Ski boot flex posts


For those who are new to the Skier’s Manifesto, welcome.

I became involved in an effort to design a new ski boot at the request of Crazy Canuck, Steve Podborski. Steve was (and I think still is) the only non-European to win the World Cup Downhill title. Steve also won a bronze medal at the 1980 Lake Placid Olympics. After he won the World Cup Downhill title he asked me if I could design a boot that would do for every skier what the boots I had built from components that used a new fit technology I had invented did for him. I saw this as an opportunity to advance skiing. I accepted.

I did not take on this project to make money. I took it on because I saw problems with equipment, especially ski boots, that were significantly affecting the enjoyment of the sport for the majority of skiers. I wanted to try and solve these problems and contribute to the betterment of a great sport.

In 1978, I started down the road to try and improve the ski boot by working with world class racers such as Steve Podborski. Today, my focus and mission remains unchanged. I am still working with skiers and racers and I am still learning. When Simon Zucchuber asked for my assistance with the Freemotion ski boot project, I did not hesitate to offer my assistance.

You can learn more about me under the HOME heading on the opening page.

Over the past week, I spent time going through my US Patent 5,265,350 trying to recall the events that influenced my thinking.

The first patent awarded to me was US 4,534,122. It was filed on Dec 1, 1983 and issued by the US Patent Office on August 13, 1985. The patent is for an innovative in-boot fit system that constrains the forefoot without obstructing the glide path of the ankle joint.

When I invented the fit system disclosed in the patent, I knew I was headed in the right direction. But I also knew that I did not have a full understanding, let alone a solution, for the flexural aspect of the ski boot. Between 1973 and 1983 I had come to understand that boot flex was affected by material stiffness, temperature and closure tension. But two of the biggest issues were that buckle boots flexed by deformation of what is a U-shaped tube (which made flex unpredictable) and the angle of the rear cuff that had minimal or no adjustment. This meant that the angle of the shank of a skier was determined by the mass of the calf muscle at the top of the shaft. Attempts by others to address flex had typically focussed on one issue at the expense of another or even caused new problems.

Devising a system for boot flex that would solve all issues and especially one that did not rely on shell deformation led me to the exo-skeleton format around 1987. A patent for this format was not filed until April 25, 1989 because of the time it took to work with lawyers and try and figure out how to define and describe the technology so it would meet the novel requirement for a patent.

Figure 1 below is from the initial patent filing for the patent that was eventually issued on November 30, 1993 as US 5,265,350. This figure and the material in the application established a priority date for the length of the eventual patent. This initial patent was later abandoned in favour of newer iterations.  All of the ‘improvements’ are described in the patent which can found by searching the patent number US 5,265,350 in the US Patent or Google Patent web sites.



The device is a exo-skeleton arrangement with a tube for the leg attached to the base by arms on each side that rotate about an axis (23) on the base structure (11). A single wide band secures the front portion of the tube (shaft) about the leg of a user to the rear portion.

A bendable spring (40) is affixed to the base on the outside (lateral aspect) of the base (11). An adjustment (42) allows the spring to be moved closer or further away from the two contact arms (43 and 46). The contact arms slide up and down in a channel on the arms so as to allow for an amount of low consistent low resistance cuff rotation before higher resistance is introduced or allow spring resistance to be introduced earlier.  Contact arm (46) can be adjusted up or down the arm so as to change the resistance curve.

An adjustment means (generally shown at 30) allows the angle of the cuff to be adjusted. This enables a user to obtain the correct forward lean angle for the shank which I knew by then was critical (see the posts on SR Stance).

Figure 1 is a rough or what is called a schematic concept of the exo-skeleton system. The next step was to try and come up with a design with aesthetic qualities. Figure 5, below, shows the exo-skeleton of Figure 1 with a soft liner. The attachment for flex spring has been incorporated into the axis journals for the arms of the exo-skeleton.


