Ski boot flex posts


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 –


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.




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.


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.


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.


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.


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,


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


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.