FIT VS. FUNCTION


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

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

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

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

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

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

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

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

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

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


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

 

10 comments

  1. Hi David et al,

    One of the biggest problems with the Intuition liner, which I skied in wrap and tongue style for about ten contiguous years, is the sole ! How do you establish a flat base to apply force to with 7-9mm of compressible foam which molds to your foot between the foot and bootboard? Intuition does make a race liner with a “strobel” sole (unpadded) for that reason. Even so, you still have to deal with all the compression and isolation from sensitivity everywhere else! Thermo foam or heat moldable EVA has its uses. Correct me if I’m wrong David but, the blue foam padding on the underside of the gray top cover of your wife’s tongue appears to be blue high density EVA. I believe this is the same tongue modeled in “Podborski’s Balancing Act”.

    David, are you suggesting that there is a pressure point on the instep where one might be able to apply enough force to hold the foot securely down and back in the boot while eliminating upward “slop” and allowing enough ankle and foot movement for proper function while skiing fast through all types of conditions. A sort of “trigger point” or a more general idea? And the sensitive circulation on top of the instep must not be obstructed. Are there limitations, such as Michael suggested, in deep heavy powder,etc.? I have been tempted to dig out my old Rosemounts and start carving and drilling for some kind of positive hold down in a boot. Or even modifying the tongue of a cabrio three piece boot with an adjustable instep hold down mechanism. Lots of work and time but, I feel, worth the effort if successful.

    It seems to all come down to the liners being the critical interface between the somewhat rigid plastic boot and the muscles and bones of the skier.

    Best regards to all,
    Herb

    1. Hi Herb,

      I apologize for the delay in responding to your comment. It’s obvious to mean that you have put a lot of thought into the issues you raise. You also come across as very knowledgeable.

      One of the biggest problems with the Intuition liner, which I skied in wrap and tongue style for about ten contiguous years, is the sole ! How do you establish a flat base to apply force to with 7-9mm of compressible foam which molds to your foot between the foot and bootboard?

      REPLY: Exactly. I open a medical text like Principles of Neural Science by Kandel and “Sensory Information is Necessary for the Control of Movement” literally jumps out at me. Sensory information is necessary for the control of movement FULL STOP. How about for the control of skis? Dr. E. Stussi, Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland says in The Shoe in Sport “…….the skier must be able to steer as well as possible, but must also have a direct (neural) feedback from the ski and from the ground (snow). So why do the makers of ski boots and liners insist on isolating and insulating the soles of the feet so they can’t receive sensory information?

      Correct me if I’m wrong David but, the blue foam padding on the underside of the gray top cover of your wife’s tongue appears to be blue high density EVA. I believe this is the same tongue modeled in “Podborski’s Balancing Act”.
      REPLY: Right and right. You know your foams. Finding the right foam is critical. I have suggested to others many times that it should be possible to design, frabicate, market and sell a dorsal tongue system that uses a non-hardening injectable silicone or other material that allows the presssure to controlled. No interest. It seems there is a lucrative market for foam injected liners that insulate the foot from plantar vibrations while adding an uncontrolled source of foreign forces to the mechanceptors of the ankle.

      David, are you suggesting that there is a pressure point on the instep where one might be able to apply enough force to hold the foot securely down and back in the boot while eliminating upward “slop” and allowing enough ankle and foot movement for proper function while skiing fast through all types of conditions.
      REPLY: Yes. But the issue is sufficient structure in the boot to resist any transient forces applied to it by the foot. In a recent post I discuss the Auto Stiffening mechanism which as far as I know I discovered. The issue is keeping the Auto Stiffening mechanism engaged which is why Podborski could compete and win with a partially healed knee. Again, it is a complete mystery to me why others have failed to recognize compression-decompression oscillation that occurs from perturbations in GRF. This stresses the hell out of the knee!

      Are there limitations, such as Michael suggested, in deep heavy powder,etc.? I have been tempted to dig out my old Rosemounts and start carving and drilling for some kind of positive hold down in a boot. Or even modifying the tongue of a cabrio three piece boot with an adjustable instep hold down mechanism. Lots of work and time but, I feel, worth the effort if successful.
      REPLY: At this point I don’t see the ski industry stepping up and addressing you raise or research being done to study the issues. So it has come down to those who want max performance to roll up their sleeps and do the heavy lifting. My role now is to offer is to provide information and offer suggestions.

