Ski boot flex posts


I haven’t had a chance to write posts for awhile. But Federica Brignone’s powerful performance in last Saturday’s Killington GS; one in which she showcased the power of the pelvis has served to inspire and motivate me. I dedicate this post to Federica Brignone and my Italian followers.

Molto Benne Federica, Molto Benne!

As a prelude, I normally study as much video as I can locate after a race in order to try and find the camera angles and clarity I need to do a proper analysis. But I could find very little video of the Killington GS. So please bear with lack of quality in some the images I will use in this post.

Right out of the Gate

As soon as Brignone came out of the start gate, extended her ankles and knees in the fall line and stood tall I knew she was going to stand tall on the podium.

A fraction of a second later, she flexed her ankles and knees while still in the fall line. This was very significant because it indicated to me that she has the ability to flex her ankles and move her shank about 12 or more degrees against low resistance within the shaft of her boots. I call this ankle-flex free play.

To find out why low resistance ankle flexion is important please read (or re-read) my post THE SHOCKING TRUTH ABOUT POWER STRAPS (1.), which remains my most viewed post ever. Then think about the implications of Brignone’s ability to extend her ankle and especially her knee for the position of COM in her pelvis in relation to her feet.

Here’s a hint: The femur is significantly longer than the tibia.

To be continued.



There has been a huge surge in interest in my post HIRSCHER AND SHIFFRIN WIN BY CROSSING THE LINE.

The reason Hirscher and Shiffrin can ski this way is that they have the ability to cross the rise line and establish balance on their outside foot and leg in milliseconds. This enables them to make what amounts to a hairpin turn. They are on and off their edges like a flat stone skipping off the water. The reason they can do what few other racers can is because their boot setup supports the requisite neurobiomechanics. I discuss this in my last post WHY HIRSCHER AND SHIFFRIN ARE DOMINATING.








It has been known for decades that an unbalanced moment of force or torque will be present on the outside ski when the center of pressure of the load applied to the ski by a skier is acting along the center of the transverse axis of the ski where it is offset from GRF acting along the inside edge. Ron LeMaster acknowledges the existence of an unbalanced moment of force on the ouside ski in both The Skier’s Edge and Ultimate Skiing (Edging the skis). LeMaster states in Ultimate Skiing;

The force on the snow is offset from the center of the skier’s and creates a torque on it that tries to flatten the ski.

Ron didn’t get the mechanics right. But he correctly shows the unbalanced torque acting on the ankle joint. LeMaster tries to rationalize that ice skates are easy to cut clean arcs into ice with because the blade is located under the center of the ankle. While this is correct, ice skaters and especially hockey players employ the Two Stage Heel-Forefoot Rocker to impulse load the skate for acceleration. Hockey players refer to this as kick.

In his comment to my post, OUTSIDE SKI BALANCE BASICS: STEP-BY-STEP, Robert Colborne said:

…..In the absence of this internal rotation movement, the center of pressure remains somewhere in the middle of the forefoot, which is some distance from the medial edge of the ski, where it is needed.

The load or weight of COM is transferred to distal tibia that forms the ankle joint. This is the lower aspect of the central load-bearing axis that transfers the load W from COM to the foot. What happens after that depends on the biomechanics. But the force will tend to be applied on the proximate center of the stance foot. This is a significant problem in skiing, (one that LeMaster doesn’t offer a solution for) when the ski is on edge and there is air under the body of the ski. The unbalanced torques will move up the vertical column where they will manifest at the knee against a well stabilized femur.

But this unbalanced torque creates another problem, one that is described in a paper published in 2005 by two Italian engineers (1.) that describes how this load deforms the base of the boot shell.

The Italian study found large amounts of deformation at mean loads of up to 164% body weight were measured on the outer ski during turning. The paper suggests that the ski boot flex index is really a distortion index for the boot shell. The lower the flex index, the greater the distortion potential.

For the ski-boot – sole joint the main problem is not material failure, but large amounts of local deformation that can affect the efficiency of the locking system and the stiffness of the overall system.

Values of drift angle of some degree (>2-3°) cannot be accepted, even for a small period of time, because it results in a direct decrease of the incidence of the ski with the ground.