About 1989, I was approached by a husband and wife radiology team. They taught radiology at a university. They were both keen skiers. They heard about my project to develop a ski boot based on anatomical principles and offered their assistance. They presented me with a copy of a recently published book called The Shoe in Sport – Supported by the Orthopedic/Traumatologic Society for Sports Medicine (OTS).  The Shoe in Sport was initially been published in Germany in 1987 as Der Schu Im Sport. They were of German background. That was how they knew about the book.

I found the knowledge contained in The Shoe in Sport invaluable, especially the article the ‘Kinematics of  the Foot in the Ski Boot’ by Professor Dr, M. Pfeiffer of the Institute for Athletic Sciences at the University of Salzburg, Salzburg, Austria. The information contained in The Shoe in Sport helped crystallize many of the issues I had been struggling with and profoundly influenced the thinking behind the Birdcage and the Birdcage experients conducted in the July of 1991 on Whistler Mountain’s glacier.

For the first time, I felt I was on solid ground with my thinking. I was ready to go boldly forward and break new ground.

… to be continued in Part 2.









The innovative aspect of the FreeMotion ski boot appears to be a U-shaped spring flex-system for the shaft of the boot that is minimally affected by temperature and buckle closure tension and an exo skeleton shaft system that does not deform significantly under load. The arms of the U-spring running along both sides of the shell lower appear to act like rails in transferring force applied to the shaft to the shovel of a ski. Given the stated importance of ski boot flex and the universally accepted position that flexing the shaft of a boot applies force to the shovel of a ski to make a ski turn, the FreeMotion should have been hailed as a breakthrough technology and widely embraced. But this does not appear to be the case.

Simon’s request for assistance, in conjunction with a recently published paper on the  flexural behaviour of ski boots has provided an opportunity to explore this aspect in detail.

The design of ski and ski touring boots should consider three key elements: performance, safety and comfort. The performance of a ski boot is often equated with its (forward) flex index (my emphasis added). A parameter used by nearly every manufacturer ranging from 50 (soft) up to 150 (very stiff). Despite the widespread usage (of the flex index) there is no regulation on how to measure these stiffness indices and it is up to the manufacturer to test and rate their models. 

Whereas industry and special interest magazines  regularly perform and publish ski performance tests, very few systematically derived knowledge is available on ski boots. This is surprising as ski and boot are influencing each other’s mechanical behaviour and should therefore be treated as a system (my emphasis added).

Flexural behavior of ski boots under realistic loads – The concept of an improved test method – Michael Knye, Timo Grill, Veit Senner

  • Technical University of Munich (TUM), Sport Equipment and Materials, Boltzmannstraße 15, D-85748 Garching, Germany – 11th conference of the International Sports Engineering Association, ISEA 2016

The authors of the above cited paper note that usually boots with high flex indices are used by more experienced and skilled skiers whereas for beginners softer boots are recommended.

Based on what we have been told for decades, this makes perfect sense. More experienced and skilled skiers have stronger muscles and are more precise than beginners. Stiff boots allow more experienced and skilled skiers to make better turns because stiff boots enable them to apply more pressure to the shovel of a ski to start it turning.

Studies cited by the authors have shown high activation levels for the m. triceps surea and m. gastrocnemius were measured for various skiing situations.

The triceps surae (aka the calf muscle) is a 3-headed muscle comprised of the m. soleus and the m. gastrocnemius. These two muscles form the major part of the muscles of the (lower leg). The two muscles share the Achilles tendon that inserts into the calcaneus.

The graphics below show the m. soleus and the m. gastrocnemius.


Based on the studies cited by the authors, it seems obvious that the m. soleus and the m. gastrocnemius muscles are instrumental in flexing the shaft of a ski boot.

But then the authors cite an apparent paradox when they state:

Muscular activity of the lower leg is also affected by the boots flexural behavior showing a higher activation with softer boots.

Why would the muscles of the triceps surae show a higher activation with softer boots than stiffer boots? In the current paradigm, this doesn’t make any sense. If the muscles of the triceps surae are responsible for flexing the shaft of a ski boot, shouldn’t they show a higher activation with stiff boots than with soft boots?

One explanation for the apparent paradox is that the paradigm of boot flex is just plain wrong.

…. to be continued.


