Ski boot modification posts

ASSESSING CHANGES TO SKI BOOTS

Since the summer and fall is a time when racers and serious skiers make changes to their ski boots, I will describe the strategy I use to assess any changes. It is important to make changes in a manner that controls variables and provides a baseline to make one-on-one comparisons against. However, after viewing video provided to me by several followers of my blog that graphically show the effect on technique of changes made to ski boots over a number of years, it became apparent to me that few, if any, racers or elite skiers have any idea of what a ski boot should ideally feel like and especially how it should affect them in terms of performance.

Without clearly defined end objectives and a sequential process for achieving and confirming successful implementation, skiers and racers can only think in relative terms of better or worse, not optimal.

The experience of Mikaela Shiffrin at the beginning of last World Cup season serves as a prime example. Prior to the start of the 2014-15 World Cup season she changed boots. Early in the season, Shiffrin struggled. Thankfully, she regained her form after changing back to her old boots at a camp over Christmas.

In a similar manner, multiple Olympic and World Championship medalist, Julia Mancuso has struggled after changing boot brands. For the 2015-16 World Cup season she will be back on the boot brand she won her medals on. Will she return to her previous form? Hopefully, she will. But there is no guarantee.

These experiences are excellent examples of the perils of fixing something that ain’t broke and especially the need to have an escape route that allows a change to be undone that doesn’t work out for the better. Getting a boot right so it enables optimal user performance involves a degree of luck in addition to skill and knowledge.  As demonstrated by Shiffrin and Mancuso, no amount of talent can overcome a ski boot that is not right.

Good ski technique starts with a strong stance. In the order of things, the starting point is ensuring optimal performance of the human system by having a comprehensive biomechanical assessment done followed by a program designed to correct any deficiencies identified. The next step is stance training done outside of the ski boot in a controlled environment. This will be the subject of a future post.

Contrary to what is inferred by marketing pitches, ski boots don’t perform. They are inanimate objects. What ski boots do is influence the performance of the human system. Skiers and racers do the equivalent of chasing the rainbow by searching for the right boot in the absence of a definitive specification for what the right boot for them needs to be. Comfort is often erroneously used as a benchmark for performance. Although a boot that supports user performance must be comfortable, comfort, in itself, does not equate with optimal user performance. A boot can be very comfortable while severely compromising skier performance. As the impediments to performance are sequentially removed, a boot that started off being comfortable typically starts to become increasingly uncomfortable as the most prominent sources of interference to function reveal themselves and does not become comfortable again until all sources of interference are removed

The following is part of general strategy strategy for assessing changes in ski boots:

  • Do not change components of a ski boot such as liners and insoles without the ability to reverse the changes.
  • When changing to a new boot model or boot brand retain the old boots. If doubts arise about the new boots  one-on-one comparisons can be made with the old boot on one foot and the new boot on the other.
  • If possible, try the shell of a new boot with the liner inserted in it from the old boot and do a one-on-one comparison with the old boot. This eliminates differences in liners and can give a good idea of how the new shell compares to the old shell.

 

REDUCING FOREFOOT CRASH SPACE

In my post, TONGUE SURGERY, I described how the tongue in my Head World Cup boot was blocking the glide path of my ankle joint by introducing an unwanted source of resistance at the lower end of my shank. By removing all the foam in the tongue below the lower end of the force distribution zone and adding a rectangular layer of foam directly in front of my shin bone, behind the existing layer of chip foam, I increased the space between the plastic tongue body and the lower end of my shank. But I also want to reduce the rearward movement at the transition of the tongue body that occurs when the tongue is bent in dorsiflexion.  I achieve this by trimming the sides of the tongue body as shown in the photo below.

Tongue trim 2

The red dashed lines show where I trimmed the sides of the tongue body and enlarged the neck at the narrowest point. Here’s a side view.

Tongue trim 1

I leave the fabric-foam outer skin run wild instead of trimming it to the shape of the tongue. The reason I do this is to lessen the tendency of the edges of the tongue to snag on my sock when I insert my foot into the boot. I also don’t re-sew the fabric-foam to the tongue body or glue it in place. Both these can stiffen the tongue at the transition bend. Putting my boot on can be a bit tricky a first. I place the tongue on my shin with my forefoot in the shaft. Then I grasp the boot shaft and shove my foot in. Once my foot is in the boot I wiggle the tongue to make sure it is in the right place.