My post GS AND KNEE INJURIES – CONNECTING THE DOTS (2.) cites studies that found that knee injuries are highest in GS in the shortest radius turns where peak transient forces are highest.

As shown in Figure 2a FR (sum of centrifugal and weight forces) and F GROUND (ground reaction force) are not acting on the same axis thus generating a moment MGR that causes a deformation of the ski-boot-sole system (Figure 2b) leading to a rotation of the ground reaction force direction. The final effect is to reduce the centripetal reaction force of the ground, causing the skier to drift to the outside of the turn (R decreases, causing the drift event).

An imperfect condition of the ski slope will emphasize this problem, leading to difficulties maintaining constant turning radius and optimal trajectory. The use of SGS ski-boot in competitions requires a particular focus on this aspect due to the larger loads that can be produced during races.

I have added a sketch showing that the moment arm M R created by the offset between the F Ground and F R is in the plane of the base of the ski where it results in an Inversion-lateral rotation torque.

The importance of sole stiffness is demonstrated with a simplified skier model…..…ski boot torsional stiffness with respect to ski longitudinal axis in particular is very important as it deeply influences the performance of the skier during turning…. A passage over a bump or a hollow may generate a sudden change in ground reaction force that may lead to a rapid change in the drift angle delta. The ski boot must be as stiff as possible going from the lower part of the boot to the ski (i.e. lower shell-joint-sole system)

As explained in the method section using the simplified model, values of some degree cannot be accepted, even for a small period of time, because the skier stability and equilibrium could be seriously compromised especially when the radius of curvature is small. A non perfect condition of the ski slope will emphasize the problem, leading to big difficulties for maintaining constant turning radius and optimal trajectory.

This excellent paper by the two Italian engineers concludes with the following statements:

Authors pushed forward the integration of experiments and modeling on ski-boots that will lead to a design environment in which the optimal compromise between stiffness and comfort can be reached.

The possibility of measuring accurately the skier kinematics on the ski slope, not addressed in the presented study, could represent a further step in the understanding of skiing dynamics and thus could provide even more insightful ideas for the ski-boot design process.

I first recognized the shell deformation, boot board instability issue in 1980, at which time I started integrating rigid structural boot boots into the bases of boot shells I prepared for racers. The improvement in ski control and balance was significant. The instability of  boot boards associated with shell/sole deformation with 2 to 3 degrees of drift at modest loads of up to 164% body weight has significant implications for footbeds.

  1. AN INNOVATIVE SKI-BOOT: DESIGN, NUMERICAL SIMULATIONS AND TESTING – Stefano Corazza 􀀍 and Claudio Cobelli Department of Information Engineering – University of Padova, Italy – Published (online): 01 September 2005 –


On December 9, 2016,  Simon Zachhuber from Austria posted the following comment on my blog.

Dear David! While researching for a university-project (FH-Salzburg, Austria) I discovered your blog! Clearly you are an expert on ski boots, and I thought maybe you can provide some feedback! At least it’s worth a try

I’m a product designer and our task is to analyze a specific ski boot and try to figure out ways to improve it! The ski boot we work on has a new concept of dealing with the occuring forces. Instead of providing stability and flex with plastic shells, the forces are applied to a metal spring. So the area of the tong and the shin doesn’t have to be covered with hard plastic, but with leather or fabric. You can see the concept on their website

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 hardplastic-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

I replied to Simon by email

Dear Simon,

Greetings from Whistler, BC Canada.
I would be glad to assist you in any way I can with your project.

Do you know of the work of Dr. Martin Pfeiffer of the University of Salzburg? He was committed to the development of a ski boot designed along anatomical principles. Two Canadian radiologists of Austrian descent made me aware of Dr. Pfeiffer’s work in 1988 when they gave me a copy of Der Schu Im Sport. Part 6, The Ski Boot, features a chapter by Dr. Pfeiffer called Kinematics of the Foot in the Ski Boot. Dr. Martin Pfeiffer was a source of valuable knowledge that influenced my work. I had personal communication with him in the early ‘90s.

Dr Pfeiffer concluded his chapter by stating, “This goal (a ski boot designed along anatomical principles) has not yet been achieved”. I do not know whether he is still with us. But I would appreciate you recognizing his contribution to skiing by making his vision a reality and by recognizing his work.

