Footwear science posts

SKIER LEG SHANK ANGLE VS. BOOT SHAFT ANGLE

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.

–  Sports Medical Criteria of the Alpine Ski Boot – W Hauser P. Schaff, Technical Surveillance Association, Munich, West Germany – The Shoe in Sport (1989) – published in German in 1987 as Der Schuh Im Sport – ISNB 0-8151-7814-X

In a conventional ski boot, the rear aspect and sides of the shaft are fixed in relation to the shell lower with the result that the angle of the rear spine of the shaft is fixed. The leading edges on either side of the shaft, overlap at the forward aspect where they are drawn together by closure means. In this configuration, the angle of the shank of a user is dependent on the degree with which the closures draw the uppermost forward aspect of the leading edges of the shaft towards each other, and towards the rear spine, in proportion to the amount of overlap created by the operation of the closure mechanism.

The graphic below shows two photos of a right ski boot shell. In the left photo, the shaft buckles are operated to the minimal closure position. In the right photo, the shaft buckles are operated to the maximal closure position. A red reference line at the rear spine indicates the fixed shaft angle. A red reference line at the front aspect of the shaft overlap indicates the variable aspect of the shaft angle is it pertains to the shank angle of the user. An arbitrary reference center with which to gauge the variance in the shank angle is shown in black. The reference shank angle in the maximal closure position (right boot) is 8 degrees less than the reference shank angle in the minimal closure position (left boot). In terms of angles of ankle flexion, the ankle of the same foot and leg in the boot shell in the right photo would be 8 degrees plantarflexed when compared to the ankle of the same foot and leg in boot shell in the left photo.

Shank difference

The implications of this arrangement are that the shank angle of the user will change in response to changes in the operation of the closures of the shaft, especially changes in the top shaft closure and/or the amount of tension in the power strap, if equipped with one. For the most part, racers are unaware of the critical nature of the correct shank angle. They have erroneously assumed, or have been taught, that a securely tightened shaft is essential for good control of the ski. The tighter the shaft is secured to the leg, the better the control. As shown in the photo below, boot makers provide shaft buckles with high leverage features that facilitate a secure closure of the shaft with the leg. Closing the boot shaft beyond the shank reference angle can have serious implications for balance and control of the ski.

Leverage boost

The angle of the shank of the user is dependent on the degree of overlap of the leading edges of the shaft, including the tension of any power strap. The cross-sectional area of the leg of the user at the boot top, in particular, the front to back dimension of the cross-sectional area, is also a factor that affects the correct angle of the shank.

The two photos below compare the shaft angle of a stock boot shell to the shaft angle of a boot shell that has been modified to reduce the shaft angle by 8 degrees in order to correct for excessive shank angle.

8 degree difference

Operating the closure mechanism of a shaft beyond a certain point creates another problem, as shown in the photo below, deformation of the interfaces of the overlap elements of the shaft.

Overlap deformation

The angle of the shank of a racer is critical. It must be maintained within a narrow range for optimal performance. Anything that has the potential to alter an optimal shank ankle should be carefully evaluated.

Related post: GETTING SHAFTED BY THE (SKI BOOT) SHAFT -https://skimoves.me/2014/04/09/getting-shafte…ski-boot-shaft/

 

 

COUPLED FORCE WHOLE LEG ROTATION

The most important event in a turn is whole leg internal rotation (Event 7) following ski flat (Event 3). But the mechanism by which whole leg internal rotation is applied to the ski is as important, if not more important, than the actual whole leg rotation.

As the outside ski changes to its new inside edge, the racer rotates the whole leg internally using top down rotation from the pelvis. The purpose of ski flat at the conclusion of the transition (Event 1) phase, is to neutralize torsion across the pelvis so it is square to the trajectory of the racer. In order to use whole leg internal rotation, the COM of the racer must be positioned on the new outside foot at ski flat in what I call monopedal stance. Monopedal stance (aka monopedal function) is a physiologic state wherein balance is achieved with the weight of the body borne on the medial plantar aspect of a fully pronated foot.

The graphic below is Figure 23 from my US Patent No. 5,265,350.

Bi-MonopedalFigure 23 A depicts bipedal stance. The points of the central load-bearing axis are stacked vertically over top of each other in the frontal plane. The load W from COM is centred between the feet with each foot carrying half the load (W2).

Figure 23 A depicts monopedal stance. In monopedal stance, the load W from COM is aligned over the proximate centre of the head of the first metatarsal in both the frontal plane (across the racer) and saggital plane (front to back). Monopedal stance at ski flat is eloquently demonstrated by Bridget Currier  in the Burke Mountain Academy YouTube video, Get Over It with commentary by Mikaela Shiffrin – (http://youtu.be/Bh7KF49GzOc).