The  3 main features that appear to be limiting the performance of the FreeMotion boot are the lack of a hard rear stop and forward lean adjustment for the exo cuff and an adjustment means for the U spring that would allow a range of low resistance rotation of the cuff before the resistance provided by the U spring is introduced. It is also important to have a monoplanar boot board with a ramp angle in the order of 2.5 to 2.7 degrees or, preferably, the ability to substitute boot boards with different ramp angles to allow experimentation to determine the optimal angle or range.

In researching the history of the FreeMotion ski boot, it appears to have evolved out of the Kneissl Rail soft boot introduced around 2002. Perhaps Simon can confirm this.

The Kneissl Rail is shown in the graphic below.kneissl-rail

Like the Freemotion, the Kneissl Rail does not appear to have a hard rear stop for the exo cuff. But the Rail appears to have a  dial on the spine that suggests some sort of adjustment for the U spring that might permit a range of low resistance rotation before it is introduced or perhaps a tension adjustment for the u spring. The Rail also has a constraint plate over the instep that is secured with a buckle, a feature the FreeMotion shown below in Figure 1 from the patent, lacks.fig-1Since the investment in prototypes and production molds is substantial, aesthetic considerations and production costs typically take priority over functional considerations.

Because of this, my preferred option is to use purely functional, low cost prototypes that are easily modified as research vehicles to prove out the functional aspects of a technology. Prototypes such as the Birdcage (shown below) can be designed and fabricated at minimal cost compared to the costs of sophisticated aesthetic and production prototypes.


The photo below is of an early research prototype called the Lab Rat that was developed for a recent project. The open architecture of Birdcage and Lab Rat formats permit instrumentation to be incorporated and visible observation of the effects of the technology on the foot and leg to be conducted, something that is difficult, if not impossible in aesthetic prototypes, especially ski boots.


The photo below shows a second generation version of the Lab Rat call The Fit. It is more compact and much lighter than the Lab Rat.


The 4 photos below show the modifications I made to address structural inadequacies and interface issues of the first generation mold generated post Birdcage prototype called the P1. The instep was reinforced with a formed stainless structure and the internal plastic components were replaced with reinforced fibreglass and Tig welded stainless steel components. While these modifications did not lead to a marketable prototype, they validated the conclusions of the analysis that explained why the prototype failed to deliver the expected performance.


In my next post, I will offer some suggestions for potential grafted-on modifications for the FreeMotion ski boot that may clarify the options required to address the issues that I flagged that were confirmed by Simon. It would be helpful if Simon can provide his comments on whether this approach is viable from his perspective.



I have never met Simon Zuchhuber or, to the best of my knowledge, any of the people associated with the FreeMotion ski boot project. Nor, do I have any involvement or financial interest in the FreeMotion ski boot project or do I expect to receive any form of compensation for any contributions I might make. My sole motivation in assisting Simon is to advance the design of ski boots based on anatomical principles and objective science and further the understanding of the mechanics. biomechanics and physics of alpine skiing, in particular, the informed analysis of skier technique.

Simons’ Response to my preliminary observations

To your summary of your observations I can make a few statements.

1.The primary innovation appears to be a U-shaped spring that opposes forward rotation of the Exo Cuff thus transferring force to the front of the shell lower.

This is completely true, even without German language knowledge you summed it up completely correct!

2.There does not appear to be any hard limit to the rearward movement of the shank of the skier.

This is perfectly true as well and was one of the first things I realized when testing the boot! I had a feeling of falling back when I was trying to lean back (both when standing and driving) In our concept we already added a “stopping element” to prevent too much shank movement backwards.

  1. There does not appear to be a forward lean (forward angle) adjustment for the exo cuff.

True again. There is no forward lean adjustment for the exo cuff. They only offer plastic elements that you can place beneath the liner inside the boot to change your forward lean angle. Would you think it necessary to have an adjustment for the exo cuff?

MY RESPONSE: Yes. An adjustment that allows the correct shank angle for isometric contraction of the soleus is essential. In a future post, I will discuss the Birdcage findings on this issue.

