ski boot

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE – PART 13

The  article that follows was published on June 18, 2010 on an internet group called EPICSKI.  I have revised the article to improve clarity and consistency with the technical terms used in the THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE series of posts.

The Birdcage Experiments

 by David MacPhail

In the summer of 1991 a science team Steve Podborski and I had assembled to develop a new ski boot conducted pioneering studies on the Blackcomb summer glacier with a device we affectionately named the “Birdcage.” The purpose of the studies was to test my hypothesis of the mechanics and biomechanics of platform angle as it pertains to skier dynamic stability and the basic premise of my hypothesis that explains how  GRF acting on the inside edge of the outski is extended out under the platform of the ski. The Birdcage is shown in the photo below.

Birdcage

The Birdcage was fit with 16 sensors each with its own channel as shown in the legend below.

Specific mechanical points of the foot, in particular the ends of the eccentric torque arm, connected to specific points of the rigid structure of the Birdcage while leaving the remaining areas of the foot substantially unconstrained. The object of the experiments was to study the effects of specific forms of constraint applied to key mechanical points of the foot we had previously identified on skier balance as it pertains to steering and edge control. The experiments also included tests that studied the effect of interfering with specific joint actions. The experiments were designed in accordance with a standard scientific protocol; one that standardized conditions from test to test while varying one factor at a time.

For example, to study the effects of cuff forward lean angle on specific muscles, the range of rotation of the cuff was kept the same from test to test while the initial angle at which the cuff was set was varied from test to test. The cuff was fit tightly about the leg so as to reduce to a minimum any effects of movement of the leg within the cuff. Other aspects of the test such as position of the heel and ball of the foot in relation to the centerline and inside edge of the ski were kept the same.

By using such test protocols the firing sequence of specific muscles and their effect on dynamic stabilty could be studied. This data could then be used to determine the sequence of events and relationship steering to edge platform angle control. It was discovered that by varying the conditions that affected the firing and effectiveness of the soleus muscle, it could be played like a musical instrument. For example, if the cuff angle were set too erect the soleus muscle would make multiple attempts at the start of each loading sequence to try and get COG over the head of the first metatarsal.

Our primary tester for the experiments was Olympic bronze medallist and World Cup Downhill Champion Steve Podborski. Steve is shown in the photos below having the Birdcage adjusted to his foot and leg.

The cable coming from the rear of the device is connected to a Toshiba optical drive computer (remember, this is 1991) that Toshiba loaned us in support of our program. The biomedical engineer and the Toshiba computer are shown in the photo below.

Since telemetry was too costly and less positive we used a 1200 ft cable that linked the Birdcage to the Toshiba computer set up in a tent. Although the technician could not see the skiers being studied within a short period of time he could easily analyze their technical competence in real time by assessing the incoming flow of data from the sensors fit to the Birdcage. This was even more remarkable considering that the technician had no background in skiing, ski teaching or coaching.

The testers wore a harness to keep the cable from interfering with their movements. A chase skier ensured that the cable remained behind the testers and did not pull on the testers. Of interest is the fact that I was unable to elicit any interest in the results of the Birdcage study

As far as I know a study of this nature had never been done before and to the best of my knowledge a similar study has never been repeated since the Birdcage experiments. The Birdcage remains one of the most sophisticated analytical sports devices ever conceived even by todays’ standards. The Birdcage research vehicle is the barefoot minimum standard for the ski boot.

SKIER BALANCE: IT’S ABOUT BALANCING OPPOSING TORQUES

The subject of my 4th post published on May 14, 2013 was the role of torques in skier balance. That this was one of my most important yet least viewed posts at 109 views suggests that the role of torques in skier balance is a concept foreign to skiers especially the authorities in the ski industry. This post is a revised version supplemented with information results from a recent study on balance control strategies.


While everyone recognizes the importance of good balance in skiing, I have yet to find an definition of what is meant by good balance, let alone a description of the neurobiomechanical conditions under which a skier is in balance during actual ski maneuvers. In order to engage in a meaningful discussion of balance, one needs to be able to describe all the forces acting on the skier, especially the opposing forces acting between the soles of the feet of the skier and the snow surface (ergo – applied and ground or snow reaction forces). Without knowing the forces involved, especially torques, any discussion of balance is pure conjecture. In 1991,  I formulated a hypothetical model that described these forces.  I designed a device with biomedical engineer to capture pressure data from the 3-dimensional forces (torques) applied by the foot and leg of the skier to the internal surfaces of the boot during actual ski maneuvers.