To reduce the crash space over my forefoot I make a new foam pad to replace the original chip foam pad. I start off making the pad bigger than it will eventually be then trim it down as necessary to enable me to get my boot on.  Here is what the foam forefoot pad for my boot looks like.

IMG_3899

I usually taper the top edges to give the tongue a shape that won’t conflict with the shape of the boot shell above. I secure the pad in place with 2-sided tape instead of gluing it. This makes it easy to reposition the pad or remove and replace it. I fold back the fabric-foam skin,  stick the foam pad to the underside of the tongue body then fold over the fabric-foam skin.

IMG_3898

I try foams of different densities and resistance to deformation to try and find the one that works best. When I did a lot of boot work I acquired such a good supply of foams that I have not bought any in years. So I can’t recall the types or sources I am using. But here’s a photo that shows half of the original chip foam tongue alongside some samples from my stock of foams.

IMG_3903

Here’s what the front of the tongue looks like with the foam pad in place. Note the gap behind the forefoot pad in the transition bend that allows the tongue to bend independently of the foam-fabric skin.

IMG_3901

In terms of reducing the crash space, I just want to take up any space between the top of the high point of my foot and the boot shell while leaving space at the back end for the glide path of my ankle joint. I don’t want to feel a significant force pressing down hard on top of my foot.  If the pad is not quite thick enough to fill the space, I add a thin layer (2-3 mm) of dense foam that compresses very little. The net ramp angle of 3 degrees in combination with 14 or 15 degrees of lead segment ankle flexion turns on the stretch reflex in my legs. The stretch reflex enables my balance system to maintain the position of my CoM over my feet on what Ken Chaddock (Ski Simply Well) calls the Magic Carpet. The stretch reflex also allows my muscles to absorb energy from perturbations in snow reaction force that would tend to disturb my equilibrium. This gives me the best ride for the least effort.

In my next post I will discuss joint angles of the legs and pelvis,

TONGUE SURGERY

In my post, TONGUE TIED, I described how a conventional ski boot tongue can block the glide path of the ankle during dorsiflexion, disrupting the physiology of the ankle joint. It is essential to avoid this especially in dynamic activities such as skiing because the ankle joint is a portal for the flow of neural information. Neural flow from the more than 200,000 nerve endings in each foot and mechanoreceptors in the ankle send a flow of proprioceptive, sensory information to the central nervous system where it is processed and used to generate postural responses that sustain balance. Disrupting the physiology of the ankle joint with physical structures can disrupt neural flow and introduce foreign forces into the ankle joint that contaminate neural information from mechanoreceptors. Disrupting the physiology of the ankle joint has a similar effect on the balance system as  taking a hammer to your computer or smart phone then expecting it to still work.

My US Patent No. 4,544,299, published almost 30 years ago, discloses an in-boot tongue fit system that restrains the foot without obstructing the glide path of the ankle joint. The short video clip below shows a section through the center of the tongue system superimposed over the actual patent figures to illustrate how this system works when used in a conventional ski boot shell. The shank and forefoot portions are separate components joined by a flexible link. This allows the components to maintain their respective positions on the shank and forefoot during ankle flexion.

 

The in-boot system in the video allows the ankle to flex while maintaining the position of the load centre on the shank. In order to keep the load on my shin centred in my Head World Cup boot work I had to perform some tongue surgery. The photo below is of the original tongue sectioned through the centre to reveal the core.

Section R

The first thing to note is the use of chip foam for the tongue padding. Chip foam is made from foam scraps that are ground up and held together in matrix with a bonding agent. It has been my experience that chip foam has very poor energy absorbing qualities. My first procedure will be a partial chip foamectomy. Since I only want the lower distribution of force on my shin to extend a little further below the load centre at the top of the front of the shaft than it extends above the load centre I don’t need any foam below this point where it could load my shin. I am also going to surgically remove a portion of the outer padded tongue skin since it is folded over adding thickness  in the glide path of the ankle, the very place where I don’t want any foam or padding. While I am at it, I am  going to remove all the chip foam from the forefoot of the tongue since it is next to useless anyway.

Here’s what the tongue looks like after removal of the foam.