  1. There does not appear to be any means to adjust the resistance curve of the spring.

True, you cannot adjust anything when it comes to the spring. Do you think there’s a possibility to make it adjustable?

MY RESPONSE: Absolutely.

  1. If the Heel Retention Mechanism is securely tensioned, it is likely to obstruct the glide path of the distal tibia on the talus. This can cause the center of force on the shank at the buckle secured to the Exo Cuff to rapidly drop down the shank.

This heel retention mechanism is in the patent, but has not been applied to the existing boot series. You can see in the video that on the outside there’s only the buckle to tighten and the zipper to close, they did not apply this heel retention mechanism.

MY RESPONSE: An instep restraint system is essential. I will present several options that can be incorporated into the FreeMotion ski boot.

6.The resistance to forward movement of the shank provided by the spring mechanism appears to be introduced too early and rises too quickly.

What countermeasures would you take? Give the spring more distance to bend for example? 

MY RESPONSE: I will sketch and post some options.

 Thank you once again for your time, I’m looking forward to hearing from you!

Greetings, Simon

Simon has advised me that the timeline for his project has been extended. I will make every effort to accommodate Simon.

For those with an interest in the application of principles of functional anatomy to the design of the ski boot and this project, I suggest that you obtain a copy of The Shoe in Sport (orginally published in German in 1987 as Der Schu Im Sport) and read the section on The Ski Boot, in particular, the paper by Dr. Martin Pfeiffer of the University of Salzburg whose teachings contributed greatly to my knowledge and the success of the Birdcage research. I would like to recognize the dedication and committment of Dr. Pfeiffer to a ski boot based on anatomical principles.


Since Simon Zachhuber is a on tight schedule, I am providing my preliminary observations on the FreeMotion ski boot.

A few days ago, Simon sent a copy of what appears to be a patent application for the FreeMotion ski boot. The application appears to have been received by the patent office on September 1, 2010.coverSince I cannot read much German, I would appreciate it if anyone who reads German and who notes errors in my interpretation to please bring them to my attention.

The initial promotional activity for the Freemotion ski boot seems to have occurred between December 28, 2011 and January 25-26, 2012 with videos posted on YouTube. In studying the videos at 0.5 and 0.25 speeds, the skiers demonstrating the FreeMotion ski boot seem to lack fluidity.

In studying the flexion of the Freemotion ski boot, as demonstrated between 0:58 seconds and 1:10 seconds (Perfekte Passform – Perfect Fit) into the video, Freemotion Skischuh Ein & Austieg, it appears that the resistance to forward shank movement is introduced too early and rises too steeply.

Patent Figure 2, below, provides some insights that might explain why the skiers demonstrating the FreeMotion ski boot appear to lack fluidity.

The mechanism that is most likely the primary subject of the patent application is the Flex Spring (annotated in red) that links the shell lower to the rear Exo Cuff. The retention mechanism for the foot appears to be a diagonal band drawn together by cords that exit at the top of the forward aspect of the tongue.


Figure 8, below, shows the draw cords (28, 29) that pull the diagonal bands together. This mechanism draws the heel of the user’s foot into the rear of the lower shell.


Summary of my Observations

  • The primary innovation appears to be a U-shaped spring that opposes forward rotation of the Exo Cuff thus transferring force to the front of the shell lower.
  • There does not appear to be any hard limit to the rearward movement of the shank of the skier.
  • There does not appear to be a forward lean (forward angle) adjustment for the exo cuff.
  • There does not appear to be any means to adjust the resistance curve of the spring.
  • If the Heel Retention Mechanism is securely tensioned, it is likely to obstruct the glide path of the distal tibia on the talus. This can cause the center of force on the shank at the buckle secured to the Exo Cuff to rapidly drop down the shank.

As observed in the videos:

  • The resistance to forward movement of the shank provided by the spring mechanism appears to be introduced too early and rises too quickly.

If Simon or others with an interest in the FreeMotion ski boot wish to comment on my preliminary observations, I will respond and try to provide some suggestions for solutions in a follow up post.


Yesterday, I found the following comment waiting in que for my review and approval.