Test subjects ranged from Olympic and World Cup champions to novice skiers. By selectively introducing constraints that interfered with the neurobiomechanics of balance even a World Cup or Olympic champion calibre skier could be reduced to the level of a struggling beginner. Alternatively configuring the research device to accommodate the neurobiomechanical associated with skiing enabled novice skiers to use  balance processes similar to those of Olympic champions. To the best of my knowledge, no one had ever done a study of this nature before and no one has ever done a similar study since.

When analyzed, the data captured using the device called into question just about everything that is accepted as fact in skiing. This study was never published. For the first time I will present the data and describe the implications in future posts. We called the device shown in the photo the Birdcage. It was fully instrumented with 17 sensors strategically placed on a 3 dimensional grid.

Birdcage

The Birdcage instrumentation package was configured to detect coordinated neuromuscularly generated multiplane torques that oppose and maintain dynamic balance against external torques acting across the running surface of the inside edge of the outside ski in contact with the source of GRF (i.e. the snow).

  1. plantarflexion-dorsiflexion
  2. inversion-eversion
  3. external/internal vertical axial tibial rotation

Ankle torques are applied to the 3 points of the tripod arch of the foot (heel, ball of big toe, ball of little toe) and can manifest as hindfoot to rearfoot torsion or twisting wherein the forefoot rotates against the rearfoot.

A recent study (1.) on the role of torques in unperturbed (static) balance and perturbed (dynamic) balance found:

During perturbed and unperturbed balance in standing, the most prevalent control strategy was an ankle strategy, which was employed for more than 90% of the time in balance.

In both postures (unperturbed and perturbed) these strategies may be described as a single segment inverted pendulum control strategy, where the multi-segment system is controlled by torque about the most inferior joint with compensatory torques about all superior joints acting in the same direction to maintain a fixed orientation between superiorsegments.

The alignment of opposing forces shown in typical force representations in discussions of ski technique is the result of the neuromuscular system effecting dynamic balance of tri-planar torques in the ankle-hip system.

NOTE: Balance does not involve knee strategies. The knee is an intermediate joint between the ankle abd hip and is controlled by ankle/hip balance synergies.

The ankle strategy is limited by the foot’s ability to exert torque in contact with the support surface, whereas the hip strategy is limited by surface friction and the ability to produce horizontal force against the support surface.

Ankle balance strategies involve what are called joint kinematics; 3 dimensional movement in space of the joint system of the ankle complex. Contrary to the widely held belief that loading the ankle in a ski boot with the intent of immobilizing the joint system will improve skier balance, impeding the joint kinematics of the ankle will disrupt or even prevent the most prevalent control strategy which is employed for more than 90% of the time in balance. In addition, this will also disrupt or even prevent the CNS from employing multi-segment balance strategies.

Regardless of which strategy is employed by the central nervous system (CNS), motion and torque about both the ankle and hip is inevitable, as accelerations of one segment will result in accelerations imposed on other segments that must be either resisted or assisted by the appropriate musculature. Ultimately, an attempt at an ankle strategy will require compensatory hip torque acting in the same direction as ankle torque to resist the load imposed on it by the acceleration of the legs. Conversely, an attempt at a hip strategy will require complementary ankle torque acting in the opposite direction to hip torque to achieve the required anti-phase rotation of the upper and lower body.

Balance is Sensory Dependent

As a final blow to skier balance supporting the arch of the foot and loading the ankle impairs and limits the transfer of vibrations from the ski to the small nerve sensory system in the balls of the feet that are activated by pressure and skin stretch resulting in a GIGO (garbage in, garbage out) adverse effect on balance.

Spectral analysis of joint kinematics during longer duration trials reveal that balance can be described as a multi-link pendulum with ankle and hip strategies viewed as ‘simultaneous coexisting excitable modes’, both always present, but one which may predominate depending upon the characteristics of the available sensory information, task or perturbation.


  1. Balance control strategies during perturbed and unperturbed balance in standing and handstand: Glen M. Blenkinsop, Matthew T. G. Pain and Michael J. Hiley – School of Sport, Exercise and Health Sciences, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK – Royal Society Open Science

THE SKI BOOT FLEX INDEX INSTABILITY PROBLEM

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 – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3887325/
  2. http://wp.me/p3vZhu-zx

THE PURPOSE OF THE SKI BOOT

As a prelude to discussing the objectives of my work with Whistler-Blackcomb Ski Pro, Matt, it is important to establish the position of various authorities on the purpose of the ski boot from the perspective of skier function.