Best regards,

Due to the urgency of Simon’s deadline (which was fortunately later extended to January 17, 2017), I made providing my assistance a priority with the following series of posts.








On January 18, 2017, the following video was posted on YouTube on the FreeMotion site with the text below translated from German.

Kooperation Freemotion mit der FH Kuchl

The results of our successful cooperation with FH Kuchl are here! We are delighted about the great input of the students and professors. We are already working on the implementation of the ideas. Once again a big thank you to the students #Feelgoodskiboot


I sincerely hope that my efforts have assisted the students of FH Kuchl. The design of the ski boot needs fresh young thinking and a new direction with design based on the functional requirements of the human system. The most satisfying aspect of the design exercise at FH Kuchl was to see the large female component with students Marlene Arabjan and Evelyn Obermuller taking an active role.

Gut gemacht Simon  Zachhuber und die anderen studenten von FH Kuchl!

Viel Gluck und Best Wunsche


A plausible reason why the FreeMotion boot has failed to gain acceptance is that the spring flex concept has not been tested in a medium that allows all the critical variables discussed in my last post to be adjusted to the individual skier. An essential prerequisite to this process is a validated physiological model; one that explains the physiologic processes of skier balance as well as the mechanics and biomechanics associated with the 3-dimensional physical environment of the activity.

While the Birdcage allowed adjustments specific to individual skier requirements to be made and data to be acquired that showed the effects on skier performance, in particular, skier balance, insufficient time and budgetary constraints limited the study of flex curve requirements.

Figures 52 and 55 A through D below show the flex assembly for the Birdcage. The details are disclosed in the section of the patent associated with aforementioned figures.


Figure 56 below shows the different components of the Birdcage shaft adjustment and flex system resistance curve. The details are disclosed in the section of the patent associated with Figure 56.


The photo below shows the actual flex system on the rear of the Birdcage. The nut under the rear stop lug allows for adjustment of the forward lean angle of the shaft. In this photo, the shaft is in the rearmost (hard stop) position with the lug seated against the rear stop nut on the spine.


The photo below shows the gap between the lug and the washer-cyclinder assembly that presses on the coil spring after a pre-set number of degrees of forward flexion.

Experiments were done with several types of coil springs, including compound coil springs where a small coil spring is nested inside a larger spring. The rubber donut at the top of the assembly prevents the ‘brick wall’ deceleration effect that occurs when the coil spring is fully compressed. Different types and assemblies of rubber donuts were tried in conjunction with different coil spring configurations.


In the photo below, the shaft of the Birdcage has rotated forward through the constant low resistance travel segment and is about to compress the coil spring assembly. This position is associated with isometric contraction of the triceps surace (calf muscles) in the SR Stance.


The photo below shows the coil spring in the initial stages of compression.


The effect of flexural qualities of the shaft of a ski boot on skier function and balance require structured studies conducted with instrumentation and data acquisition. Fortunately, papers on such studies have recently been published. I will discuss them in my next post.


In The Shoe in Sport, the detached, objective assessment of the conventional ski boot articulated in Part 6, The Ski Boot, by preeminent authorities on biomechanics and safety served to support my conclusions of the past 10 years that little of what forms the basis of knowledge in skiing can be defended. This made me acutely aware of the dangers of going forward with a concept for a new ski boot that could not defended with a validated hypothesis based on principles of applied science and especially data captured during actual ski maneuvers that could be readily and consistently reproduced by others.

Key critical comments made in Part 6 of The Shoe in Sport, (first published in 1987 in Germany as Der Schuh im Sport – ISNB 0-8151-7814-X ) follow below as a prelude to the discussion of the Birdcage format as a basis for flex options for the FreeMotion ski boot and ski boots in general.

Biomechanical Considerations of the Ski Boot (Alpine)

Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland

  • Correct flexibility (of the boot shaft) is not the whole answer. The functional anatomy of the ankle joint must also be an important consideration. If the axis of flexion of the ankle joint is not precisely in line with the axis of flexion of the shaft, an unfavorable lever ratio will ensue, and the advantages of flexibility will be lost.
  • 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.
  • 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.
  • From a technical (skiing) point of view, the ski boot must represent an interface between the human body and the ski. This implies first of all an exchange of steering function, i.e., 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). In this way, the skier can adapt to the requirements of the skiing surface and snow conditions. 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.
  • The modern ski boot must be designed from a functional point of view, i.e., the design must take into consideration the realities of functional anatomy (axes etc.).
  • It (the design) should not make compromises at the expense of other joints (length of shaft, flexibility and positioning).
  • It (the ski boot) must represent the ideal connecting link between man and ski (steering and feedback).

