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.

7 comments

  1. Hi
    Many thanks for replying to my questions.
    It will help me take a step by step process in setting up my boots.

    Shell work first
    Cuff angle adjustments with stock insole
    Adjustments to ramp angle as it is at present 4°so may lift the toe of zeppa?

    When standing on a flat floor my knees track straight .
    The big toe is straight on both feet .

    Lower legs curve/bow outwards.

    Thanks
    Eddy

    1. When standing on a flat floor my knees track straight.
      > In an erect stance on 2 feet with weight equally distributed on each foot, the knee caps should align facing forward. As you flex the ankles and move forward in the waist, both knee caps should track forward.

      The big toe is straight on both feet.
      > Perfect.

      Lower legs curve/bow outwards.
      > The lower third normally typically angle inward towards the feet (tibial varum) on males, some more than others.

  2. Hi
    I have been reading many of your posts and I am going to follow them to help set upmy boots especially paying attention to making room for the foot to function inside the ski boot.
    What are your thoughts on solecanting ?

    1. “What are your thoughts on solecanting ?”

      That’s an excellent and very pertinent question. I don’t think in terms of canting as a good or bad thing. Instead, I think in terms of end objectives; what do I want to achieve, how will the proposed course of action affect the function of the end user and what are the circumstances driving my options?

      When plastic ski boots were first made, the designers were aware that the leg angle of the lower part of the average male leg angled inward (varum) about 3 degrees. So boots were designed with the shaft canted outward in the area of 3 degrees. Soon after I started modifying ski boots, it became apparent that some skiers, especially females, could not stand flat on the floor on the soles of the ski boots because the shaft of the boot was canted at the wrong angle for their legs. Since it was not easy to change the angle of the shaft, the only viable solution was to cant the sole with shims. Warren Witherall offered a viable solution with sets of canted shims that could be put under bindings or on boot soles. Later, some techicians started planning the boot sole at a cant angle.

      I experimented with cants with mixed success. When I became aware that Stenmark had cants on this boot soles high side out, I knew what he was doing and why. Some coaches had noted that racers who were knockkneed were better technical skiers than those whose knees were aligned straight. Stenmark was everting the angle of the soles of his feet in what amounted to pre-pronating them in a manner that potentiated pronation. This literally gave Stenmark an edge over his competition.

      As to your question, “What are my thoughts on sole canting?”, I don’t have any thoughts or position one way or the other on sole canting. The human foot has a multiaxial joint system in the ankle foot complex so the tripod base of the foot can sit flat on a supporting surface. I can see no reason, given adequate shell clearances, why the foot should not sit flat in a ski boot. However, this not rule out the use of sole canting under some conditions.

      What would be achieved by solecanting?

      1. Hi

        Would sole canting improve?

        Better balance
        To stand flat on the skis
        Improve tracking of the leg

        I have very curved lower legs I would say more than average and when standing in my ski boots my knees seem to track inwards and that I feel is making me consider sole canting.

        My forefoot width is 110 mm and in the past I have been crammed in to 98mm shells with no adjustments.

        Would one make adjustments to the shell first ski and test ?
        Then cant if needed ski and test ?
        Or make adjustments to shell and cant at the same time ski and test?

        How do they measure to see if one may need canting ?
        On snow testing I feel would be needed to check results.
        I can get the shell adjustments done easy on my current boots nordica gp 130
        And adjust the cuff outwards a little more to try and match the shape of my lower leg.
        But its a long drive to have the canting checked and if wrong to refix a problem.

        To check if needed they seem to use a straight edge towards the top of the tibia to center of boot as a stating point ?

        Regards
        Eddy

      2. Would sole canting improve?

        Better balance
        > It Depends
        To stand flat on the skis
        > To a degree

        Improve tracking of the leg
        > Unlikely. Knee tracking is controlled by muscles. A number of things affect this. The natural alignment of the feet is slightly toed out in an erect stance on 2 feet; becoming aligned straight ahead and parallel during dorsiflexion. When feet are significantly toed out, it is usually caused by imbalanced rotators with the external rotators shortened and the internal rotators lengthened. When the feet are forced parallel, the knees and pelvis will not be in their natural position. A big toe turned outward, (Hallux Valgus) will cause what is called knee kick where the affected knee turns inward. A professional assessment is a good start.

        I have very curved lower legs
        > I assume you mean the lower part is angled inward. This is called varus.
        I would say more than average and when standing in my ski boots my knees seem to track inwards and that I feel is making me consider sole canting.
        > What about standing on the floor with your feet parallel? It is normal for the knees to track inward in ankle flexion. A good kineosologic assessment can identify issues and the cause and usually prescribe correct exercises.

        My forefoot width is 110 mm and in the past I have been crammed in to 98mm shells with no adjustments.
        > Not good. I always start with natural alignment of the big toe and adequate width across the forefoot.

        Would one make adjustments to the shell first ski and test?
        > I always start with the shell then work my way through the liner. Ski testing is for confirmation and fine tuning.
        Then cant if needed ski and test ?

        Or make adjustments to shell and cant at the same time ski and test?
        > Usually shell first. But if the cant is way off as indicated by standing on 2 feet in the shells only, I make a quick coarse adjustment. Shell interference with the foot can affect cant angle adjustment.

        How do they measure to see if one may need canting ?
        > I start by having a skier stand on both feet in bare shells with the shells parallel and feet under the hips in their natural position. I want to see equal space on either side of the legs at the top of the boot shaft. Most boots have cant adjustment means on both sides. This usually gives sufficient adjustment for most legs. Once the boot and liner are ready, I put the boots together, have the skier put them on and do tests to see if they can step on to one foot (in the boot) and balance quickly and easily. If this test is good, I have them ski test to confirm the setting. If issues are indicated, I make small cant adjustments and have the skier test again. I sometimes find adjusting in what seems to be the wrong direction is a better indicator because it is usually obvious that it is wrong.

        On snow testing I feel would be needed to check results.
        > Yes. Do the shell shaft cant test and let me know if you cannot get equal space on your legs.

        To check if needed they seem to use a straight edge towards the top of the tibia to center of boot as a stating point ?
        > Not in my books.

  3. Great post!! Another great help in my adventures of trying to help others!!

    In one PSIA clinic I was critiqued for initiating my turns with the ‘big toe’. My misalignment was such that I finished the turn with heel pressure and never thought twice about it because that’s what we were taught to teach. I never ran and rarely hiked because of a knee injury so I couldn’t transfer non skiing athletic activity to skiing. I do remember one hike which I finally figured out that placing my toes down first helped the knee going down hill but it never clicked in that what made that almost impossible was the high heel of my hiking shoes so never ‘ran’ with it. After reading “Born to Run” I tried to run on flat land with little success; the high heels (excessive ‘toe drop’) requires a lot of energy and strain on the system especially when one hasn’t run in years. Some tweaks forced me to unbuckle my upper cuffs of the ski boots and remove one foot bed which cured the tweaks and allowed me to keep skiing. Then came this site from which I learned how to systematically apply foot wear fitting techniques to free the feet and have been able to function at the highest level skiing and in ‘real’ life ever since the injury with the least amount of wear, pain, and tear! The forefoot emphasis related here IS unique and very key to great skiing because it is combined with how to attain the ability to use that area of the foot which most skiers can’t crammed into too small ski boots by conventional methods. I never could!!

    I’ve ‘crowed’ in the past how great this info is and only repeat this for ‘newbies’ who may not have read those comments. David’s methods really work; more scientific than any other foot wear fitting system I’ve ever come across because one tests at every stage for function/interference.

    Thank you very much for sharing all this info, this helps revolutionize footwear world wide!!

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