Dear David! I am currently working on a project as a student for the FH Salzburg, Austria! We have the task of analyzing a specific ski boot and implementing improvements! Since this is a short project and we are not experts on ski boots, I wanted to ask you for profound feedback on this boot! It can be seen on the website

The boot’s concept is that instead of having a hard plastic-shell to deal with the forces, it uses a metal spring that starts at the forefoot and goes through the ankle-axis all the way back over the heel! We already tested it and were quite surprised, how well it worked, but maybe you can add a few thoughts that come to your mind when you check the boot?
I know, it’s very hard to give feedback without having the boot to test, but maybe you can still give some feedback? Thank you very much!!!

With greetings from Austria
Simon Zachhuber

It goes without saying that I offered my assistance to Simon. As Simon said, “it’s very hard to give feedback without having the boot to test”.

While it is hard to assess the FreeMotion boot without being able to examine it and test and evaluate it during ski maneuvers, from what I am able to see in the photos and videos. I believe that the concept holds promise. In important ways, the FreeMotion is remarkably similar to the exoskeleton format of the Rise ski boot that was based on the Birdcage shown below.


My vision for a a new ski boot was embodied in the minimalist concept, exo-skeleton Birdcage that I designed in 1991 with biomedical engineer, Alex Sochaniwskyj. MACPOD engaged Alex to consult on the ski boot project. Since the Birdcage was a research vehicle designed to test my hypothesis on the mechanics, biomechanics and physics of skiing and acquire the data needed to design a ski boot based on anatomical principles, aesthetics were not a consideration.

Here is what the Rise ski boot looked like that eventully evolved out of the Birdcage.


Here is a design concept sketch of the Rise ski boot.


Here is a photo of the FreeMotion ski boot.


The red arrow in the photo below shows the rubber wedge at the rear of the shaft of the Rise concept. The wedge was compressed by rotation of the shaft as to arrest forward travel after a specified amount of rotation. The actual Rise boot used a rubber stop in front of the cuff to perform the same function.


The FreeMotion boot uses spring mechanism running along each side of the shell lower body to the back of the hinged shaft. The spring is bent as the cuff rotates forward so as to arrest forward rotation.

Here is the FreeMotion promotional video.

Here is the Rise promotional video.

Here is the promotional material for the Rise ski boot.



Unfortunately, due to limitations in materials and manufacturing processes that could not be overcome, the Rise ski boot never made it to market.

In my next post, I will review the development process that saw the principles of the Birdcage applied to the Rise ski boot and how the FreeMotion project can benefit from the knowledge gleaned from the Rise project.


When used appropriately, power straps can be very effective in decelerating forward movement of the shank when transient perturbations in snow reaction force exceed the limits of the balance system. But Power straps are typically used to provide a very snug fit of the leg with the rear spine of the boot shaft by reducing space between the calf muscle and the rear spine. As shaft buckles are increasingly tensioned, volume and fore-aft space within the confines of the shaft is proportionally reduced. But by acting directly on the leading edge and wrapping around the sides of the shank, a securely tightened power strap can severely limit ankle dorsiflexion by fixing the forward most position of the shank and eliminating any free space between the calf muscle and the spine of the boot shaft. By binding the shank to the structurally stiffest element of the shaft, the spine acts to rigidly splint the shank while impinging on the soft tissue that is normally effective in absorbing energy from transient shock loads from perturbations in snow reaction force.  The unavoidable consequence of a securely tightened  power strap is that flexion of the ankle joint is greatly constricted or substantially eliminated.

The two photos below use a skeleton leg to graphically simulate the effect of a single lap power strap on shank position without the shaft buckles being operated. In the left photo, neither the shaft buckles or the power strap are operated. In the right photo, only the power strap is operated. Operating the power strap with moderately light force had the effect of reducing the angle dorsiflexion of the shaft by 11 degrees.

