Birdcage Experiments

THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: HEEL/FOREFOOT ROCKER

An essential mechanism to the ability to create a platform under the outside ski to stand and balance on using the same processes used to stand and balance on stable ground, is the Heel to Forefoot Rocker. A slide presentation called Clinical Biomechanics of Gait (1.) by Stephen Robinovitch, Ph.D. (Simon Fraser University – Kin 201) is a good reference for the various aspects of gait.

Slide 19 of the Gait presentation describes the ankle Inversion-Eversion-Inversion sequence of the ankle. The sequence begins with heel strike (HS), followed by forefoot loading (FF), followed by heel off (HO) followed by toe off (TO).

The normal foot is slightly inverted in the swing phase (unloaded) and at heel strike. It is everted through most of the stance phase. The ankle begins to invert in late stance. The kinetic flow of pressure is from the heel to the ball of the foot and big toe. This is what should happen in the transition phase of a turn sequence when a skier begins to transfer more weight to the inside foot and ski from the outside foot and ski. Up until the start of the transition, the skier’s center of mass is behind the inside foot with the majority of pressure under the heel on the transverse center of the foot and ski where is exerts an inversion torque that is tending to rotate the ski into contact with the surface of the snow. The skier maintains the edge angle by applying a countering eversion torque with a combination of external rotation-abduction of the inside leg.

When the skier begins to transfer more weight from the outside ski to the inside ski, the leg releases the countering eversion torque and the ski begins to invert in relation to the surface of the snow.

The presentation on the Clinical Biomechanics of Gait did not include important aspects of the stance phase that occurs in late stance. Nor, did it mention Achilles forefoot load transfer.

The Three Rockers

Slide 23 shows the Three Rockers associated with the gait cycle.

First Rocker – occurs at heel strike. It causes the ankle to plantarflex and rock the forefoot downward about the heel into contact with the ground. The rocker movement is controlled by eccentric dorsiflexor torque.

Second Rocker – shifts the center of pressure from the heel to the forefoot. Eccentric plantarflexor torque controls dorsiflexion of the ankle.

Third Rocker – occurs at heel separation from the ground that occurs in terminal phase of stance.

Slide 13 shows how the knee shifts gears and transitions from flexion in early stance to extension in late stance. In late stance, the Achilles goes into isometric traction. At this point, further dorsiflexion of the ankle passively tensions the plantar ligaments to intiate forefoot load transfer. Load transfer is accentuated when the knee shifts gears and goes into extension moving COM closer to the ball of the foot increasing the length of the lever arm.

Two Phase Second Rocker

Classic descriptions of stance and the associated rockers do not include a lateral-medial forefoot rocker component that occurs across the balls of the feet from the little toe side to the big toe side in conjunction with the heel to forefoot rocker creating what amounts to a Two Phase Second Rocker.

In his comment to my post, OUTSIDE SKI BALANCE BASICS: STEP-BY-STEP (2.), Robert Colborne said:

….… regardless of where the centre of mass is located relative to the centre of pressure in the above-described mechanism, when you go into a stable monopedal stance, as you would when you are in a turn, the ankle is dorsiflexed forward and as this occurs the tibia rotates internally several degrees.

COMMENT: The tibia rotates internally (i.e. into the turn) as a consequence of ankle dorsiflexion. It does not require conscious action by the skier.

This means that the main muscle forces acting across the ankle (the plantarflexors) are no longer acting along the long axis of the foot, but rather partly across it, medially toward the big toe.

So, the beneficial effect of that muscle force is to force the base of the big toe into the ground, and that becomes the centre of the turn (centre of pressure).

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 photo below shows a skier in bipedal stance with weight distributed equally between the two feet standing on a plush carpet with foam underlay. Black hash marks show the positions in space of key aspects of the right foot and leg.

The photo below shows the same skier in monopedal stance with all the weight on the right foot. Forefoot loading from the Two Phase Second Rocker has pushed the toes down into the carpet by compressing the underlay.

The video below shows the dynamic action of the Two Phase Second Rocker.

The Two Phase Second Rocker results in a heel to ball of foot diagonal rocker action acting towards the centerline of the body; i.e. diagonally across the long axis of the ski with the load acting inside the shovel.