      1. Thank you for your responce David.

        So, the boot must be able to hold the foot in the compressed state at all times, and not “decompress” during the transition phase of the turn or over terrain, for example? Thus, the test for forefoot hold would be to put a ski or weight on with the boot and lift. The sensation of lightness should remain with the added weight.? Ski to confirm, of course.

        I believe the Zip-fit tongue has a separate fore tongue bladder but, I haven’t found anyone who will sell me just the tongue of any liner. One would, probably, have to convince Sven Coomer that it was commercially viable. Like the footbeds he has marketed for years.

        Thanks again for the help!

      2. Thank you for continuing to pursue these important issues which the industry has so far failed to address.

        It was about this time in 1980 that I was riding an ancient double chair chair lift on Whistler when right foot slipped off the foot rest and dropped down hard causing the mass of my boot and ski to take up the slack between the dorsum of my foot and the top of the shell of the ski boot sending a hard shock to my knee. Up to this point I was unaware of the crash space between the dorsum of my foot and boot tongue and the top of the shell.

        When I got home I put on my boots, set the buckles to the normal position then undid the shaft buckles, firmly grasped the top of the tongue and pushed down. I could push it down about an inch of free before I felt firm pressure on the dorsum of my foot. It was then that I realized this had to be happening while skiers were skiing. When the foot lifts off the base of the boot the tension in the muscles is released as is what is called tonus. Worse, the arch decompresses and recompresses in a sine cycle. I thought it reasonable that this had to be stressibng the knee and making it vulnerable to injury in the decompressed state. My theory turned out to be correct when Podborski was able to ski and win less than 4 months post ACL reconstruction with the dorsal loading system I invented but not with a regular tongue.

        In retrospect, what I discovered back in 1980 should have been drop dead to anyone involved in the design of ski equipment. But even after Podborski did the impossible there was no interest in even knowing why. I can only do so much. At some point there has to be movement to get issues like arch compression-decompression cycling addressed.

        The tongue test is a good pre-ski test.

    1. Loading the dorsum of the foot is far more complex than a simple matter of placing padding over the area. In the loading phase the dorsum of the foot can apply high forces to the foot in response to the transfer of achilles force to the forefoot. The dorsal loading system needs to be able to provide reaction force. It’s not a good idea to just do stuff with ski boots unless one has a very good idea of what to do and how it will affect skier performance.

  2. The intuition liners are very modifiable. Once molded, the exterior cloth can be cut away and they are easily modified with a grinder like would be used for grinding foot beds.

    1. I have checked out the Intuition liners including a thin version. As with pretty much every liner I have investigared here are too many issues to make them a viable option. Ideally liners should not offer any interference with the essential joint actions and especially reception of sensory information.

  3. Hoping that your next post will have some solutions that allow for modification of existing products in order to acheive the result you describe. Is this even possible? I await the next article eagerly.

    1. With rare exceptions I have been able to create a functional environment for the foot and leg in existing products. The single biggest problem today is the very ability to successfully modify liners.

      Comment from Michael below (I am adding a comment sent to me from a follower who is experiencing problems posting comments with his PC.)

      “The Birdcage research vehicle is the barefoot minimum standard for the ski boot.”

      I’d have to argue the “minimum standard!” After reading your blog for a while, I believe around 2014 i first did the ridiculous and removed the portion of the liner around the toes. I tried it out with great fear on a below zero Fahrenheit day. My toes were the warmest ever and it skied even better than that:) So I kept whittling but never reached the point of Steve Podborski’s liner you pictured in one of the posts. Not really knowing what I was doing and feeling there had to be a better way and since you reported that “The Birdcage” skied very well, i went back and looked at the key points of contact that seemed to hold the leg in the ‘cage’ and carved things out in between on my liner. To be honest it worked fantastically, the only part I’m not satisfied with is the ‘instep pressure device’ which I haven’t had luck in replicating.

      So my opinion is that the Birdcage represents the MAXIMUM leg/ankle/foot contact needed to function the best on a pair of skis without the interference that ski boots impose upon us by blocking the natural balancing mechanism of the leg/ankle/foot:)

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