Here is what the authorities cited below said in 1987

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).

Biomechanical Considerations of the Ski Boot (Alpine) Dr. E. Stussi,  Member of GOTS –

Chief of Biomechanical Laboratory ETH, Zurich, Switzerland – 1987

_________________________________

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.

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

_________________________________

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 may 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.

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

_________________________________

Here is what I said in 1992

The interaction between the foot and the footwear necessary to elicit optimum response from the medium to which it is attached is not well understood. 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. 
Alpine ski boots, ice skate footwear and cycling shoes are among the many types of sports footwear known. As with all sports footwear, the objectives in design and construction are to facilitate and enhance performance in the particular sport and to provide comfort to the wearer.

US Patent No 5,265, 350 February 1992 – MacPhail, David Michael

_________________________________

After the skier, the most important piece of equipment in the skier/ski equipment system is the ski boot. The conventional ski boot has the biggest influence on who rises to the top technically. Those who are able to connect with the snow through their feet so as to enable the use of their natural  mechanisms of balance are unlikely to lose the resulting kinaesthetic association. When these skiers try on a new boot, they usually know within seconds whether it will work for them or not without even having to go on snow. But for nine out of ten never-evers, the initial skiing experience involves such a severe disconnect from familiar sensations, especially a compromise of balance, that is so unsettling that their first day on skis is also their last day.

It is for this very reason that the consensus of the previously cited authorities is that a ski boot should be adapted to the functional requirements of the user and not the other way around. It is particularly important that the ski boot not incur functional compromises on the part of the user. A properly designed ski boot should enable the user to utilize mechanisms of ski control that are complimentary to and consistent with, their innate mechanisms of balance.

In my next post, I will  use video clips and annotated screen shots to describe how Matt’s ski boots were compromising his function and the process by which his boots were modified so as to adapt them to Matt’s functional functional requirements.

 

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.

 

 

PRONATION AND THE BIOMECHANICS OF EDGE CONTROL

The stick man sketch below are Figures 23 A and 23 B from US Patent No. 5,265,350 (expired) awarded to the writer.  The stick man in FIG. 23 A is engaged in quiet standing with the weight equally distributed between the left and right feet. This is called Bipedal (two-footed) Stance. The force vector W emanating from the Centre of Mass or CoM  is the ‘disturbing force’ of gravity.  W is called a disturbing force because it is tending to disturb the equilibrium of the stick man and cause him to topple.

Bi-Monopedal

Gravity is an ‘attractive force’ like magnetism. CoM is where you are in relation to the supporting surface. In this case, ground. But W is not the force applied to ground by the stick man. The applied force occurs at the contact points of the foot or feet with ground.  In FIG 23 A,  W lies equidistant between the two feet in the transverse plane.

In normal Bipedal Stance, each foot supports equal proportions of the bodyʼs weight W, assuming equal leg lengths. Approximately 50 percent of the load is borne by the heel. The remaining 50 percent is borne by the heads of the long metatarsal bones. The load on the head of the first metatarsal (aka the ‘ball of the foot’) is twice that of each of the heads of the other four metatarsals. The anteroposterior (ergo, front to back) distribution  of the load through the foot is due to the position of the CoM of the body above.  The point on the foot where the centre of the applied force appears to act is called the Centre of Pressure or CoP. I say ‘appears to act’ because CoP could lie somewhere in the vault of the arches of the foot. In Bipedal Stance CoP lies on an axis that runs through the proximate centre of the heel and the head (ball) of the 2nd metatarsal. In ice skates, this is the ‘balance point’ where the ice blade is mounted. The forces shown as w2 are the centres of the ground reaction force or  that opposes CoP.

The footwear industry’s dirty little secret is that shoes are made on lasts that approximate the shape of the human foot in Bipedal Stance; standing on two feet and not moving. When you start to walk in a shoe, the structures deform and distort to accommodate changes in the architecture of the foot. Ski boots are worse. Not only are they built on lasts that are the approximate the shape of your feet and legs in quiet Bipedal Stance, the structures impede or even prevent the user from attaining a dynamically balanced base of support on one foot. Claims in relation to skiing are made that the human foot functions best in skiing when its joints are immobilized, preferably in a neutral position. In a neutral position, joint actions of the foot and knee and hip are limited to flexion and extension with transverse and orbital movement of the leg in hip joint within its normal range of motion.