A primary objective of the Birdcage studies was to validate my hypothetical model of the Two Stage Diagonal (heel – forefoot) Second Rocker in creating a balance platform under the outside ski for a skier to stand and balance on.

The graphic below shows the alignment of the Two Stage Diagonal (heel – forefoot) Second Rocker.

In my next post, I will discuss the Two Stage Diagonal (heel – forefoot) Second Rocker Turntable Effect.


  1. http://www.sfu.ca/~stever/kin201/lecture_outlines/lecture_17_clinical_biomechanics_of_gait.pdf
  2. http://wp.me/p3vZhu-29n

CARVE, MEET BIRDCAGE – BIRDCAGE, MEET CARV

July 1991: Birdcage Research Vehicle – Cost approximately $140,000

Secret  Toshiba Prototype Portable Computer used for Birdcage studies – Value? Priceless!

Birdcage Co-Designer and Team Science Leader, Alex Sochaniwskyj, P. Eng.

After interviewing a number of candidates in the spring of 1991 for the science component of the MACPOD project to develop a ski boot based on anatomical principles, I chose Alex Sochaniwskyj, P. Eng. as the most qualified candidate and one of the most intelligent and creative persons I have ever had the privilege of meeting.

Alex provided the CV that follows in his letter in support of my nomination for the Gold Medal in the categories of Applied Science and Engineering in the 1995 British Columbia Science & Engineering Awards.

Alex Sochaniwskyj, P. Eng.

Alex is a professional engineer with 12 years of biomedical and rehabilitation engineering research experience in the Human Movement and Motor Functions Research Programmes at the Hugh MacMillan Rehabilitation Centre in Toronto, Ontario, Canada. The principle aim of these labs is to provide detailed information and objective analysis of movement, dynamics and motor function of persons with various physical disabilities. The information is used to objectively assess the effects of a variety of therapeutic and surgical interventions.

Alex holds a Bachelor of Science degree from the University of Toronto in Human Physiology and a Bachelor of Applied Science from the University of Toronto. Most recently, Alex has worked with several companies including ADCOM ELectronics Limited in Toronto, where he was responsible for the design and development of video conferencing and multi-media communication systems, and the Arnott Design Group, where he focused on physiological human factors in product system design, prototyping and testing.

Currently, as a principal at designfarm inc., he consults to design and manufacturing firms on the development of programs to evaluate human physiological, biomechanical, ergonomic and environmental response for product and interface design, and the planning of comprehensive technology implementation strategies for the integration of computing, telecommunication and telepresence technologies. Alex is also a Certified Alias Instructor in the Information Technology Design Centre in the School of Architecture and Landscape Architecture at the University of Toronto, where he teaches courses in computer literacy, three-dimensional design, modelling, simulation and animation.

Alex is a member of the Association of Professional Engineers of Ontario (APEO), the Institute of Electrical and Electronics Engineers (IEEE), the Association of Computing Machinery (ACM) and the University of Toronto, Department of Rehabilitation Medicine Ethics Review Committee. He is co-author of numerous publications in refereed medical and engineering journals and has produced several video productions regarding biomedical and rehabilitation engineering.

              – March 24, 1995

Team Birdcage


2000 – Novel Pedar In-Shoe pressure technology used by Synergy Sports Performance Consultants – Cost, approximately $60,000 with 2 Sony VAIO laptop computers

 


2017 – CARV: Cost? Approximately $300 US – See footnote re special price

 

Birdcage to CARV: “Where have you been? I’ve been waiting 26 years for you. Welcome! The future of skiing has arrived.”


FOOTNOTE
CARV is currently taking pre-orders at $249 at 
 http://carv.ai

SKIING BIOMECHANICS EXPLAINED BY DAVID MACPHAIL – EPICSKI 2002

While going through my files, I found an article that Joan Rostad compiled and edited in September 2006 from my writings to the Ski Balance forum that spun off from EpicSki in 2002. Since it no longer appears to be available on the internet, I have decided to put up it as a post.