Post foam re

In addition to removing the foam, I also trimmed off the front of the plastic tongue body that would normally be used to stitch the tongue to the liner. I want my tongue to be able to ‘float’ in the forefoot area to reduce any possibility of the transition blocking the glide path of my ankle joint. As my shin approaches the front of the boot cuff in the lead segment of flexion, I want to decelerate the movement as opposed to having my shin slam into the top edge of the shaft (aka Lange Bang). I also want to create more space below the pressure distribution zone to help maintain the centre of pressure while reducing the possibility of any load at the bottom of my shin that could block the glide path of my ankle. The solution? Add a band of foam to the tongue in front of my shin bone as shown in the photo below.

Foam add

I don’t want to add foam to the entire area of the tongue. I only want to add a rectangular band of foam that is directly in front of my shin bone. The reason for this is that there is usually a gap where the sides of the liner overlap the sides of the tongue. Placing a band of foam in front of the shin bone draws the sides of the tongue inward as my shin pushes through the gap. This assists the deceleration of the forward movement of my shin during ankle dorsiflexion while helping keep the force centred. The photo below shows the gap.

Wedge

When I am skiing, the only time I would ever have any perception of contact of my shin with the front of my cuff is if I were to get momentarily pitched forward. Even then, any sensation of any contact is minimal. When I am skiing, the only sensation I am consciously aware of is the considerable tension in the soles of my feet.

In my next post I will discuss final tongue modifications including how I reduce the crash space over my forefoot.

 

ANKLE GLIDE PATH BLOCK

The first thing I look for in a ski boot I am considering is a shaft with sufficient stiffness to create a defined oval shape that will accommodate 14-15 degrees of lead segment low resistance ankle flexion before firm contact of my shank with the front of the shaft occurs. Because my shank is free to move fore and aft up to 14-15 degrees within the shaft, I tend to be acutely aware of what the tongue is doing on my shin. This is much harder to sense in boots with flexible shaft overlap segments that won’t assume a defined shape and especially in a boot with the shaft buckles and power strap cinched tight. When I took my Head World Cup boots out of the box and put them on I immediately sensed the tongue pressing firmly against the base of my shank. This was before I even tried to dorsiflex my ankle (rotate my shank forward). The curve of the transition of the tongue felt like a block pushing against the base of my shank.

Most plastic tongues amount to bent half tubes. One of the stiffest shapes known is a tube. When the trailing edges of a ski boot tongue are loaded by the leading edges of the boot liner and the curved interface of the boot shaft, the shank portion of the tongue becomes substantially rigid. When the shank presses against the tongue it bends at its transition with the forefoot portion. When it bends, the curve at the transition flattens and the tongue body moves rearward towards the shank. Unless the tongue is sewn to the toe box of the liner so it is too far forward, it can press on the lower end of the shank and block the glide path of the ankle joint. Here is a simulation of what happens. The black line represents the profile of the tongue.

 

This issue was identified in my US No. 4,534,122 and in a series of X-ray video studies done by Professor M. Pfeiffer (Kinematics of the Foot in the Ski Boot – The Shoe in Sport).  In the Type C study Dr. Pfeiffer observed that, among other things, the physiologic function of the ankle is stopped prematurely (blocked) with the effect that the talus (the bone that forms the ankle joint with the tibia) is levered backward and upward within the boot shell. The previous short video clip and the clip that follows below show this effect. If you pause the videos before and after shank loading you can see the extent of the effect of the tongue bending at the transition and pressing against the base of the shank. The flattening effect at the transition is due to the manner in which the stiffness of the half tube shape of the tongue influences the deformation.

 

Note how the foot is forced backward in the boot and the entire forefoot lifts off the boot board. This effect is easy to demonstrate with foot pressure technology by having the subject apply firm pressure to the balls of the feet and then flex the boot. As boot flex progresses, the pressure seen on the monitor under the balls of the feet will progressively decrease then disappear. The reason for this is that ski boots are flexed by decreasing the contraction of the soleus muscle. This has the effect of turning off the connection of the tibia with the balls of the feet. In his article, Dr. Pfeiffer stresses the importance of the forces on the shank in the fore aft plane being the result of active muscle participation and tonic muscular tension and that if muscular function is inhibited in the ankle area, greater loads will be placed on the knee. Tonus in a muscle is a reflex state where the muscle is primed and ready to rapidly respond to a neural signal to contract.