Pwr Diff

Rigidly connecting the leg to the ski has its origins in the widely held view that the leg should be used as a lever with which to apply force to the ski. Power straps became ubiquitous in race boots when self-turning, fixed-radius skis spawned the technique of skiing on two skis and holding the skis on edge with the legs and later, the short-lived slip-catch technique that placed high loads on the lower limbs. But interfering with ankle flexion and especially shank position in relation to the proximate center of the head of the first metatarsal, can have serious implications.

A skier in motion across the surface of the snow is standing on a moving platform that is simultaneously being perturbed in two planes (saggital and frontal). The situation is similar to those that exist in balance studies conducted in laboratories where a subject is standing on a platform that is suddenly tilted without warning, perturbing the subject’s balance. The difference is that in skiing the COM of a skier has momentum that tends to smooth gross perturbations of COM. In the management of perturbations in skiing, the ankle is the primary joint at which perturbations in GRF are modulated by dorsiflexion/plantarflexion primarily through changes in the magnitude of contraction of the soleus muscle.  This is the balance strategy used to maintain upright postures. The pull of gravity on COM disturbs balance by causing the ankle to dorsiflex. The CNS modulates forward sway by regulating contraction of the triceps surae in an ankle plantarflexion  strategy that maintains balance by opposing ankle dorsiflexion. Shaft resistance to the shank movement associated with ankle dorsiflexion can greatly diminish muscle contraction and degrade the mechanism that maintains balance.

In a similar manner, perturbing forces travelling along the length of the outside ski of a turn are modulated primarily by the soleus muscle. But this is only possible when the ankle joint is in the Resistive Shank Angle and has a range of motion sufficient to allow the soleus to modulate perturbations in GRF. Without the ability to move, the shank becomes a vertical shock transmitter. In addition to modulating perturbing forces, the soleus acts as a powerful shock absorber in dissipating perturbations in GRF.

Shock Absorbers

Securing the shank of the user to the rear spine by drawing it rearward suppresses the 3 degrees of freedom in the ankle/foot complex. Depending on how linear the alignment of the shank with the femur is, transient shocks from peak perturbations in GRF may bypass knee and go straight to pelvis and lower back where they can cause gross disturbances in skier equilibrium compromising pressure control of the skis. Limiting flexion of the ankle joint limits the suspension travel from coordinated ankle, knee and hip flexion that maintains contact of the skis with the snow over terrain changes and also the control of pressure exerted on the snow by the skis.

For reasons I will explain in a future post called, STANCE BASICS 101: RESISTIVE SHANK ANGLE, the boot shaft angle should allow the shank angle that occurs in late stance. This shank angle allows the load from the central load-bearing axis to be transferred to the heads of the first and second metatarsals. Power straps can be used to advantage by adjusting them so they help decelerate forward movement of the shank beyond the limits of eccentric gastrocnemius-soleus muscle contraction. But the margin for error is narrow.

Long before the introduction of power straps, the importance of ankle flexion was stressed in the chapter on The Ski Boot in the book, The Shoe in Sport (1989), published in German in 1987 as Der Schuh Im Sport– ISNB 0-8151-7814-X

“If flexion resistance stays the same over the entire range of flexion of the ski boot, the resulting flexion on the tibia will be decreased. With respect to the safety of the knee, however, this is a very poor solution. The increasing stiffness of the flexion joint of the boot decreases the ability of the ankle to compensate for the load and places the entire load on the knee”. – Biomechanical Considerations of the Ski Boot (Alpine) – Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland

“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., (14 degrees) 20 to 22 degrees. Up to that point, the normal, physiologic function of the ankle should not be impeded”.

“Previous misconceptions concerning its role in absorbing energy must be replaced by the realization that shaft pressure generates impulses affecting the motion patterns of the upper body, which in turn profoundly affect acceleration and balance.