It was around 2002, after someone introduced me to the EpicSki forum, that I connected with Joan Rostad. Joan was a Professional Ski Instructors of America (PSIA) executive vice president and served as a board member from 1989 until 2002. She was a writer, editor and publisher of EpicSki, a popular skiing website. Joan and I shared a common passion for skiing and a commitment to making a contribution to the advancement and enjoyment of the sport.

By 2002, the Birdcage/Rise project that started in 1991 to attempt to bring a ski boot to market based on principles of anatomy had become insolvent. A more recent attempt to introduce pressure analysis to skiing to diagnose boot problems as the cause of technical faults had stalled. Although I was optimistic that my ski boot design would become a reality after the unprecedented success of the Birdcage experiments that validated my hypothesis of the mechanism by which the world’s best skiers were able to truly balance on their outside ski, my optimism turned out to be short-lived. The project suffered one setback after another, eventually becoming insolvent in 2000.

Newton’s Laws and Product Marketing

Shortly after the successful Birdcage studies, a friend who worked in marketing told me that while I had gotten the right answer in terms of what a ski boot should be, it was the wrong answer for the current players who were seeing a growing revenue stream from the wrong answer. He told me to expect pushback. He turned out to be right

Like Newton’s body in motion that continues in motion until acted on by an external force, a successful product  acquires economic inertia that resists new thinking. This issue aside, accepting that my ski boot was founded on the right principles would constitute a tacit admission that current ski boots were based on the wrong principles. This is not a good marketing story.

In Honor of Joan Rostad

By 2002, Joan Rostad (aka nolo in EpicSki), myself and others were witnessing a growing erosion of the passion that had long been the essence of skiing as the increasing corporatization shifted attention away from skiing and the skier to the skier’s wallet. Rostad, myself and others were attempting to create a common platform in EpicSki  that would unite those who were facing an uphill battle in their attempts to retain a semblance of the passion that made skiing special.

I am both flattered and privileged to have been the benefactor of Joan Rostad’s considerable passion and purpose. Although the concepts presented below are mine, the format and editing are due to Joan’s talent and efforts for which she deserves full credit.

Thank you Joan Rostad for all you done for skiing. This is for you.


Note to the Reader: Since this article was written, my thinking on some issues has evolved and matured. This is what should ideally happen if one is truly committed to the truth.


Skiing Biomechanics Explained By David Macphail

By: nolo and stins – Posted 9/21/09 • Last updated 4/26/11

Table of Contents

  1. Introduction
  2. Part I. The Secret of Effortless Skiing
  3. Part II. Four Key Biomechanical Principles
  4. Post Script

Introduction

David MacPhail was the innovative engineer who created the “MacPod” prototype boot in partnership with “Crazy Canuck” Canadian downhill champion Steve Podborski as an ergonomic advancement in ski boot technology. The MacPod “birdcage” design, which was applauded by scientists in the human performance community, was met with utter indifference by boot manufacturers who never gave it any serious consideration.

The following remarks were selected and compiled from David MacPhail’s writings to the Ski Balance forum that spun off from EpicSki in 2002. At its height of activity, the group consisted of David and me (nolo), Rick Schnellmann (Fastman), Ric Blevins (RicB), Hans Kosak (Biowolf), Lou Rosenfeld (lou rosenfeld), Martin Olson (Martino), and Steve Hultquist (ssh). The forum was active from late 2002 until the spring of 2005, when David’s creative energies turned to golf and the discussion lost its center of gravity.

Part I. The Secret of Effortless Skiing

If you understand the following principles and rules you will understand the secret of effortless skiing. More importantly, you will be able to teach it to others, and someday there may be enough people with this understanding to influence the ski industry to produce equipment designed primarily for good biomechanics instead of good looks.

1. Basic physics

Newton’s Law says, “A body in motion in one direction will tend to stay in motion in that direction unless acted upon by an outside force.” In skiing, gravity is always trying to pull us down to the center of the earth. The low friction base of the skis sliding on snow facilitates a shear force component of gravity that acts parallel to the fall line. You acquire inertia due to the acceleration. The turn effort creates centrifugal force, which wants to eject the skier off the tangent of the arc. The only thing stopping the forces from pulling you downhill is the internal force expressed in the outside leg against a ground reaction force (GRF), assuming balance exists.

2. Balance

By my definition, balance is equal and opposite vertical forces aligned in opposition along the same force path with one net external force and one net internal force.