In my next post I will discuss the modifications I make to my boot tongue to try and minimize ankle glide path block.

 

 

TONGUE TIED ERRATA

In reference to the photo of the tongue, which I surgically removed from the liner of my Head World Cup ski boot with a tonguectomy procedure, I also performed a bilateral resection of the tongue for the purpose of exposing the core structure. The photo on the right is of the resected tongue . I will discuss this tongue in the next post. I apologize for any confusion this omission may have caused.

Tongue section

TONGUE TIED

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.

 

GETTING SHAFTED BY THE (SKI BOOT) SHAFT

In order to appreciate how and why I fabricate a tongue system that works with my minimal shell, a requisite knowledge of the key aspects of the underlying issues and fundamentals of the science of human balance are essential.

By 1979, through a series of experiments, I had arrived at a tentative conclusion that the concept of attempting to immobilize the joints of the foot and support it within the confines of a rigid shell ski boot was unsound and not conducive to physiologic function. One of the issues that I had identified was the incompatibility of the fixed plane of the front of the shaft (aka the cuff) of the ski boot with the dynamic plane of the front of the shin or shank of the skier’s leg. There was also the issue of inadequate or even the absence of loading of the instep of the foot within the forefoot portion of the ski boot shell. It is one thing to arrive at a conclusion that a concept is flawed. But unless one can come up with a better solution, a tentative conclusion is moot.

In the spring of 1980 I came up with a solution that addressed both issues. It was an innovative, in-boot technology that was granted US Patent No 4,534,122.  The effect of this technology on Podborski’s skiing far exceeded any expectations I held. Although it appears I was first out of the gate in recognizing problems associated with the ski boot shaft, it was soon to turn out that I was not alone in identifying this issue. Here is what I said in my patent filed on December 1, 1983, granted on August 13, 1985 and assigned to Macpod Enterprises Ltd. (Squamish) MACPOD was David MACPhail and Steve PODborski.

Designers of ski boots intended for downhill (alpine) skiing have recognized the need to provide support for the leg, ankle and foot, but have tended to produce boots that are uncomfortable, that do not give the skier proper control, and that restrict those movements of the ankle joint that are necessary during skiing.

Fore and aft movements of the leg at the ankle joint (i.e. plantarflexion and dorsiflexion of the foot) are often restricted or prevented in prior art ski boot by the boot tongue or other structure designed to restrain movements of the foot. Typically, a boot tongue extends from near the toes to the lower shin and, in order to provide good padding and support, is relatively inflexible. Such a tongue presents considerable resistance to dorsiflexion of the foot.”

It is important to note that at the time that the patent was filed I was still in the paradigm of immobilizing the foot and the use of supportive footbeds.

Four years after the filing of the patent my position on the shaft of boot interfering with the physiologic function of the ankle joint was confirmed in four articles contained in the section, The Ski Boot, in the book, The Shoe in Sport (1989) – Published in Germany in 1987 as Der Schuh im Sport. ISNB 0-8151-7814-X (27 years ago). It appeared that as a Canadian I had laid down a gauntlet on issues with the shaft of the ski boot and, in so doing, had led the world in drawing attention to this issue. The response from boot makers? Deafening silence.

In the first article, Biomechanical Considerations of the Ski Boot (Alpine), Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland, explains that the ski boot must represent an interface between the human body and the ski and that more than simply enabling the skier to steer the ski as well as possible, the boot must also allow direct (neural) feedback from the ski and from the ground (snow) to the skier. In other words, in order to function in a rapidly changing dynamic environment, the balance system must have access to accurate neural feedback from the snow in order to generate what are called postural responses (ergo – balancing processes). Dr. Stussi states, These conditions can be met if the height, stiffness, angle  and functions (rotational axes, ankle joint (AJ)/shaft) of the shaft are adapted, as well as possible  to the individual skier (my emphasis added). Dr. Stussi warns of the problems associated with the loading of the ankle such as occurs when a boot is tightly fit in what is often referred to as ‘The Holy Grail of skiing; the perfect fit of the boot with the foot and leg,, Improvements in the load acting on the ankle make it biomechanically very likely that the problems arising in the rather delicate knee joint will increase.” Dr. Stussi seems to have called this right. Knee injuries did increase. But the loading of the ankle not only continues unabated today, the state-of-the-art in ankle loading continues to evolve.

