Ski boot flex posts


This post is about how tongues in ski boot can affect balance.

Every ski boot has some sort of tongue. In the case of rear entry boots or liners like the Intuition, a portion of the liner acts in the capacity of a tongue. So what exactly does the tongue do? The obvious job of the tongue is to the pad the shin and spread the load applied by the shank to the front of the boot shaft.

What about the forefoot portion of the tongue over the instep of the foot? What does it do? As far as I have been able to ascertain, for most skiers, not much. Seriogram X-Ray studies done for me in 1995 found that in the boots of some skiers, there was a significant crash space between the top of the forefoot portion of the tongue and the inner surface of the boot shell. A lack of constraint or load applied to the instep of the foot of a skier means that the entire foot can float within the boot shell in response to perturbations in snow reaction force. Typically, when a skier’s CoM is perturbed, the plantar foot separates from the insole on the liner. If the skier is thrown off balance and pitches forward, the heel of the foot moves up as the foot rotates about the balls of the foot. This is an issue that the in-boot technology in my US Patent No. 4,534,122 addressed.

But ski boot tongues can do other things that you may not be aware of. The tongue can act in the capacity of a spring that opposes and progressively loads the shank in ankle flexion. Worse, it can  obstruct the glide path of the ankle joint. When the now ubiquitous power strap that is present on most boots today is cinched up tight, the tongue can act as an effective splint for the ankle.

In my last post, MOMENT OF THE SHANK IN THE SHAFT,  I used a simulation to show how my shank can move with little resistance from the shaft for about 14-16 degrees within the front to back free space within the shaft. In his article, Kinematics of the foot in the ski boot, Dr. M. Pfeiffer refers to this as the lead segment of shank flexion. Here is what it looks like in my Head World Cup ski boot.

Lead segment

The red line emanating from the fixation of the shaft of the boot indicates the proximate point about which deformation of the front of the cuff will occur. As my shank encounters the front of the shaft I want the load centre to remain substantially fixed and the resistance to predictably increase so my balance system can work with it.

The load applied by my shank is to the top edge of the front of the shaft of the boot. This is the centre of the load. The load is distributed by the tongue above and below the load centre. I like to have a little more load on my shank below the load center than above the load centre. The red arrows and bar with the dots in the photos below show this. I don’t want to have any load on my shank below the lower aspect of the load distribution.

C of Force

Here is what the stock tongue from my boots looked like after I performed a tonguectomy procedure that removed it from the liner.

Tongue section

Here is what the tongue looks like overlaid on my ski boot.

Tongue overlaid

Note the flat profile. In order for the tongue to conform to my foot and leg either my ankle has to severely plantarflex or my the tongue has to bend. I suspect that tongue is made this way to act as a sort of shank-shaft  shoehorn to facilitate entry of the foot into the boot. Since I can’t stand up let alone ski with my ankle plantarflexed, the tongue has to bend. By what? By my shank applying a force to it. In this configuration the tongue is acting like a spring pushing against the shank of my leg in places where I don’t want any load.


I push on the tongue, the tongue pushes back. But it can be worse than that especially if the tongue is too far back as it was in my boots. The tongue is fixed (usually sewn) to the toe box of the liner. The first time I put my boots on (the liners were intact then) and operated the buckles it felt like a steel rod was jammed into the base of my shank. If I tried to flex my ankle I could feel that the glide path of the joint was impeded. So I would get an initial load on my shank at its base followed by a secondary load at the top of the shaft superimposed over the first load. To me, the feeling is like running up a flight of stairs and catching the toe of my lead foot on a stair nosing. I call this kind of unpredictable loading the ‘trip effect’ because it feels similar to tripping in terms of the effect on my balance.

In my next post I will discuss the tongue modifications I typically make.