3. Skiers are naturally two-footed

Skiing involves more than just standing in place. We are moving from one limb (outside leg) to another. This basic form of locomotion is one of alternating single limb support, the same as walking. It wasn’t until I connected the movements of walking to those of skiing that I was able to perform and coordinate the movements with ease.

When we walk and when we ski we have a stance foot and a swing foot. When you lift your foot to take a step the movement comes from the inertia of the movement of your center of gravity (CoG), but support from the new stance foot makes the movement possible. If you look at a skier in the middle of a turn the inside leg exaggerates the characteristics of the swing leg in walking. When walking, as you swing the unloaded leg forward, this foot naturally inverts. This is the same movement of release of the old stance foot in skiing. Even when constrained by the ski boot the foot will always try to invert when unloaded.

Similarly, as you shift weight to your new stance foot in walking, so do you shift the weight in skiing: the weight first goes to the outside of the foot, and additional weight tips the foot to the inside, which helps to engage the inside edge of the outside ski. The foot’s anatomy is such that it must adapt to the transverse aspect first before it can fully accept the weight of the body. The lateral arch must make contact first and cause the foot to roll into pronation (i.e., evert) in order to tension the arches in the correct sequence.

In the last third (bottom) of the turn, CoG is behind the uphill or swing foot. So the initial active weighting will be on the heel. If the skier initiates movement down the hill by relaxing the support or stance leg while starting to extend on the swing leg, the force will start to drive the foot into eversion. The new stance foot needs to fully adapt to the supporting surface in order to generate a dynamically rigid base of support for CoG. Then the skier has to simultaneously extend both legs to get CoG up and over the ankle of the swing foot in order to unequivocally make it the new stance foot. It is important to note that the extension is gravity-assisted.

4. Extension drives pronation

Because (at the end of the turn) the uphill or swing leg is flexed in relation to the support or stance leg, we have an opportunity to extend away from the inertia of CoG–which is being pulled down the hill and down towards the center of the earth. It is important to understand that this is a lateral extension that moves our hips and core towards the center of the next turn, without up motion. We need this extensor effort to create the force in the foot against GRF to drive the outside foot into a pronated, stance position.

This critical moment is sort of a twist on the line of the song New York, New York: “If you don’t make it here you won’t make it anywhere.” If you don’t drive the force in your foot to the ball of the big toe, it will be on the wrong side of the inside edge of the ski at its waist. This will create a situation where you will be using the boot cuff to indirectly transfer power to the ski, and doing so will cause a disruption to the balance system feedback and interfere with effectively skiing from the bottom of the foot.

If you do not drive the force in your foot to the ball of the big toe, as soon as the external forces begin to build, the pronated position of your foot will reverse into supination and your foot will revert back to the adaptive state. Please be clear on this because the end result justifies the effort of extension. Try and visualize what is happening during extension of the swing leg. Your body is pivoting about the uphill or lateral aspect of the foot as CoG moves downhill and over the ski. You have to press with enough force to cause the foot to rotate faster. Additionally, elevating the inside hip greatly assists both rotation around the foot and loading the foot. In this scenario, both internal and external mechanics are pulling the stance foot into eversion and holding the edge. The whole lower limb is turning into the hill.

5. External forces can reinforce stability

By simply positioning CoG over the line of force where the ball of the foot is acting within the sidecut of the ski (think of it as literally a line extending from the point the flare of the ski starts to the point where it stops), the forces acting on the skier will reinforce the stability of the base of support even as the forces grow in magnitude. In other words, external forces that would normally disrupt the skier’s balance will have the complete opposite effect in this configuration.

If the skier is using the inner (uphill side) of the boot cuff to hold the ski on edge then the action of the skier is opposing a force that is trying to rotate the stance foot into inversion away from the slope of the hill, which compromises her hold on the edge.

If you get bumped around, you can absorb and fill the surface gaps by letting the ground push toward you on convex surfaces and letting your leg push toward the ground on concave surfaces. The body will do this reflexively if the forces are balanced and the soleus muscle is in eccentric contraction. You don’t even have to think about it. The only thing you have to do is keep CoG in the right place. (This assumes your equipment will actually allow you to do this.)

