing, my thinking was so far from convention that I insisted on retaining two scientists to provide oversight on the project. This included reviewing everything I put in writing but especially my patent. This process was intended to ensure that the principles I was using were both sound and correct. One of the scientist was G. Robert Colborne, Ph.D, an expert in the biomechanics of the human lower limbs. After reviewing my hypothesis, the initial impression of these scientists was that if it were correct it meant the whole world was wrong. Because I was in uncharted territory it was critical to me to have my findings confirmed before going forward. Once the wheels of a new technology are set in motion and significant money has been invested, it is hard to change direction, and especially to reverse direction. For this reason, we did a series of on snow studies in 1991 on Whistler’s glacier to confirm my hypothesis. I will provide details of the results in future posts.

The image below shows the model engaged in quiet standing Bipedal stance. There four positions of Centre of Mass in relation to the feet with weight distribution as follows from 1 to 4:

1. Centre of Mass is just in front of the base of the shin. The heel which is carrying about 60-70% of the load of each foot. This represents the rearmost limit of Centre of Mass. Should CoM fall behind the base of the shin, a rearward fall will result.

2. Centre of Mass is in the proximate centre of the span of the longitudinal arch. The heel is carrying approximately 50% of the load of each foot. The balls of the feet are carrying the remaining 50% of the load. The ball of the great toe is carrying twice as much load as the other 4 balls of each foot. This position represents the most stable and efficient form of bipedal stance.

3. Centre of Mass is approaching the balls of the feet. The contraction of the muscles that plantarflex (push down) the feet is increasing. The balls of the feet are carrying the remaining 60-70% of the load of each foot.

4. Centre of Mass is almost over the balls of the feet. The contraction of the muscles that plantarflex the feet has further increased. The muscles that push the toes down are now contracting forcefully, pushing the toes against the floor. This is the absolute forward limit of Centre of Mass in quiet standing. The toes act as a fail safe by pressing down onto the support surface in what is called the Reverse Windlass Mechanism. This mechanism tensions the forefoot into a rigid lever in preparation for propulsive phase of gait. At this point, almost all the weight is being carried on the balls of the feet and the toes. Should CoM pass the balls of the feet without evoking plantarflexion of the ankle, a forward fall will occur.

Screen Shot 2014-02-10 at 10.03.33 AMBIPEDAL DRILLS

These should be done in bare feet on a hard, flat, level surface. Start with the second position.

Drill 1. Stand erect with your feet a natural hip width apart and with a small angle of flexion at the knee joint. Release any tension from your body and allow your feet to settle onto the surface of the floor. Do not consciously apply force with your feet. Tune in to the pressures in your feet and buttocks. Sway back and forth slightly using only ankle flexion. Find the point at which the weight feels even between your heels and balls of your feet. You should feel slightly more pressure under the ball of your big toe than under the balls of your other toes. This is normal. Look down at your knees. They should be aligned straight ahead.

Drill 2. Using only the ankle joint, press down on the balls of your feet until you feel most of the weight on under your heels. Do not go too far. This is the limit of the rearward movement of C0P. At this point you are on the verge of a backward fall. Look down at your knees. They should be aligned straight ahead.

Drill 3. Using only the ankle joint, release the pressure under the balls of your feet until you feel more of the weight on the balls of feet than your heel. Look down at your knees. They should be aligned straight ahead.

Drill 4. Using only the ankle joint, release more pressure under the balls of your feet until you feel the weight pressing down hard on the balls of your feet and your toes. Do not go too far. This is the limit of the forward movement of C0P. At this point you are on the verge of a forward fall. Look down at your knees. They should be aligned straight ahead.

FUNCTIONAL PRONATION MONOPEDAL DRILL

1. Start from position 2 above.

2. Move your Centre of Mass slowly towards whichever one of your feet you most comfortable and confident with.

3. As you move towards one foot allow your ankle and leg to relax and roll inward, towards the L-R centre of your body.

4. When you feel the pressure strongly under the ball of your foot move, allow the ankle to relax and your Centre of Mass to move forward into position 3 above. As this happens lift the other foot off the floor. You will feel a pronounced change in the tension of the gluteus muscles in same side as your support foot in your buttocks. If your foot is functionally pronated you will feel most of the pressure under the ball of your foot.

Congratulations. You have achieved functional pronation and a dynamically tensioned base of support. Now try putting insoles and arch supports under your foot and feet. Do the same drills and see what happens.

PRONATION: FORCES IN THE FEET – CONTINUED

Before I can discuss the role of pronation in enabling a skier to develop a dynamically tensioned base of support, I need to clarify the implications of a neutral foot.

The 2 big buzz words in boot-fitting are neutral foot and neutral alignment meaning that the knees track straight forward during ankle flexion.

