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

SCHEDULE OF POSTS

The link below will take you to a page with a link to a PDF of all 298 posts I have made since my first post of May 11, 2013.

https://wp.me/P3vZhu-2sM

The schedule lists posts in the order of newest (Jul 10, 2018) to oldest (May 11, 2013). The image below shows what the schedule looks like. The date and time of the post and the views and likes are listed below the title of each post.

 

The top 10 posts to date are shown in the graphic below.

I am in the process of reviewing and analyzing post subjects based on ranking with the objective of better directing my efforts to my readers. If there are any subjects you would like addressed please post them in the comments section.

 

WHAT’S YOUR PQ? [PERFORMANCE QUOTIENT]

After my disastrous experience in 1977 with the mythical Perfect Fit with Crazy Canuck, Dave Murray (.1); one that transformed Mur from a World Cup racer to a struggling beginner, my work on ski boots became focussed on removing instead of adding material and making room to allow a skier’s foot to assume its natural configuration in the shell of the ski boot. As I improved the accommodation of a skiers’ neurobiomechanical functional requirements in the ski boot, skier performance improved in lockstep. I was merely reducing the structures of the boot that interfered with performance to enable a skier/racer to use the performance they already had.

Fit: The Antithesis of Human Function

Fit, by it’s definition of joining or causing to join together two or more elements so as to form a whole, is the antithesis (def: the direct opposite) of enabling the function of the human foot and lower limbs as one of the most dynamic organs in the human body. Fitting a ski boot to the foot and leg of a skier, especially a racer, equates with imposing a disability on them (2.). Although I didn’t realize it until I read The Shoe in Sport and learned of the barefoot studies done at the Human Performance Laboratory at the University of Calgary, my work on ski boots had transitioned from Fitting (disabling), by adding materials to liners to fill voids between the foot and leg and shell wall, to UnFitting (abling) by removing materials from liners and expanding and grinding boot shells so as to accommodate the neurobiomechanical functional requirements of the foot and leg of a skier.

But the big breakthrough for me came when Steve Podborksi won the 1981-81 World Cup Downhill title using the dorsal constraint system (Dorthotic) I developed and later patented. The Lange boot shells the device was used in had the least constraint of any ski boot I had ever worked with. The instantaneous quantum leap in Steve’s performance compared to the same shell using a conventional liner raised the question of how could a skier’s maximum performance be achieved and was there a way to compare to a skier’s performance in different ski boot/liner configurations to an optimal reference standard?

A reliable indicator that my un-fitting was trending in the right direction was that skiers consistently found that skiing became easier. For racers, coaches would typically report that the racer was skiing better. Improved race results served as further confirmation of my efforts. But these indicators were subjective. I wanted a way to not just measure performance with quantifiable metrics generated from data specific to the activity, I wanted to be able to compare the same metrics to a reference or baseline standard that represented the optimal performance of a skier or racer at a given moment in time. Without a way to measure and compare performance there is no way of knowing how a ski boot is affecting a skier or racer and especially no way of knowing how close they are to skiing at their maximum level of performance. I wanted to develop a skier Performance Quotient or PQ.

Definition of Quotient

  • Mathematics: – a result obtained by dividing one quantity by another.
  • a degree or amount of a specified quality or characteristic.

A skier Performance Quotient would capture baseline metrics from a skier’s performance in a ski boot that provides the optimal functional environment for the foot and lower limbs to the skier’s peformance in different ski boots including a skier’s current ski boot. The ski boot that provides the optimal functional environment for the foot and lower limbs would be designated as 100%. If the same metrics captured in a different ski boot were 78% of the reference standard, the skier’s PQ in the ski boot would represent a PQ of 78% against a possible 100% or 78/100.

Raising the bar of skier/racer function with body work and/or conditioning will raise the PQ. But it cannot close the PQ gap created by the performance limitations of the interference with neurobiomechanical function caused by their ski boot. Nor can trying harder or training more intensely overcome the limitations of a ski boot. Assuming 2 ski racers of equal athletic ability and mental strength, the racer with the ski boot that enables a higher PQ will dominate in competition. The only way to improve a skier’s PQ when it is less than 100% is to improve the functional environment of the ski boot.