“When the lateral stability of the shaft (the leg) is properly maintained, the forces acting in the sagittal direction should not be merely passive but should be the result of active muscle participation and tonic muscular tension. If muscular function is inhibited in the ankle area, greater loads will be placed on the knee”. – Kinematics of the Foot in the Ski Boot – Professor  Dr. M. Pfeiffer – Institute for the Athletic Science, University of Salzburg, Salzburg, Austria

“Many alpine skiers have insufficient mobility in their knees and ankle. The range of motion, particularly in the ankles, is much too small. The lack of proper technique seem so often is not due to a lack of ability, but to an unsatisfactory functional configuration of the shaft in so many ski boots”. – 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



Biomechanics in sports, can be stated as the muscular, joint and skeletal actions of the body during the execution of a given task, skill and/or technique.  Athletic performance is governed by these actions. Coordinated, uninhibited, fluid execution of these actions leads to efficient superior performance. Interference or inhibition of these synchronized body mechanics leads to poor performance and injury. These interferences may be caused by inherent structural limitations in our own bodies, injury induced, training deficiencies or equipment related.

In an attempt to enhance your understanding of David’s piece on power straps, let’s review the functional anatomy and mechanics in the skier’s foot/ankle complex in a ski turn.  Ankle dorsiflexion is critical to stance and balance on a stable turning foot in a ski boot. Dorsiflexion enhances pronation and leg rotation. This combination of forces controls the edge angle. Edge angle is increased by increasing pressure on the inside (medial) aspect of the foot by pronation.  As pronation increases, an obligatory 1:1 internal rotation of the lower leg (tibia) occurs. Whole leg internal rotation with hip joint stabilization completes the rotary response.

3 degrees of freedom r1

We can see that interfering with ankle flexion and especially lower leg (shank) position in relation to the center of the head of the first metatarsal of the foot can have serious implications. Compression of the foot in normal pronation stretches the plantar aponeurosis (plantar fascia) which is a primary source of sensory feedback.  The ankle joint is also a source of sensory feedback modulated by dorsiflexion/plantarflexion through stretch receptors in the soleus muscle of the calf.  As the soleus muscle contracts or relaxes, its combined Achilles tendon insertion to the calcaneus (heel bone) lowers or elevates the rear foot in association with ankle dorsiflexion or plantarflexion. The other calf muscle, the gastrocnemius, reflexly flexes the knee joint during ankle dorsiflexion, since it crosses the knee joint. TRY FLEXING YOUR ANKLE WITHOUT FLEXING YOUR KNEE!

Active muscle contraction does not flex a ski boot,  Leg pressure from COM driven tibial flexion is used to create ankle dorsiflexion in a ski boot. Here are some variables that interfere with this mechanism:

  • The power strap is a 5th buckle. It increases the height of the boot cuff anteriorly in some boots by as much as 45mm. In short-legged individuals, the lever of the high cuff is most instrumental in preventing ankle flexion. This is exacerbated in the vertical cuffs of more contemporary boots. This makes lower leg (shank) length a major factor in overcoming a higher boot cuff. In women with shorter legs and larger lower calf diameters fitted in a higher volume boot shell, the lever of the higher cuff inhibits ankle flexion.
  • Power straps inhibit knee flexion. As a result lower leg flexion, lower leg rotation and ankle    flexion are restricted and pronation is impaired.
  • Boot flex (stiffness) is another ankle motion modifying factor that varies greatly from each boot manufacturer and model.  All boot flex indices should be standardized to accurately inform boot fitters and boot buyers.

The natural reflex interaction among the foot, ankle and knee joint muscles should expose the misconception of adding structural supports that interfere with normal anatomic function.  When a power strap inhibits knee flexion, lower leg flexion, lower leg rotation and ankle flexion are restricted and pronation is impaired.

CONCLUSION:  Synergistic reflex responses and muscle co-contractions cannot occur when their sources of neural sensory input such as stretch or positional proprioception are blocked by mechanical interference.  This is especially true in the foot, the body’s base of support. Interference in sensory input leads to poor skeletal alignment and loss of balance. Good balance minimizes the stress on the body while maximizing the efficiency of movement. In skiing, loss of alignment and balance leads to poor performance and at times, severe falls and injury.

Dr. Kim Hewson is an Orthopaedic Surgeon and former Director of Orthopaedic Sports Medicine  at the University of Arizona.  He is currently a veteran Telluride Ski School Alpine Instructor and Staff Trainer in the Biomechanics of Alpine Skiing.