In the article, Biomechanical Considerations of the Ski Boot, in the book, The Shoe in Sport, Dr. Stussi raises the issue of the relationship of the levers or moment arms between the rotational axis of the shaft of the ski boot and the proximate axis of the ankle joint. As far as I know, few, if any, ski boots actually have a shaft with a functional axis of rotation. Shafts are typically fixed in place by an interface detail with the lower boot shell.  What appears to be a axis pin is actually a fixation means that secures the shaft to the lower shell. While the fixation means does not create a rotational axis, it doe influence where and how the shaft will deform when the skier’s shank applies force to it. For this reason, I prefer to have the fixation means of the shaft slightly behind the proximate centre of rotation of my ankle joint.

In general, the ski industry seems to be unaware of the fact that the ankle joint is not a fixed hinge but a gliding hinge with a centre of rotation that not only varies during plantarflexion and dorsiflexion but which has a variable axis that changes continuously throughout the range of motion of the ankle joint. In addition, there can be considerable variation in the proximate centre of the axis of rotation from the ankle joint of one person to another person. In view of this, it is not practical to design a ski boot with an axis of rotation that is congruent with the axis of rotation of the ankle. The issue I focus on in a ski boot is keeping the load centre of the shank on the shaft consistent. Here’s a short video clip that shows how the movement of the shank of my leg would look within the shaft of my boot if we could watch with an imaging device.


Although there is no flesh on the skeleton and no liner and especially a conventional boot tongue in place, the video clearly shows that the centre of force of the shank of my shin on the front of the shaft will be on the top of the shaft.

The 3 photos below show 3 different boots with all the buckles undone. If the interface of the overlap of the shaft stays together with the buckles undone I consider that the material and shape of the shaft is stiff enough to substantially maintain its shape while skiing with the buckles in the first bale catch position. The first photo is my Head World Cup boot.  The integrity of lower shell-shank interface and the overlap is good.

Good 1

The problem is that as a boot gets stiffer it becomes increasingly difficult to insert the foot into. The upside is that a stiff cuff shape provides sufficient front-to -back space for the range of low resistance ankle flexion I need with the buckles engaged in the first bale catch.

The photo below is of my spouse’s Head boot. Although not as well defined as the World Cup the shape without the buckles engaged is acceptable.


The photo below is of a vintage Lange XLR race boot. Even though the shell material is in a race stiffness it does not provide the defined shape of the cuff I need to provide a defined shape for the movement of my shank within the shaft in ankle flexion.


The problem with my Head World Cup and most boots is that even if the shaft stiffness and shape is good the tongue typically introduces a secondary source of resistance to the movement of the shank of a skier that is variable. I will explain why in my next post.






The title refers to the ability of the shaft of a ski boot to disrupt and contaminate the neuromuscular processes of balance and, in particular, to diminish or extinguish the contraction of muscles that would normally act to oppose forces that tend to disturb balance. As a prelude to discussing the measures that I use in my ski boots to mitigate these effects I will address the widespread perception among ski professionals and coaches that the ski boot does not signficantly affect skier performance.

Although the design of the modern rigid plastic ski boot has some serious shortcomings, the fact of the matter is that some skiers can ski reasonably well in these boots. It is also a fact that many World Cup and Olympic medals have been won on them. This has spawned the erroneous assumption that if a skier is having problems with their skiing skills the problem lies with them, not their equipment and especially not their ski boots. The reasoning of many ski professionals and coaches is that if they can  ski without apparent difficulty in stock ski boots then acquiring skill in skiing is a simple matter of technical training and practice.  Unless one regressed, as I did after switching from low-cut leather boots in which I was an expert skier to higher, rigid plastic boots in which I was reduced to the level of a beginner, they would not have the benefit of the perspective of the ski boot as the problem. Further, until one gets very close to the optimal ski boot configuration any interference with the function of the feet and lower limbs, in particular the processes of balance, caused by the ski boot is unlikely to be perceived.