In the second article, Kinematics of the Foot in the Ski Boot, Professor  Dr. M. Pfeiffer of the Institute for the Athletic Sciences at University of Salzburg, Salzburg, Austria, presents the results of a number studies using  x-ray video tape imaging on the effects of the shaft of the boot on the shape of the foot and the displacement of bones towards and away from each other during flexion of the ankle. These changes disrupt the normal physiologic function of the ankle necessary for balance. Based on these studies Dr. Pfeiffer concludes, “The shaft of the boot should provide the leg with good support, but not with great resistance for about two thirds of the possible arc, i.e., (12 degrees) 20 to 22 degrees. Up to that point, the normal, physiologic function of the ankle should not be impeded.” The response of the ski industry? Power straps to further impede the normal physiologic function of the ankle, the very thing Dr. Pfeiffer warned against.

Dr. Pfeiffer points out that it is misconception that the role that the role of the shaft is to absorb energy and that this misconception must be replaced with the realization that, shaft pressure generates impulses affecting the motion patterns of the upper body, which in turn profoundly affect acceleration and balance. He advises that the lateral stability of the leg should result from active muscle participation and tonic muscular tension and that if muscle function is inhibited in the ankle area (which is the seat of balance – my comment added), greater loads will be placed on the knee (my emphasis added).

Dr. Pfeiffer concludes his article by stating that “the ski boot and it’s shaft must be adapted to the technical skill of the skier, and the technical skills of the skier must be adapted to the preexisting biomechanical functions of the leg and the foot.” Dr. Pfeiffer ends by expressing the hope that his studies will lead to the development of a ski boot design based on anatomical principles. It seems that Dr. Pfeiffer’s hope was in vain.

In the third article, Ski-Specific Injuries and Overload Problems – Orthopedic Design of the Ski Boot –  Dr. med. H.W. Bar, Orthopedics-Sportsmedicine, member of GOTS, Murnau, West Germany mentions that Dr. Pfeiffer’s studies have found that the foot maintains some spontaneous mobility in the ski boot and that because of this, the total immobilization by foam injection or compression by tight buckles are unphysiologic“. Translation? Tightly fitting and compressing the foot especially with foam injected or form fit liners screws up the function of the foot. This is not a good thing. Along this line Dr. Bar goes on to state, Only in the case of major congenital or post traumatic deformities should foam injection with elastic plastic materials be used to provide a satisfactory fixation of the foot in the boot.” Based on the amount of foam injection being done these days it seems that there must be a lot skiers with major congenital or post traumatic deformities.

In the final article,  Sports Medical Criteria of the Alpine Ski Boot – W Hauser & P. Schaff, Technical Surveillance Association, Munich, West Germany, Schaff and Hauser discuss the problems caused by insufficient mobility in the knees and ankles of most skiers and especially much too small a range of motion in the ankles. The authors speculate that “in the future, ski boots will be designed rationally and according to the increasing requirements of the ski performance target groups.”

I’ll conclude this post with some excerpts from my US Patent 5,265,350 filed on February 3, 1992.

Skis, ice skate blades, roller skate wheels and the like represent a medium designed to produce specific performance characteristics when interacting with an appropriate surface. The performance of such mediums is largely dependent on the ability of the user to accurately and consistently apply forces to them as required to produce the desired effect.

In addition, in situations where the user must interact with external forces, for example gravity, the footwear must restrain movements of the user’s foot and leg in a manner which maintains the biomechanical references with the medium with which it is interacting. It is proposed that in such circumstances, the footwear must serve as both an adaptive and a linking device in connecting the biomechanics of the user to a specific medium, such as a ski, for example. This connective function is in addition to any type of fixation employed, in this instance, to secure the footwear to the ski.

Existing footwear does not provide for the dynamic nature of the architecture of the foot by providing a fit system with dynamic and predictable qualities to substantially match those of the foot and lower leg.

More that 2o years later, existing footwear (ski boots) still do not provide for the dynamic nature of the architecture of the foot by providing a fit system with dynamic and predictable qualities to substantially match those of the foot and lower leg. Since it is unlikely that ski boots will be available any time in the near future that meet the preceding requirements, I had to find a way to work within the limits of presently available ski boots. In my next post I will explain how I avoid getting shafted by the shaft of my ski boot.