A whole industry has been established on methods of immobilizing the foot and stabilizing it in a neutral position with custom formed boot shells, custom formed liners and custom formed footbeds and orthotics that significantly restrict or prevent pronation. The indirect effect of preventing pronation is that the position of CoP on the axis running through the proximate centre of the heel and head of the 2nd metatarsal becomes fixed.  For reasons that will be explained in future posts, this can have the effect of preventing the user from being able to establish the over-centre edge control mechanics that the best skiers use and especially an inability to establish a dynamically stable base of support on which to move from ski to ski.

SKI RACING: AN UNLEVEL PLAYING FIELD

Ski racing is a lot like a lottery. With rare exceptions, equipment, but especially ski boots, can create what amounts to an unlevel playing field, one that can prevent racers from performing to their full potential. Because of the often significant physical differences in the feet and lower limbs from one competitor to another, the degree to which racers can be compromised by equipment varies greatly. This can be especially true if a racer happens to stumble upon the right combination. In ski racing, although luck is a factor, having the right feet with ski boots that allow them to function the way they were intended to can be everything.

Back in 1977 Lange was the only boot I had found of that allowed me to literally build a pair of race boots from the ground up. Like  Alan Trimble (boot tech for Lange USA), I was starting the process with a shell bottom and adjusting cuff side cant and forward lean angle of the rear spoiler to conform to the racer’s legs. In other words, I was building every pair of boots to each racer’s individual functional specification. But I was adjusting two other things that no one even seemed to even be aware were issues; net ramp angle and position of the ball of the foot and big toe in relation to the ski edge underfoot.

Net ramp angle is the inclination of the sole of the foot in relation to the surface of the snow. It affects the muscles a racer can use and especially the ability to apply force to the fore body of a ski and the ability to activate what I call the auto processes of edge control and turning forces. Two factors contribute to forward inclination; 1) the ramp angle of the boot board that the sole of the foot is supported on and , 2) the angle created by the heel and toe plates of the ski binding. I don’t know where they are today, but back in 1977 ramp angles were all over the map in both boot and binding brands and even models. Because of this, both the base angle of the boot sole plate and binding had to be considered as a unit in determining net ramp angle. Although few if any were aware of this issue, to me it explained why a racer’s skiing and results sometimes went downhill when they changed to a different ski binding or boot or worse, changed both. The generally accepted assumption, one that persists even today, is that a good athlete can ‘adjust’ to their equipment. Because of this, boots are not considered a factor. Fast skis are everything. While it is true that a talented racer can ski in just about any equipment, adaptation comes at a cost. The cost comes in the form of a reduced ability to perform.

If a NASCAR team were to announce that they intended to enter a stock sedan right off the showroom floor with no modifications in the Daytona 500 and that they would be competitive because their driver would ‘adapt’ to the lack of power and handling of the car they would be laughed off the circuit. If a world class sprinter announced that he or she that was going to compete wearing stiff leather dress shoes and that they would ‘adapt’ to the considerable limitations of the footwear no one would take them seriously. Yet most coaches, and even some racers, tend to minimize the role of the ski boot. When a racer dominates his or her competitors it is attributed to ‘exceptional talent’ and they are elevated to the status of a god with mystical powers.

In a November 13, 1990 Globe & Mail article appropriately titled, Boyd putting best foot forward,  Whistler’s Rob Boyd describes how the boots I had built for him changed his skiing and renewed his enthusiasm for racing. Said Boyd, “In Chile, I skied easily. It was fun again. It rekindled my love of skiing. Everything was so smooth…….” Boyd went on to say how he used to only worry about a solid fit in his boots, how he skied from the ankle up and that after skiing in the boots I had made for him he realized, “how much the foot can be used and should be used.”  Earlier that year my coaching counterpart, Glen (Meister) Wurtele (head coach of the men’s team), had summoned me to action in the World Cup Wars to work on Rob’s boots. After preparing his boots in my Whistler shop I flew to Portillo, Chile to hook up with the Canadian Ski Team who were training on the summer glacier.

The next day I was standing on a knoll about half way down a 45 second downhill course behind the lodge with the team coaches when Boyd took his first run in his new boots. When he came into view Boyd was skiing so differently that the coaches standing with the Meister and I didn’t recognize him. They were sure it was another racer. As he drew near, it became obvious that it really was Rob. When the coaches began to speculate as to what to what had caused such a dramatic change in Boyd’s technical skiing Wurtele said, “It’s his new boots”. The coaches were emphatic, ski boots could not possibly affect a racer’s skiing to that degree. Yes, they really can. In future posts I will explain why.