ski boot

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 FREEMOTION SKI BOOT: FLEXURAL BEHAVIOUR

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

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

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

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

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

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

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

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

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

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

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

triceps-surae

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

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

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

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

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

…. to be continued.

THE EMERGING REVOLUTION UNDERFOOT

What if you used your hands like you used your feet?

Bind your hands with tape and then try typing. Instead of using the muscles and joints of the fingers (intrinsic), you’d peck at the keyboard using excessive motion of the wrists (and extrinsic musculature), leading to intrinsic muscle atrophy and extrinsic muscle (and related joint) overuse. – Katy Bowman, Nutritious Movement FaceBook Group, April 20, 2016 and author of Whole Body Barefoot: Transitioning Well to Minimal Footwear

Bowman wasn’t talking about what happens when you put your foot in tightly-fitting ski boots. She was only talking about what happens to most people’s feet in their everyday shoes. Binding your feet in tightly-fitting ski boots is infinitely worse in terms of making it impossible to effectively use all the muscles in your legs to coordinate balance and initiate precise movement. But in fact, this is the whole idea behind the ski boot. The holy grail in skiing is the fit of a ski boot that perfectly mirrors the shape of the foot and lower leg. In other words, the objective is to render the human foot completely dysfunctional.

“The bootmaker, ignorant of the relative use and importance of the different parts of the foot, has steadily persisted for centuries, and at this day usually persists, in so shaping the shoe that the great toe is forced upon the other toes more or less out of its right line with the heel. (1)

“The Problems in Existing Ski Boots

Existing ski boots 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. Thus, the problem with existing ski boots arises due to the dynamic nature of the architecture of the foot”. (2)

One hundred and thirty years after the article was published in the Scientific American, not much has changed. But it gets worse.

“The foot may have been distorted by wearing improperly made shoes, and the person may have become accustomed to the bad-shaped shoe.” (2)

“The less a shoe does to a foot, the better for the foot. A shoe should stay out of the foot’s way. In it’s most elemental form, shoe has only two functions; as a non-intrusive protective foot covering and as an ornamental dressing. The moment a shoe assumes a therapeutic function for the average foot, the foot is in trouble. (3)

LIFT ME UP?
“The worst thing shoes do, according to Bowman, Rossi and others? Tilt the whole body forward by elevating the heel so it is higher than the forefoot. In ski boots, I call this the Net Ramp Angle.

Last ski season, I learned more about skiing than in all the years since I started down this road in 1973. Prior to last season, I knew that a Net Ramp Angle of more than about 3 degrees had an adverse effect on stance, associated balance, ski control and especially the ability to move with precision. But I had no idea just how critical ramp angle is until I reduced the ramp angle of the boot boards in my own ski boots from 3.0 degrees to 2.5 (my bindings are zero ramp). Then the whole world changed. In working with other skiers and racers I found that they were sensitive to changes of one tenth of a degree.

Jessi Stensland, a high performance athlete, talks about how shoes affected her performance and served as the inspiration for FOOT FREEX in a the video in the following link – http://www.feetfreex.com/#!about/c1nx6

After working with Whistler Ski Pro Matt since skiing started last fall it is only in the past few days that Matt’s feet were finally set free and after thousands of hours on skis, he finally knows, for the first time, what it is like to truly ski. Mat’s problem was not a lack of fitness and/or athletic ability. Matt’s problem is his large boned feet that cannot function in a ski boot without extensive modifications. I will explain, in graphic detail, what had to be done to Matt’s boots in my next post.


1. NATURAL SHAPE OF THE FOOT—HOW BOOTS AND SHOES OUGHT TO BE MADE – Scientific American, October 25, 1862

2. US Patent 5,265,350 1992 – MACPHAIL, November 30, 1993

3. Footwear: The Primary Cause of Foot Disorders – Willam A. Rossi – “http://www.feetfreex.com/#!references/c19n”>http://www.feetfreex.com/#!references/c19n

 

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.