Part II. Four Key Biomechanical Principles

1. The pelvis always rotates about the ankle of the stance foot

In the first photo Norm Kreutz is rotating his pelvis towards the outside of the turn about the ankle of his stance foot. This is driving the inside or swing leg in the opposite direction, i.e., into the hill.

In the second photo Norm is relaxing the stance leg and extending the swing leg. This transfers the support of CoG to that leg making it the new stance leg. This action transfers the point that the pelvis is rotating about to the ankle of the (new) stance foot. As Norm extends his legs the pelvis will draw his feet in the opposite direction that it was moving in the first photo. It is kind of like a power assisted rotary move. For this reason Norm does not want to plant his pole until after the extension has begun. This helps stabilize the position of his CoG to ensure rotation occurs in his femurs.

The interesting thing is that this all happens on its own as part of the kinetic flow which should also flow in the direction that starts when the stance leg changes. Once you get the movement pattern right there is nothing to think about aside from deciding when to start a new turn.

2. Kinetic Flow
If there is one area where there seems to be general agreement it is that edging and pivoting occur as a unit movement pattern and not as a series of separate events spliced together. This is why the timing and sequence of the initial move to start a turn is important.

Every turn has a start/end movement sequence that is consistently towards the inside of the turn just as there is a consistent flow in walking. Kick starting the flow is always contrived. This is why the first turn is the most difficult.

The movement that sets up the direction of the flow starts in the feet. At the completion of a turn the flow is to the inside of the turn. To turn in the opposite direction requires that the kinetic flow of the joints of the body be reversed. The outside foot will be in pronation and the inside foot will tend to pronate. In other words the inside foot is already primed to flow to the inside of the new turn.

 

 

When Maier relaxes his outside foot (first photo of the turn) his pelvis starts to unwind. This lets CoG drift behind his inside foot. Now CoG is behind his new stance foot and the foot is supported on its lateral or outer border. It is in the same position as it is at heel strike in the adaptive phase of walking.

If you look at the movement of his upper body as he comes out of the fall line you will note that it seems to be coming at you (like a 3D movie) as opposed to going across the hill. If you were standing opposite the gate looking across the hill this would be more readily seen. As his CoG begins to move downhill Maier extends on his (new) outside foot. The foot is initially on its outer aspect but is tending to pronate. Applying force to the foot in combination with the movement of CoG starts the movement of the foot and leg in the direction of pronation. As CoG crosses over his skis his body will become erect with the slope of the hill. In effect, he is ‘standing up’.

Remember, his inside femur was previously moving in a direction in relation to its position with the pelvis that was consistent with supination of the foot (i.e. as the swing leg in walking). The movement of the femur is in opposition to the joint movement of the foot (supination vs. pronation). Maier has to change the relationship of the pelvis to match the flow of the feet.

3. The pelvis always rotates into the stance foot whether in walking or skiing
As he extends on the new inside foot the tension of the rotator muscles in the pelvis unwinds the legs and aligns them with the pelvis, turning the legs into the fall line. As the turning progresses Maier applies rotational effort simultaneously to both legs to pivot the legs across the pelvis so that the inside hip leads the outside hip. In effect Maier creates the natural flow of the pelvis that takes place when one steps R foot – L foot, etc. At the same time he uses the extension movement to bring CoG up and ahead of the ankle of his new outside foot. By this movement Maier synchronizes the flow of the legs to match the kinetic flow of the joints of the outside foot (foot everted, leg turning into the turn, pelvis turned into the outside leg or towards the outside of the turn). Once he has synchronized the flow he exaggerates the rotation of the pelvis to reinforce the pronation of the outside foot as he relaxes onto the outside foot and stretches the muscles in eccentric contraction.