The ankle complex consists of two major joints. The base of the tibia forms a joint with a bone called the talus. The resulting joint is called the tibial-talar joint, commonly referred to simply as the ankle joint. A second joint below the talus is the sub-talar joint. This joint underlies the tibial-talar joint. It allows the foot to rotate about its long axis in eversion and inversion. The tibial-talar joint is the joint that flexes the foot in plantar-flexion (toes move away from the shin) and dorsi-flexion (toes move closer to the shin).

TT -STJ

Due to the confusion and misinformation that has arisen surrounding pronation, a common perception exists that anything other than flexion of the ankle is abnormal and problematic. The solution is footbeds that maintain the foot in a neutral axis and alignment procedures typically follow intended to correct abnormal tracking of the knees and ensure that they move straight ahead on a neutral axis when the ankle joint is flexed.

None of these positions are supported in sound principles of biomechanics. But I will save further discussion of this issue for a future post. The image below shows the same neutral configuration of the feet in bipedal neutral stance as my last post. In this image I show the straight ahead (neutral) excursion trajectories of Centre of Mass and Centre of Pressure in each foot. I also show the axis of the ankle joint as square (at right angles) to the excursion trajectories. This does not happen in reality. But in order to differentiate between neutral Sub-Talar Joint and pronation I need to indulge the neutral camp.

Neutral axis

The limits of the Base of Support that define what is called the sway zone is shown in grey. Because of the large area of the Base of Support at one time or another some formulators of ski methodologies have advocated a wide track stance with  weight and steering on both skis because it is more stable than a narrow stance where the weight is predominantly on the outside ski of a turn. In fact, as I will show in future posts, a wide track stance is not only highly unstable, it precludes the ability to develop a dynamically tensioned base of support on which to move from one foot to another foot.

PRONATION AND THE BIOMECHANICS OF EDGE CONTROL

The stick man sketch below are Figures 23 A and 23 B from US Patent No. 5,265,350 (expired) awarded to the writer.  The stick man in FIG. 23 A is engaged in quiet standing with the weight equally distributed between the left and right feet. This is called Bipedal (two-footed) Stance. The force vector W emanating from the Centre of Mass or CoM  is the ‘disturbing force’ of gravity.  W is called a disturbing force because it is tending to disturb the equilibrium of the stick man and cause him to topple.

Bi-Monopedal

Gravity is an ‘attractive force’ like magnetism. CoM is where you are in relation to the supporting surface. In this case, ground. But W is not the force applied to ground by the stick man. The applied force occurs at the contact points of the foot or feet with ground.  In FIG 23 A,  W lies equidistant between the two feet in the transverse plane. In normal Bipedal Stance, each foot supports equal proportions of the bodyʼs weight W, assuming equal leg lengths. Approximately 50 percent of the load is borne by the heel. The remaining 50 percent is borne by the heads of the long metatarsal bones. The load on the head of the first metatarsal (aka the ‘ball of the foot’) is twice that of each of the heads of the other four metatarsals. The anteroposterior (ergo, front to back) distribution  of the load through the foot is due to the position of the CoM of the body above.  The point on the foot where the centre of the applied force appears to act is called the Centre of Pressure or CoP. I say ‘appears to act’ because CoP could lie somewhere in the vault of the arches of the foot. In Bipedal Stance CoP lies on an axis that runs through the proximate centre of the heel and the head (ball) of the 2nd metatarsal. In ice skates, this is the ‘balance point’ where the ice blade is mounted. The forces shown as w2 are the centres of the ground reaction force or  that opposes CoP.

The footwear industry’s dirty little secret is that shoes are made on lasts that approximate the shape of the human foot in Bipedal Stance; standing on two feet and not moving. When you start to walk in a shoe, the structures deform and distort to accommodate changes in the architecture of the foot. Ski boots are worse. Not only do they approximate shape of your feet and legs in quiet Bipedal Stance, they prevent the user from obtaining a dynamically balanced base of support on one foot. Claims in relation to skiing are made that the human foot functions best in skiing when its joints are immobilized, preferably in a neutral position. In a neutral position, joint actions of the foot and knee and hip are limited to flexion and extension with transverse and orbital movement of the leg in hip joint within its normal range of motion.

A whole industry has been established on methods of immobilizing the foot and stabilizing it in a neutral position with custom formed boot shells, custom formed liners and custom formed footbeds and orthotics that significantly restrict or prevent pronation. The indirect effect of preventing pronation is that the position of CoP on the axis running through the proximate centre of the heel and head of the 2nd metatarsal becomes fixed.  For reasons that will be explained in future posts, this can have the effect of preventing the user from being able to establish the over-centre edge control mechanics that the best skiers use and especially an inability to establish a dynamically stable base of support on which to move from ski to ski.