The opening graphics advise the racer to Get Over It and Stay Over It, meaning maintain the alignment of W from COM, over the proximate centre of the head of the first metatarsal in the frontal and saggital planes throughout the entire turn. But few racers can Get Over It, let alone Stay Over It, because the structures of their ski boots prevent them from assuming monopedal stance. This is especially true of racers whose boots are closely formed to the shape of their foot and leg in what amounts to perfect envelopment.

The graphic below is a re-creation of the stick person in Figure 23 above. The notations have been revised to reflect the terminology used in blog posts. The left stick person is depicted in bipedal stance. The centre stick person is depicted in monopedal stance. The right stick person is depicted in fixed neutral stance. When the foot is fixed in neutral, pronation is not possible and the foot is prevented from everting (the sole turns outward). In order for W emanating from COM to be positioned over the proximate centre of the head of the first metatarsal, the foot must evert approximately 7 to 8 degrees as depicted in the centre stick person.

The graphic below shows the effect of fixing the foot in neutral. When a racer attempts to balance on the new outside limb at ski flat, the inability to align W with GRF at the inside edge of the outside ski will cause the racer to fall into the new turn or consciously move away from the outside ski. .

Falls into turnPreventing the foot from pronating within a ski boot causes other problems. When the leg is rotated internally relative to the foot by the hip rotators, a torsional load is applied to the foot. Conventional ski boots do not provide surfaces for the foot to transfer biomechanically generated forces such as torque to. In addition, the structures of conventional ski boots present sources of resistance which interfere with the movements necessary to establish force transfer connections of discrete aspects of the foot with the boot shell.

Figures 22 A through 22 D below are from US350. Figures 22 A and 22 B depict the architecture of a foot in bipedal stance. Figures 22 C and 22 D depict the architecture of a foot in monopedal stance. Changes in the length of the foot in bipedal and monopedal stances are annotated as  L1 (bipedal) and L2 (monopedal). Changes in the angle of dorsiflexion of the ankle joint in bipedal and monopedal stances are annotated as  A1 (bipedal) and A2 (monopedal). Changes in the height of the arch in bipedal and monopedal stances are annotated as H1 (bipedal) and H2 (monopedal). Internal rotation of the leg in monopedal stance is annotated at 6. Changes in the length of the foot in bipedal and monopedal stances are annotated as  L1 (bipedal) and L2 (monopedal).  Changes in the position of the head of the first metatarsal in bipedal and monopedal stances are annotated as  2. Changes in the position of the medial tarsal bone in bipedal and monopedal stances are annotated as  3. Changes in the width across the heads of the metatarsals in bipedal and monopedal stances are annotated as  4. Shear forces, which will be the subject of a future post, are shown in Figure 22 D.

Screen Shot 2015-01-08 at 2.12.34 PMIn order to apply top down internal rotation, the racer has to move the load W to the ball of the foot as shown in the graphic below.

IdealThe short video clip below shows how the foot must be able to pronate within the confines of the ski boot without interference in order to set up the force couple required to transfer whole leg internal rotation to the new outside ski. The typical most significant source of interference is the structures of the ski boot in front of the ankle joint on the inner aspect of the boot.

 

The red bars in the BIPEDAL foot define common sources of interference created by structures of the ski boot that prevent the foot from pronating and establishing force transfer connections with the shell as shown in the MONOPEDAL foot. While the connection of the two transfer points suggests that the centre of rotation lies within the confines of the foot its true centre is not intuitive. This will be the subject of the next post.

BIO-MEDICAL PERSPECTIVE

Normal medial STJ movement of the talus is followed by a mandatory normal 1:1 coupling of the tibia to encourage normal internal leg rotation and normal dorsiflexion of the ankle. This normal coupling mechanism produces a synergistic postural response enhancing internal rotation of the entire leg.  Pelvic counter ensures hip capsule tightening which stabilizes the hip joint during the turn.

Screen Shot 2015-01-28 at 1.09.11 PM

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.

 

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.

 

 

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.

 

 

SKIING IS BELIEVING – MAKING THE BOOT WORK

Since the end of ski season is rapidly approaching in many parts of the world, I am going to do a series of posts on the modifications that I typically make to my ski boots that allow my lower limbs to function for skiing. The best way to appreciate my position on the biomechanics of lower limb function required for effortless skiing is to experience it. But experiencing what I experience requires a functional environment for the foot and leg within the ski boot.