4. The flow of the feet is reinforced by the pelvis
Unless one has a reasonable knowledge of biomechanics the effect of the rotation of the pelvis on edging usually makes no sense because it seems to have nothing to do with the mechanics of the feet. The effect of this movement is that it shortens the muscles that drive the foot into pronation. This not only applies edging force to the ski, it also torques (twists) the ski about its long axis. This is why Maier’s ski bites positively at the shovel. If you compare him to lesser skiers you will typically see skiffs of snow being thrown up off the edges instead of flowing along the ski. This is caused by the percussion of the ski as it oscillates about its edge (into the hill – away from the hill). This is due to insufficient loading of the shovel.
Post Script

Although it is common to speak of kinetic flow starting in the feet, science has proven that the movement really originates out of the pelvic floor, hips, and the abdomen. These muscles will fire first in a functionally fit individual. They do this because they are required not only for stability but also to initiate the pelvic rotation. I think this is why lifting and tilting the pelvis as an exercise is so effective. – Ric Blevins

Compiled and edited by Joan Rostad, September 2006

 

FEATURE POST: THE EMERGING REVOLUTION UNDERFOOT

There is a rapidly emerging convergence of electronic wearable technologies such as CARV, in ski boot digital sensor technology, and sport performance equipment. This years ISPO dedicated a large space to this segment with the larger mobile phone companies all showing concepts and apps.

There is a similar convergence emerging on the science front with attention increasingly focussed on the human foot as rapidly evolving micro-sensor technologies allow the study of metrics associated with foot function such as arch height and specific muscle activity. These technologies enable users to look inside footwear and study its effect on the foot that have previously been hidden out of sight and free from investigation and scrutiny. In due time, some of the premises that form the cornerstone of knowledge in skiing, such as the foot functions best in skiing when its joints are immobilized in a ski boot and, the ski boot is a handle that is used to apply force to a ski, will be scrutinized, revealed and discredited as fabricated nonsense.

THE EMERGING REVOLUTION UNDERFOOT

CARV IN BOOT DIGITAL SENSOR TECHNOLOGY

CARV – The world’s first wearable that helps you ski better. – https://carv.ai

Notch – 3D Motion Sensor Technology captures movements and shares and replays them in 3D in any modern browser – https://wearnotch.com

The equivalent of DNA, as an investigative tool with which to study the effects of footwear on the human foot, has arrived.

 

 

A SKIER ASKS QUESTIONS ON BOOT-FITTING 101: THE ESSENTIALS

After my last post BOOT-FITTING 101: THE ESSENTIALS – SHELL FIT, I received an email from a Whistler skier asking a number of questions. I have copied and pasted the questions into the post below and inserted my answers


Whistler Skier: After reading your last two posts and going through all the information about boot fit, the tongue and where your shin should contact the boot/liner, I probably need  to punch the shells a bit wider in the ankle area of the navicular bone (it definitely needs more room when I go from bipedal to monopedal stance).

Answer: Make sure forefoot width is adequate first.


Whistler Skier: 

  • Do you want a full finger width between all parts of the shell and your foot?

Answer: No. Just behind the heel. A few mm clearance to 1st and 5th toe joints and inside ankle bones is usually sufficient if the liner is thin enough in those areas.


Whistler Skier: 

  • Should I just experiment with padding of different thicknesses over my forefoot to try and keep my foot in contact with the bottom of the boot?

Answer: TONGUE CHECK will be the subject of a future BOOT-FITTING 101: THE ESSENTIALS post.


Whistler Skier: 

  • When I put the boot on and lightly buckled, I can still ‘stand on my toes’, so:
    • Is that because my foot is not sitting in the heel pocket?

Answer: I suspect you aren’t standing in an SR Stance. If you were, you would not be able to stand on your toes.


Whistler Skier: 

  • I gather from your postings that I don’t want to add foam to the front ‘crook’ of my ankle to hold it in the heel pocket because it will impede the natural ankle movement on flexion?

Answer: Yes. It might not impede the natural ankle movement. But at the same time, L-pads do nothing useful if the foot is stiffened by fascial tension. It is the age old problem of what is easier to nail to a tree, liquid Jello or frozen Jello?


Whistler Skier: 

  • If so, how do I get my heel to stay in the heel pocket? (do I want it there?)

Answer: Wedge fit loading of the instep of the foot with forefoot portion of the boot tongue. The key is ensuring fascial tensioning can occur in the boot because it makes the foot behave as it were solid, not malleable. Conventional boot fitting strategies attempt to achieve this objective by encasing the foot with a form fitting medium. This has the exact opposite effect. It actually prevents fascial tensioning.