 

FLEX YOUR SKI BOOT FLEX MUSCLES

I believe that the best way to understand how something works is to actually experience it myself, not just take someone’s word for it. When I read articles in ski magazines years ago about how important boot flex is to skiing I just had to try it myself. I was especially intrigued by the claim that only the best skiers have muscles strong enough to flex a stiff race boot. To me this was saying that if you can flex with the best, you can ski with the best. Game on! So I went up on the ski hill one day and tried flexing my boots as deeply as I could under different conditions and in different parts of a turn. I had to try and figure out when to flex and when not to flex because the experts who stress the importance of having boots with the right flex index didn’t say how and when flex should be used. But if the experts said flex is important, I had to find out if they were right. And that is exactly what I did.

Going up on a ski hill and flexing boots during actual ski maneuvers is a lot of work. And to be quite honest, I found it more than a little tricky. Maybe it was just me. But I wasn’t very good at flexing my boots in order to put pressure on the shovel of my ski to start it turning. It didn’t help my understanding of how to use boot flex when I studied the best skiers like Ligety and noticed that the shank angle of their outside leg changes very little especially in the high load phase in the bottom of a  turn. It must be an illusion. It’s probably caused by the distortion of the long camera lenses.

So how good a skier you are? Here’s a easy way to find out.  Put your boots on and buckle them up. Sit down and place your feet (with the boots on) on a hassock or even a box. Now flex your boot flex muscles and see how much you can flex your boots.  If your boots are too stiff  you could perhaps prevail on a local ski shop to see if they will let you flex tests some boots. You could even make an afternoon of it. Have a boot flexing competition with your buddies. Find out who’s the best.

If, you can’t flex your boots, better hit the gym. Maybe hire a fitness instructor. Pump those boot flex muscles up. Remember, boot flex is critical in skiing.

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.

 

 

EPICSKI ARTICLE – THE BIRDCAGE EXPERIMENTS

The following is an article that I posted on EPICSKI (www.epicski.com) on June 18, 2010. The article states that the tests were done on the Blackcomb Glacier. In fact, the tests were done on Whistler Mountain’s summer glacier • Last updated 12/15/12 • 1,380 views

The Birdcage Experiments by David MacPhail

In the summer of 1991 a science team that Steve Podborski and I had assembled to develop a new ski boot conducted pioneering experiments on the Blackcomb Glacier with a device we affectionately named the “Birdcage.” The Birdcage connected a rigid structure to the foot at very specific points while leaving the remaining areas of the foot free of any constraint. The object of the experiments was to study the effects on balance of specific forms of constraint applied to the mechanical points of the foot we had previously identified. The experiments were designed according to a scientific protocol that standardized conditions from test to test while varying one component at a time.

Birdcage
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 we could study the firing order and pattern of specific muscles. This data could then be used to determine the order of events such as balancing, steering and edging. 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 so it was too erect the soleus would make multiple attempts at the start of each turn to try and get CM over top of and in front of the ankle.

Our primary tester for the experiments was Olympic bronze medallist and World Cup downhill champion Steve Podborski. Steve is shown to the left trying on the Birdcage. The cable coming out of 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. Since telemetry was too costly and less positive we used a 1200 ft cable that linked the Birdcage to a computer that was set up in a tent. Although the technician could not see the skier, he could easily assess their technical competence within a short period of time by assessing the data generated. This was even more remarkable considering that the technician had no background in skiing or ski teaching.

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 them. Of interest is the fact that this work was universally regarded within the ski industry as completely irrelevant to skiing.

Here is the link to the original article – http://www.epicski.com/a/the-birdcage-boot-experiment-by-david-macphail

In a series of future posts I will discuss the findings from these ground-breaking, on-snow, real-time experiments. As far as I know they had never been done by anyone else before I did them and have never been done by anyone else since the Birdcage experiments.