In current ski boot design process, manufacturing and aesthetic considerations override skier functional requirements. An innovative approach to the design of the ski boot is needed. This is the subject of my next post.

  1. IN THE BEGINNING: HOW I GOT STARTED IN SKI BOOT MODIFICATIONS, May 12, 2013 – https://wp.me/p3vZhu-y
  2. LESS REALLY IS MORE, May 13, 2013 – https://wp.me/p3vZhu-N

 

SKI BOOTS: WHY LESS IS MORE

At the time I filed an application for my second patent in April of 1989 , I had some ideas of what a ski boot should do for the user from what I had learned from the dorsal containment system I was granted a patent for in 1983. But I was still a long way from being able to answer the question.

A watershed moment came for me in 1990 when I read a medical textbook published in 1989 called The Shoe in Sport on what is referred to in the text as ‘the shoe problem’.

The Shoe in Sport, supported by the Orthopedic/Traumatologic Society for Sports Medicine, was originally published in German in 1987 as Der Schuh im Sport. The textbook is a compilation of the collective efforts of 44 international experts, including Professor Peter Cavanagh, Director of the Center for Locomotion Studies at Penn State University, biomechanics experts from the Biomechanical Laboratories at ETH Zurich and the University of Calgary, Professor Dr. M. Pfeiffer of the Institute for Athletic Sciences at the University of Salzburg, Dr. A. Vogel of the Ski Research Syndicate, Dr. W. Hauser and P. Schaff of the Technical Surveillance Association Munich and many other experts in orthopedic and sportsmedicine on  ‘the shoe problem’.

The buyers of athletic shoes are always looking for the “ideal shoe”. They encounter a bewildering variety of options and are largely dependent for information on the more or less aggressive sales pitches that directed at all athletes in all possible ways. (1.)

This volume should assist in defining the role and the contributions of science in the further development of the athletic shoe and in the recognizing of the contributions made by the various research groups, who are all interested in the problems of the athletic shoe. (1.)

Dazzled by the fancy names, the buyers believe that they can match the athletic performance of the champion who wears “that shoe,” or after whom the shoe is named. The choice is not made easier by the plethora of promises and a roster of specific advantages, most of which the merchant cannot even explain. (2.)

When The Shoe in Sport was first published in 1987, the field of biomechanics was in its infancy as was the associated terminology. This created an opportunity for a new marketing narrative of techno buzzwords. Since the consumer had no way to understand, let alone assess, the validity of any claims,  the only limits to claims made for performance was the imagination of the marketers. Consumers were increasingly bombarded with features that far from recognising the human foot as a masterpiece of engineering and a work of art as espoused by Leonardo da Vinci, suggested the human foot is seriously flawed and in need of support even for mundane day-to-day activities. These marketing messages distract attention away from the real problem, the design and construction of shoes and their negative effect on the function of the user; the modern ski boot being one of the worst examples.

The Shoe Problem

For this reason, the “shoe problem”as it exists in the various fields of athletic endeavour, will be studied with respect to the biomechanical, medical , and technical aspects of shoemaking. The findings (criteria) should enable the interested reader to distinguish between hucksterism and humbug on the one side and the scientifically sound improvements in the athletic shoe on the other. (1.)

Form follows Human Function

The Shoe in Sport focusses on the medical orthopedic criteria in offering guidelines for the design of shoes for specific athletic activities including skiing and ice skating.

Less attention will be paid to the technical and material aspects of the running surface and shoe, and more to the medical and orthopedic criteria for the (design of) athletic shoe. For this reason, the “shoe problem”as it exists in the various fields of athletic endeavour, will be studied with respect to the biomechanical, medical , and technical aspects of shoemaking. 

This volume should assist in defining the role and the contributions of science in the further development of the athletic shoe and in the recognizing of the contributions made by the various research groups, who are all interested in the problems of the athletic shoe.