Contrary to what some ski magazines and boot-fitters would have people believe, ski boots do not function. They are inanimate objects. In my US Patent No. 5,265,350 I go into great detail on the structural aspects of a ski boot required to create a functional environment for the foot and leg.  While it is possible to apply the information contained in the patent to a conventional ski boot, the intent was to apply it to the design of an entirely new ski boot.  If you want to ski like Ligety, Shiffrin or any skier who engages the external forces to drive their outside ski into a turn you need to have ski boots that allow the same 3-dimensional movements of the elements of the foot and leg that they use. But the reality is that without a foot and leg shape that is compatible with the internal constraints typical of most rigid ski boot shells the task of finding and modifying a boot is made more difficult.

Here are some excerpts from my US Patent No. 5,265,350 on the problems with conventional ski boots aka the prior art.

The (joints of the) foot articulates in order to facilitate muscle function. Muscles respond in opposition to loads imposed upon the foot. A process ensues wherein the chain of articulations, initiated at the foot, are continuously mobilized so as to maintain a state of balance.

COMMENT: This is an immutable principle that for some reason ski boot designers and boot-fitters seem to ignore.

Existing footwear (i.e. the conventional ski boot) 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.

Although somewhat vaguely stated, a generally accepted theme has arisen over the years, one of indiscriminate envelopment and “overall restraint” applied to the foot and leg within the footwear. The stated position of various authorities skilled in the art of the design and fabrication of footwear for skiing is that the foot functions best when movement about its articulations is substantially prevented or restricted. 

To serve this end, inner ski boot liners are usually formed around inanimate lasts or, alternatively, the foot and leg are inserted into an inner liner within the ski boot shell and foam is introduced into a bladder in the liner so as to totally occupy any free space between the foot and leg and the outer ski boot shell. The outer shell of the footwear is closed around this inner envelopment forming an encasement with which to secure and substantially immobilize the foot and leg. This is considered the optimum and, therefore, ideal form of envelopment. The perspective is that the physiologic structures of the foot are inherently weak and thus, unsuited for skiing. Enveloping the foot within an enclosure which makes it more rigid is thought to add the necessary strength with which to suitably adapt it for skiing. The reasoning being, that the foot and leg now having being suitably strengthened, can form a solid connection with the ski while the leg, now made more rigid, can better serve as a lever with which to apply edging force to the ski. To some degree, the prior art has acknowledged a need for the ankle joint to articulate in flexion. However, the prior art has not differentiated exactly how articulation of the ankle joint might be separated from the object of generalized and indiscriminate envelopment and thus made possible. Therefore, the theme of prior art is inconsistent and lacks continuity.

COMMENT: The reality is that ski boot designers and boot-fitters have no definitive idea of how the function of the foot and lower limb as a whole is impacted when a skier clamps their foot inside the confines of a rigid plastic ski boot. The effect may be quite different from one skier to another and even between the left and right feet of the same skier. And it can and does change from one ski day to another. Hence the reference in my patent to indiscriminate envelopment and  overall restraint. This is an important point because the tacit assumption on which most ski teaching and coaching methodologies are based is that skiing like the best skiers is a simple matter of  watching and copying the best. This ‘theory’ is complicated by the fact that what expert skiers, including ski pros, coaches and even the world’s racers, say they are doing and what they are actually doing can be two very different things.

In my first year on skis in low-cut leather boots I distinctly recall watching in awe as expert skiers skied effortlessly on icy slopes, their edges gripping the slippery surface like magnets. Even though I skied with ease and could make elegant parallel turns on powdery snow,  I could not hold an edge on ice. Among the explanations offered by the experts for my ineptness; my boots weren’t stiff enough, my edges weren’t sharp enough, I needed more practice or I lacked the athletic ability. None of these explanations helped. One explanation that made sense to me was that the experts, who could hold on ice, established ‘an early edge’. But no one could explain how they did this. It was considered mystical, beyond explanation. The new rigid plastic boots promised universal edge hold for all skiers. But they not only made edging harder for me, they made skiing in general harder, much harder. By about 1985 I began to observe what I came to call the Ski Move. It was subtle. But it was clear to me that the expert skiers who skied with incredible ease and finesse were making different moves than lesser skiers. If this subtle move made skiing incredibly easy for a select few why wasn’t everyone doing it? What I eventually discovered was that for most skiers it wasn’t possible for them to make the Ski Move because their ski boots would not let them.