Kinematics of the Foot in the Ski Boot – Professor Dr. M. Pfeiffer – Institute for the Athletic Science, University of Salzburg, Salzburg, Austria

  • 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.
  • Because of its effects on the foot, the arc described by the shaft is divided into a “lead segment” and into a later “lever segment”.
  • Forward sliding of the foot should not be possible. There should also be no loss of contact of the sole and no decrease in the “feel”.
  • Previous misconceptions concerning its (the boot shafts) 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.
  • Correct positioning of the foot is more important than forced constraint and “squeezing” the foot. This will prevent the misuse of the ligaments and weakening, particularly of the fibular musculature and ligaments. (This will also explain why even competitive skiers can suffer ankle sprains while engaged in light athletics or even just in everyday activities.)
  • 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.
  • 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.
  • The medical requirements with respect to sports should not be construed as criticism of the boot industry. It is hoped that they are contribution to the development of a ski boot designed along anatomical principles. This goal has not yet been achieved.

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

  • Few forms of athletics place as high demands on the footwear used in their performance as alpine skiing. It (the ski boot) functions as a connecting link between the binding and the body and performs a series of difficult complex tasks.
  • Investigations by Pfeiffer have shown that the foot maintains some spontaneous mobility in the ski boot. Thus the total immobilization by foam injection or compression by tight buckles are unphysiologic.
  • The boot must assure freedom of mobility to the toes. This is accomplished by having a large enough inner shoe. 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.

Sports Medical Criteria of the Alpine Ski Boot – W Hauser, P. Schaff, Technical Surveillance Association, Munich, West Germany

  • Many alpine skiers have insufficient mobility in their knees and ankle. The range of motion, particularly in the ankles, is much too small. This results in a static, stiff run. It does not correspond at all to the ideal of a wide range of mobility in the area of the knee and ankle, which was proposed and taught during early alpine ski lessons. Even the best diadactic (patronizing) methodology is not always successful in imparting to the student the full range of motion. 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. This is particularly true in models designed for children, adolescent and women.
  • In the future, ski boots will be designed rationally and according to the increasing requirements of the ski performance target groups.

The comment Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland 30 years ago has turned out to be prescient:

  • 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.

Unfortunately, Stussi’s warning does not appear to have been heeded.

In an article called Getting Serious About Skier’s Knee (Ski Tech, October 1993), Andy Bigford states, “Horror stories about ACL injuries are a dime a dozen” and “Victims tell of the cost involved (the whole package can run to $50,00 per injury), the difficulties of rehab, the lost work time and the fear of never being able to ski again”. Bigford goes on to point out that not everyone is convinced that the stiff boot-aggressive ski combination is totally at fault; that ACL injuries in the late ’80s and early ’90s came when softer (flex) and ‘more forgiving’ rear entry boots were popular.

Bigford ends his article by stating that the (knee) problem needs to be solved without scarring away skiers, but “If something isn’t done, they (skiers) will have plenty of reason to be scared”.

More than 20 years later, the cover heading What’s New With Knees (December 2016 Ski Canada magazine) is the lead-in to a feature article called Tear and Repair. The article header  states, “With knee injuries so commonplace, especially among skiers, the medical world is constantly updating procedures and surgical techniques for the big fix. The take away message from Tear and Repair  appears to be that the solution to knee injuries is not addressing the cause, it is repairing the resulting damage.

In the same magazine, a comment under Short Turns (Save Your Knees) states: “If you haven’t had a serious knee injury, you likely know someone who has”.  The take away message appears to be that knee injuries are part of skiing. Get used to it!

If my next post, I will discuss how the knowledge gleaned from The Shoe In Sport influenced my thinking on the design criteria for a new ski boot, in particular, shaft configuration and resistance to the rotation while creating an awareness of the need to be able to defend my design criteria with principles of applied science data acquired during actual ski maneuvers.