Barefoot as the Reference Standard

Research done at the Human Performance Laboratory at the University of Calgary found that optimal human performance is produced with the unshod foot and that human performance is compromised by the degree of interference; the greater the interference caused by any structure appended to the foot, the greater the compromise of performance. This is true even for a thin sock.

The authors of The Shoe in Sport ask:

Is there really a need for shoes? The examples of athletes like Zola Budd and Abebe Bikila suggest in a technologic environment the evolution of the athletic shoe parallels the decline of our organs of locomotion. (1.)

The Future of the Ski Boot

The shoe affects the athlete’s performance and serves to support the foot as a tool, as a shock absorber, and as a launching pad. Giving serious consideration to our organs of locomotion opens up an enormous area of activity to the athletic shoe industry. (1.)

This is especially true of the ski boot. The questions that needs to be asked is how does the structure of the ski boot affect the human performance of skier and what is the minimal combination of structure that will enable maximum skier performance.

Few forms of athletics place as high demands on the footwear used in their performance as alpine skiing. It (the ski boot) functions as a connecting link between the binding and the body and performs a series of difficult complex tasks. (3.)

Before the question of what structure of a ski boot will maximize skier performance can be answered, the functional mode of the human system in the complex physical environment associated with skiing must be known. The first and most important and fundamantal component of this question is explaining the mechanism by which the human system is able to achieve a state of balance on the outside ski characterized by neuromuscular control of torques in all 3 planes across the joints of the lower limb and pelvis.

  1. Introduction by Dr. med. B. Segesser, Prof. Dr. med. W. Pforringer
  2. 2. Specific Running Injuries and Complaints Related to Excessive Loads – Medical Criteria of the Running Shoe by Dr. med. N. L. Becker – Orthopedic Surgeon
  3. 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

WHY HIRSCHER AND SHIFFRIN CAN CROSS THE LINE

There has been a huge surge in interest in my post HIRSCHER AND SHIFFRIN WIN BY CROSSING THE LINE.

The reason Hirscher and Shiffrin can ski this way is that they have the ability to cross the rise line and establish balance on their outside foot and leg in milliseconds. This enables them to make what amounts to a hairpin turn. They are on and off their edges like a flat stone skipping off the water. The reason they can do what few other racers can is because their boot setup supports the requisite neurobiomechanics. I discuss this in my last post WHY HIRSCHER AND SHIFFRIN ARE DOMINATING.

WHY SHIFFRIN AND HIRSCHER ARE DOMINATING

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.

MacPhail, US Patent 5,265,350 – November 30, 1993

Of all the figures who have influenced the development of the plastic shell ski boot over the years, the Australian, Sven Coomer, stands tall as one of the most significant and innovative. More recently, Coomer was involved with the development of Atomic’s race boot, the Redster, used by Marcel Hirscher and Mikaela Shiffrin. Coomer claims that the Redster allows the skier’s forefoot to flex and move naturally within the confines of the shell.

A 2014 article by Jackson Hogen quoted Coomer as saying:

This liberation of the previously stunted, frozen and crushed forefoot is what allows for the subtle edging and foot steering that initiates the slalom turns of World Cup champions Marcel Hirscher and Mikaela Shiffrin. (1.)

Four years, later Hirscher and Shiffrin are dominating the technical disciplines of the World Cup circuit.

The ability to establish balance on the outside foot and ski in milliseconds is dependent on the ability of the forefoot to fully spread and acquire fascial tensioning that extends to the ankle and knee. This is called time-to-stabilization. Although Coomer doesn’t mention them, a myriad of other factors are also critical; including the alignment of the big toe on the long axis of the foot and the optimal ramp angle.

Coomer suspects that if racers would only fit their boots more accurately, coupled with a dynamic molding inner boot medium between the foot and shell, and without down-sizing into short, narrow, thick-sidewall shells, their results just might improve. (1.)

In order to realize their maximum potential it is critical that racers and even recreational skiers have a ski boot fit with dynamic and predictable qualities that substantially match those of the foot and lower leg. Yet Coomer readily acknowledges:

Many racers believe they need downsized, super-stiff, ultra-narrow boots. The most accomplished alpine ski boot designer of the plastic era, Sven Coomer, believes that’s changing.(1.)