In my next post I will talk about what Ligety, Shiffrin, Vonn and the rest of the world’s best skiers have in common; the right foot and leg shape and structure.

 

 

 

THE EFFECT OF LIFT PLATES ON SKI MANEUVERS

In THE EFFECTS OF LIFT PLATES – CONTINUED, I used a simple model to show how the presence of joints with rotational capability below the lower end of the mechanical line at the tibia affect the initial length of the torque arm acting on the outside foot stabilized in a neutral position and the force vector of the mechanical line as the foot rotates about its long axis about a pivot under the inner aspect of the foot. In this post, I am going to set this basic model in the context of the forces acting on a skier in the arc of a turn.

When it comes to discussions of the forces involved in ski maneuvers, most of the force diagrams I have been able to find by others are those that show components of gravity (G) and centrifugal forces (C) with a resultant force (R) acting at the inside edge of the outside ski. If a force diagram is really sophisticated, it might show a ground reaction force (GRF) acting in opposition to the resultant force (R) similar to what is shown in the annotated photo below.

Forces on skier

The inference of such simplistic explanations is that, far from being complicated, the forces involved in skiing are really quite simple.

Gravity (G) and centrifugal force (C) are components of a resultant force (R). The resultant force merely has to be shown aligned in opposition to a ground reaction force (GRF) at the inside edge of the outside ski in order to satisfy an explanation of the mechanics of  edge hold. Forces applied by the foot? No need to complicate things. Keep it simple. Ignore them. If other forces are ignored they aren’t important.

Riser plates? Torques? Ignore them too. It would be nice if things were that simple. But when things like centre of pressure (CoP) and the torsional effects of lift plates are added to the discussion, it quickly begins to become obvious that the forces in skiing are anything but simple. Significant forces other than gravity and centrifugal force are present. And they do affect the skier.

In THE EFFECTS OF LIFT PLATES – CONTINUED, I used a simple model to show how the presence of joints with rotational capability below the lower end of the mechanical line at the tibia affect the initial length of the torque arm acting on the outside foot stabilized in a neutral position and the force vector of the mechanical line as the foot rotates about its long axis about a pivot under the inner aspect of the foot. In this post, I am going to set this basic model in the context of the forces acting on a skier in the arc of a turn as shown in the above sketch.

The sketch below shows the forces applied to the outside foot and ski of a turn by the foot through the mechanical line in conjunction with a resultant force acting at the inside edge before a load is applied. This situation would exist if a skier were to get caught inside and lose contact of the outside ski with the snow. The tendency of a limb that unloads from a force applied to it is to release muscle tension and unwind into a supinated position. In the situation described in the sketches the foot has been stabilized in a neutral configuration with arch supports or custom insoles and/or a form fitted liner or possibly a form fitted shell. When the foot is in a neutral position the force applied to the foot will act on the proximate centre of a line that runs through the ball of the 2nd toe and the heel.

1

The sketch below shows force applied to the outside foot causing the rotation described in THE EFFECTS OF LIFT PLATES – CONTINUED. There is way more going on than shown in the sketch. But I will get to the other issues in future posts.

2

The sketch below shows and overlay of the first and second sketches to show the changes. As rotation occurs in the subtalar joint the force vector of the mechanical line shifts towards the outside of the turn. As it does the transverse angle of the ski base flattens. These changes will tend to cause the ski to slip out of the turn forcing the skier to increase the angle of the resultant force R by increasing the angle of inclination.

3

Increasing the angle of inclination makes a bad situation worse because the forces become more aligned with the slope of the hill thus increasing the magnitude of the forces that tend to make the ski slip out of the turn.

20 degreesThe NY Times video – Ligety on GS commented, “The trace of his (Ligety’s) path is smoother than that of his foes, who ski in somewhat violent fits and starts, making adjustments that spray snow.” What are Ligety’s foes doing that is different from what Ligety is doing? The forces on Ligety’s outside ski are consistently rotating into the turn. The forces on the outside ski of his foes are rotating out of the turn. But changes in the consistency of the snow texture and the forces acting on the outside ski cause changes in the edge angle. The animated video clip below show this effect.

Changes in edge angle cause the outside ski to oscillate into and out of the turn. Ligety’s foes, which includes the majority of World Cup racers, are unable to develop a dynamically tensioned base of support on their outside foot and ski because the forces they are applying are on the wrong side on the inside edge and they are unable to apply a countering torque with internal rotation of the outside leg from the pelvis. The result is the outside ski is unstable. It makes small oscillations in response to perturbations in ground reaction force necessitating a corresponding series of small adjustments by the racer. Edge angle oscillation can also cause the ski to suddenly hook into turn without warning causing a fall. In a future post I will include some video clips showing edge angle oscillation.