Whistler Skier: 

  • Should I hold off on punching the ankle area?

Answer: For the time being. If it is close, skiing will confirm whether or not the space is adequate.

Whistler Skier: 

  • i.e., Is the space already large enough?

Answer:  I don’t know. See above.

Whistler Skier: 

  • Is it the cuff of the boot that provides control in conjunction with the sole of the foot?  My shin seems to contact the front cuff of the boot right about where your diagrams indicate it should and there is no pressure further down the cuff.

Answer: Sounds good.


The key that has literally been under everyone’s feet for decades, while they threw out nonsensical theories on skier balance, is what I call Ground Control. Given the correct sequencing of events, fascial tensioning in combination with pronation of the outside foot in a turn, enable pelvic rotation of the femur to extend the ground (snow) under the inside edge of the outside ski up under the base of the ski.

The mechanics of Ground Control has the effect of bringing the ground (snow) up under the entire ski base and foot thus allowing a skier to actually balance on the outside ski as if they were standing with their outside foot in full contact with solid ground (snow). This was my hypothesis that the Birdcage experiments confirmed in 1991.

 

THE POLISH SKIER BALANCE STUDY: IMPLICATIONS FOR THE FUTURE OF SKIING

The foot functions best in skiing when it’s joints are immobilized in a tightly fitting ski boot, preferably in a neutral position with the arch fully supported by a footbed.

This widely held position within the ranks of the ski industry implies that immobilizing the joints of the foot in a ski boot has positive benefits for skiers.

But the authors of the Polish study (1) that was the subject of a recent post cite research that indicates otherwise:

Wearing ski boots for a few hours can lead to a weakening of the muscles that operate within the ankle joint. This works as though one joint was excluded from the locomotive function.

………. according to Caplan et al. [3], the muscle groups that determine strength and are responsible for the function of stability in the ankle joint are very sensitive to changes caused by immobilisation. They found that immediately after immobilising the ankle joint for a week, the balance parameters were 50% lower than before the immobilisation.

The authors of an earlier Polish study (2) on skier balance also cite research that indicates otherwise:

It must be mentioned that the stiff ski boots of skiers facilitate the transfer of power to the skis, but they also increase the difficulty in maintaining postural control. Mildner et al. (2010) showed that balance performance on the MFT S3-Check was negatively influenced when wearing ski boots.

The authors of the recent Polish study (1) further commented:

Publications on issues related to the biomechanics of a descent, with particular emphasis on the balance parameters, are rare in the literature on alpine skiing.

The authors of a 2014 Polish study (2) on skier balance, similarly commented:

Our results were in agreement with the scarce information available regarding balance changes during or after a ski training camp.

The authors of a 2013 Italian study (3) made a similar comment with regard to ski equipment.

Despite the large market of ski equipment, not many scientific papers have been published on this subject in the past.

If research on balance in alpine skiing is rare and scarce in the literature and not many scientific papers have been published on ski equipmentwhere did the foot functions best in skiing when its joints are immobilized in a ski boot story come from?

The most plausible explanation is that the story was simply invented to distract attention away from the fact that no one knows what happens when the foot is constrained within the rigid shell of a ski boot.

Inventing a cover story is a typical strategy used when an issue can’t be explained. Using fact-deficient or obfuscating generalities to appear knowledgeable on a subject is not the same as being knowledgeable. Yet, few question this tactic. Instead, they assume that they’re just not smart enough to understand it and they ignore their judgment and common sense. Once people buy into a cover story, information bias sets in and they unconsciously filter out any information that challenges what they have chosen to believe.

The two Polish studies (1), (2) should be taken seriously by the ski industry because that the authors used barefoot balance as a reference against which to assess the effects of the ski boot and balance training on skier balance. In addition, the recent Polish  study (1) employed a systematic protocol; one that standardized conditions, controlled variables and acquired data that could be objectively quantified as opposed to subjectively interpreted based on uninformed observation. If balance was worse or improved after training, or in tests done with subjects wearing ski boots compared to the barefoot baseline, the protocol provided compelling evidence of the cause of the change.

The test subjects were closely matched in terms of physical characteristics and included both skiers and non skiers.