My experience with my spouses’ ski boots serves an example of how fine the line is that separates performance from dysfunction. This season I made what I thought were minor changes to her boots. But these minor changes had negative effects on her skiing far in excess of anything I could have expected. I replaced the soft fabric Lange liners in her 10 year old Head boot shells with identical new Lange liners. The reason for this change was that the old liners were disintegrating. I removed the stock Lange tongue from the new liners so she could use the tongue fit system from her old liners. But I did not cut away the seams along the sides in the front of her liner as I had done with her old liners. The reason I do this is to ensure that the metatarsals can spread under load. One significant change that I did make was to use a stock Lange insole in place of the much thinner insole in her old liners. The special tongue fit system I invented in her boots is the principal fit component. This did not change.

What happened on snow was startling.  In the first few meters of her first run my spouse went from an expert to a struggling beginner. She appeared to have little balance or control. She told me her boots felt completely wrong and that she was disoriented and could not find her balance. Her situation was so bad that we left the ski hill after one run and went home to assess the situation. After we got home I cut away most of the front of her liners. I also heated the Lange insoles and pressed them dead flat because she complained they were digging into her arch. This seemed incredible to me because the Lange insoles have almost no arch form.

The photos below show the new unmodified Lange liner on the left with the same liner modified after her skiing experience on the right. I left a small amount of the toe box of the liner to help keep the insole from creeping forward. On the left liner I also cut away some of the plastic backing of the cuff to allow her calf muscle to sit properly.

Screen Shot 2014-04-08 at 2.13.58 PM

When we went back on the ski hill the change in her skiing was dramatic and instantaneous. A similar thing happened to me with interference with the arch of my left foot. When I am in a moderate to high speed GS turn the tension in the sole  of my outside foot is so great that it feels as if my foot is made of steel and the base of support on my ski feels as if I am standing on a concrete surface. With this amount of tension anything that impinges on my arch feels like a sharp stone in my shoe. I had an interference problem in the arch of my left foot that was not addressed by shaving down an already flat insole. I eventually traced the problem to the detail of the sole of the liner that rises up along the inner aspect. Cutting away this section of the sole resolved the problem. The photo below shows the portion of the liner that was causing the problem.

Arch int

The ability of the ski boot to significantly affect skier performance was described in The Shoe in Sport 1989 – Published in Germany in 1987 as Der Schuh im Sport. ISNB 0-8151-7814-X (27 years ago). Despite the call by scientists for a ski boot based on a design that from a functional point of view that takes into consideration the realities of functional anatomy (axes etc.) a significant component of the design of ski equipment and the formulation of ski teaching and coaching methodologies continues to be influenced by opinion and subjective observation. Ski pros and coaches tend to interpret what they see in the context of what they know or what they believe they know. Although athletic prowess is arguably a factor, a significant but unrecognized factor confounding the analysis of technical faults is that the ski boot acts as a selective filter that literally determines how well someone can ski based on the physical characteristics of their feet and legs. Those who are able to ski with reasonable efficiency within the constraints of a ski boot become ski instructors. Those who ski really well become racers. The remainder languish as terminal intermediates. Levels of competition act as selective filters that eliminate those more compromised by the constraints imposed by their ski boots. Ski pros and coaches tend to view their charges based on a paradigm in which the ski boot has no effect on skier performance. Even today some coaches continue to argue that the in-boot technology that I invented and that Podborski used to compete and win on a partially healed knee was not a factor in this success, that it was training or some other factor.

A  book that I just finished reading, one  that substantiates my position that with rare exceptions ski teaching methods tend to overlook the effect of the boot on the skier, is Ski Simply Well by Ken Chaddock ( Chaddock is a local Ski Pro who teaches on Whistler-Blackcomb. With the exception of a few omissions, Ski Well Simply is simply one of the best books I have ever read on ski teaching, bar none. But while Chaddock raises the issue of cuff cant angle which is important, he appears to make the common assumption that those who read his book will be able to ski the same way he does. What Chaddock gets right, and he really gets it right, is his description of how he develops plantar tension in the soles of his feet and skis with minimal use of the cuff of the boot. Chaddock gets so much right that in a future post I will fill in the missing pieces and connect the dots he missed.

In my next post I will describe how I build a tongue that lets me ski in a minimal boot shell.