THE EFFECT OF LIFT PLATES – CONTINUED

In skiing,  a myriad of complex issues are associated with riser plates that elevate the foot above the base of s ski. FIS regulations for 2013-2014 permit a maximum stack height of 100 mm total between the base of a ski and the sole of a racer’s foot.  Because of the complex nature of the issues, I am going to use a simplistic model to explain the primary effects of riser plates.

NOTE: Check FIS regulations for current stack heights.

Platform shoes, high heels and similar footwear that elevate the sole of the foot above the supporting surface, tend to be make the wearer susceptible to ankle sprains when a lateral thrust or cutting move is made off the stance foot. The high centre of the ankle joint in relation to the supporting surface makes the ankle susceptible to lateral rotation and twisting above the contact surface of the sole when angular forces are applied. As the ankle progressively rolls over, the twisting force dramatically increases due to the resulting over-centre mechanism. This is called an inversion sprain because the sole of the foot turns inward towards the centre of the body. In the days when skiers used low-cut leather boots, the experts, who could make their edges hold on hard pistes, would call it ‘falling off the edge’ when their outside foot and ski rolled downhill, away from an edge set.

The model I am going use for my explanation assumes that the feet and legs have been aligned and fixed in the magical neutral position. Technically, a neutral position means that the joint that underlies the subtalar joint (what most people think of as the ankle joint) has been effectively rendered non-functional.

In a neutral position it doesn’t really matter whether the foot is a block of wood or a marvel of anatomy, the force applied by the weight of the body resulting from the force of gravity applied on the mechanical line will impress a portion of the weight to the proximate transverse centre line of the foot. For the following explanation the foot will be considered as solid entity like a block of wood. In a neutral position, both feet would be under the femoral heads. For the sake of simplicity I am showing one foot with the mechanical line vertical to gravity. Static references are shown with dashed red lines.  In skiing the force are more complicated.

The sketch below shows a schematic model of a right leg. As shown in my previous post on this subject, the mechanical line has a ball joint at the top (pelvis) with the subtalar joint below the ankle at the bottom. Both joints allow for rotation in the plane facing the reader. The subtalar joint acts in two coupled planes. For the following explanation only the effects associated with rotation of the foot about an axis below the edge of its inner aspect are considered. Fa is the force applied to the base of foot from the force of gravity acting on the mechanical line. Ma is the length of the moment or torque arm resulting from a line from the pivot axis that is perpendicular to the force vector of Fa.

1In the sketch below, the foot model has rotated counterclockwise 10 degrees about the pivot from the original configuration. The original configuration prior to rotation is superimposed in light grey over the new rotated configuration. Note that the mechanical line as a whole and the centres of the ball and subtalar joints have dropped in relation to the pivot point. The vector of force Fa has shifted to the left and is now angled towards the left hand side of the base of the foot model. The moment arm, Ma, that drives the rotation, has grown longer.  This is called an over-centre mechanism because reversing or unwinding the rotated configuration requires that the load that created it be overcome. As the rotation progresses the mechanics associated with reversing direction become increasingly unfavourable.

2In the sketch below a lift plate has been added to the bottom of the foot model. The previous rotated configuration is superimposed in light grey over the new configuration. Note that the mechanical line as a whole and the centre of the subtalar joint have not changed significantly in relation to the position in the previous sketch. But the vector of force Fa has shifted significantly further to the left and is now almost at the left hand edge of the base of the foot model. In addition moment arm Ma, that drives the rotation, has grown significantly longer.

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The sketch below compares the original unrotated configuration (light grey)  to the rotated configuration with the lift.

4Preventing the outside foot of a turn from pronating by fixing the foot in neutral or otherwise obstructing pronation with arch supporting insoles and/or injected and heat formable liners ensures that any force applied by the balls of the feet will be on the outside turn aspect of the inside edge of the outside ski and that the tibia cannot rotate into the turn. The mechanics described above would be similar to that of a situation where a skier gets inside, ends up losing contact with the snow of their outside ski then re-establishes contact. Although the constraints imposed on the foot and leg by the structures of a rigid ski boot would probably make 10 degrees of rotation unlikely, having the applied force on the outside turn aspect of the inside edge of the outside ski will almost guarantee mechanics similar to those described above.  Clearly lift plates can have a positive effect but only if the moments forces acting on the ski are going into the turn and can be dynamically balanced by muscular effort mediated by the skier.