But then, he seems to retract his optimism when he says that after forty-five years as the Cassandra of the ski boot world, he knows all too well that just because you can prove you’re right, it doesn’t mean your advice will be heeded.

My observation is that since Hogen’s 2014 article, the situation with downsized, hyper-restrictive ski boots that severely compromise the dynamic nature of the architecture of the foot, has gotten worse. I have seen instances where after having ski boots properly fit, it took several full seasons for the competence of the balance to be fully restored after a skier or racer’s feet and legs were constrained for years in ski boots that were too small and too tightly fit.

Marcel Hirscher and Mikaela Shiffrin have heeded Coomer’s advice. Others choose to ignore him at their own peril. In so doing, they handicap their efforts and limit their race results.

In my next post I will start a series of posts on how to build a ski boot from the snow up; one that provides a fit with dynamic and predictable qualities that substantially match those of the foot and lower leg.

  1. The Master Boot Laster by Jackson Hogen: The International Skiing History Association – Article Date: Tuesday, June 3, 2014

(NEURAL) BIO (MECHANICAL) ENGINEERING: DOES IT WORK?

The proof of a performance concept lies in the data. If neural bio mechanical engineering can improve human performance in a specific application such as skiing, skating or cycling then it should be possible to demonstrate meaningful performance improvement with quantifiable metrics generated from data captured from the actual activity. In the case of cycling, meaningful improvement would be shown by an increase in metrics such as peak force transferred to the pedal spindle that cannot be explained by other factors. In order to attribute any change in performance, whether positive or negative, to neural bio mechanical engineering the effect must be immediate, reversible and reproducible. In the case of the improvement seen with Podborski’s performance using ski boots fit with the dorsal fit system, reverting to identical boot shells fit with a conventional liner reversed the improvement in performance.

Where possible, standard protocols should be used and testing performed by experts in the field. In the case of the cycling shoe Tekscan F Scan data comparison, my only involvement was to analyze the human lower limb requirements for cycling and generate the design and specification for the device that produced the neural bio mechanical engineering effect. I played no role in designing the test protocol or conducting the tests. I was not even an observer.

So how did the neural bio mechanical engineering system work in the application to cycling?

Performance Metrics

There are numerous metrics that can be used to assess and compare performance. The subject test was limited to pressure analysis using the Tekscan F Scan system.

In the graphic below the upper image outlined in red is the F Scan pressure image of an elite cyclist captured at 3 o’clock in the crank cycle at moderate to high load using their own conventional cycling shoe. The F Scan pressure image outlined in green below the first image is of the same cyclist using the device that bio engineered the foot and lower limb.

Results

There are a number of significant differences between the forces applied with the same cyclist with the conventional cycling shoe and the bio-engineering. In the 2 F Scan images, the contact area of the application of force across the heads of the metatarsals over the pedal spindle with bio-engineering is much greater and the force much higher than the force applied to the heads of the metatarsals in the conventional shoe.

The two significant, quantifiable (measurable) metrics that relate directly to cycling performance are: Peak Force and Anterior-Posterior (forward-backward) Excursion of the Center of Force.

Peak Force

The peak (maximum) force with the device that bio-engineered the foot and lower limb was 140% of the peak force applied with the conventional cycling shoe.

Anterior-Posterior (fore-aft) Force Excursion

This is the range of forward-backward movement of center of force through the crank cycle.

The graphic below shows the tracking of center of force forward and backward in the pedal stroke. Notice how straight the force tracks in the lower image with bio engineering compared to the upper image captured from the conventional cycling shoe.

The ability to move the center of force forward and backward, not just down and, more important, substantially aligned with the crank rotation is both more consistent and efficient than the excursion tracking with the conventional shoe. The bio engineering device improved excursion by 175% in rearward tracking force (long bars) and 200% in forward tracking force (over the top) as shown by the short bars below the images.

In my next post I will discuss how I designed a ski boot from the snow up using principles of neural bio-mechanical engineering.