The findings of the study were as follows:

In the trials involving standing barefoot, there were no significant differences between the measurements taken at the beginning and at the end of the training programme. 

In none of the tests conducted on both feet were significant differences in the length of the COP path observed between the group of beginners and the group of advanced skiers.

In the case of standing on one foot, no signifcant differences were observed in the sway range in the frontal and sagittal planes between the measurements taken before and after the training camp (Table 3).

In both groups, a statistically significant improvement in stability was observed after the training camp only while standing in ski boots, both with the eyes open and the eyes closed (Fig. 2). 

The earlier Polish study (2) also did balance tests without subjects wearing shoes. The authors commented that:

It must be mentioned that the stiff ski boots of skiers facilitate the transfer of power to the skis, but they also increase the difficulty in maintaining postural control. Mildner et al. (2010) showed that balance performance on the MFT S3-Check was negatively influenced when wearing ski boots.

A study by Noé et al. (2009) found that mechanical effects of wearing ski boots resulted in changes in postural strategy through the reorganization of muscle coordination in experienced skiers. The improvements in balance performance in our study could also be explained by guided skiing including a number of lateral and fore-aft drills over a week of skiing. Exercises such as skiing only on the outside ski with the inside leg raised or skiing without poles are part of the curricula of ski instructor associations.

While this may sound like a good thing, the Polish study (1) found that the normal balance process was worsened:

What is interesting is that in the measurements involving the participants standing barefoot with their eyes open, significantly greater sways in the sagittal plane were observed after the training camp than before it.

My Hypothesis on How Elite Skiers Balance on the Outside Ski

In 1991, after having spent more than 10 years trying to solve the mystery of how the world’s best skiers are able to balance on their outside ski, I was about to embark on a project to design and produce a radical new ski boot. The design of the ski boot was based on my theory that the world’s best skiers balance on the outside ski through a sequential tightening of the bio kinetic chain that engages the processes of pronation followed by the application of internal axial rotation of the femur of the outside leg of a turn from the pelvis. The  bio kinetic chain is closed through inclination. Once the bio kinetic chain is closed by locking the inside edge of the ski into the snow,  internal axial rotation of the femur applied to the outside leg is translated through the subtalar joint into dual plane torque that opposes the torque that is inverting the outside ski (i.e. rotating it away from the turn). In effect, this bio kinetic mechanism enables the world’s best skiers to truly balance on the outside ski by balancing multi plane torques. The problem I faced was that I had no way to prove my theory. The technology I needed did not exist.

With the immobilization works best cover story already under a microscope as I was poised to move forward to try and design and develop a new ski boot, I found myself staring down the barrel of a loaded gun. I needed to prove my theory. But since the technology  to do this didn’t exist, I insisted that MACPOD retain a science team to work with me to see if we could develop a technology with which to confirm my theory and the bio kinetic sequence it predicted. The process resulted in the Birdcage and the on-hill studies done in the summer of 1991 using elite, intermediate and novice skiers. The most significant aspect of the Birdcage research vehicle was that it allowed the capture of baseline skier data equivalent to barefoot function and the study the effect of constraining specific joints.

The increasing use of protocols such as the one used for the Polish study (1) in combination with the rapidly evolving field of micro sensor technology and data analysis  is making quickly making the vision of the Birdcage as an analytical tool for activities like skiing and skating a reality. As this unfolds, widely held beliefs that are the foundation of skiing will increasingly come under the lens of a microscope.


(1) Influence of a nine-day alpine ski training programme on the postural stability of people with different levels of skills  (April 2016, Biomedical Human Kinetics (DOI: 10.1515/bhk-2016-0004) – Michał Staniszewski, Przemysław Zybko and  Ida Wiszomirska,  Józef Piłsudski University, Warsaw, Poland.

(2) Changes in the Balance Performance of Polish Recreational Skiers after Seven Days of Alpine Skiing – Beata Wojtyczek, Małgorzata Pasławska, Christian Raschner

(3) Materials, Designs and Standards Used in Ski-Boots for Alpine Skiing: Martino Colonna *, Marco Nicotra and Matteo Moncalero

FREEMOTION FLEX CURVE OPTIONS – PART 3

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

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

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

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

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

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

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

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

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

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