Ski Equipment posts

EDGE CHANGE INERTIA + ROCKER ROTATION INERTIA

As I was in the process of writing this post, a FaceBook group on skiing posted a link to an article From PSIA: Examining Transitions. The article is based on a presentation last fall by US Ski Team Head Men’s Coach, Sasha Rearick, in which he shed new light on transitions (1.).  While Rearick did shed light on some events associated with transitions, as with previous efforts by others on this subject, Rearick failed to shed light on the mechanics and physics associated with edge change.

As I explained in my last post, transferring the weight from the outside foot and ski of a turn to the inside foot and ski in the transition phase sets in motion what I call the Eversion/Internal Rotation Cascade that rotates the base of the ski into a transient moment of full contact with the surface of the snow between changing to the new (downhill) edge.

At the start of the transition leading up to ski flat between edge change, the center of pressure (COP) of the weight of the body applied by the sole of the inside foot will be under the heel where it is aligned on the proximate center of the ski. In this configuration, the force applied to the ski by the skier is working with gravity to rotate the ski.

The post left off by showing how rotational inertia will tend to make the ski continue rotating about the uphill edge past ski flat and penetrate into the snow surface on its downhill aspect as shown in the graphic below.

Rotational inertia will tend to make the inside edge of the new outside ski automatically rotate into the turn except for the fact that the force FW applied by the skier is on the wrong side of the new edge.

The graphic below has a dashed red reference that is parallel with the snow surface.

If the force FW applied by the skier is still aligned on the transverse center of the ski, it act will act to oppose edge change as shown in the graphic below. When the axis of rotation of the body of the ski changes with a change in edges, the transverse aspect of the base of the ski and the platform under the skier’s foot will tend to accelerate into an eversion translation. But this can only happen if the associated biomechanics are not interfered with by the structures of the ski boot.

The graphic below shows the change in the mechanics of rotation associated with edge change.

At the start of the transition, movement of the mass of the skier’s upper body is in phase with the downhill rotation of the ski and the force FW applied to it. But when the ski changes pivots at edge change and the mass of the skier continues to move downhill, the force FW applied to the ski will tend to rotate it back to ski flat; i.e out of the turn, unless the point of application of force FW changes during ski flat as shown in the graphic below and COM of the skier is aligned with force FW.

………. the angle between the platform and force you apply to it, the platform angle, must be 90 degrees or smaller.  – page 19, The Ski’s Platform Angle, Ultimate skiing; Le Master

The shift in center of pressure from the heel to the ball of the foot in a turn sequence seen in pressure studies of expert skiers is well documented (2., 3., 4). What the studies are really confirming is the use by expert skiers of the Two Phase Second Rocker mechanism to rock (tip) the outside ski on edge and control the edge angle during the load phase of a turn sequence.

Since the limit of the position of the application of force by the foot in relation to the inside edge of the outside ski is the center of the ball of the foot the effect of ski width underfoot and stand height should be obvious. Both rotational inertia and torque will increase as the width of a ski underfoot (profile width) is reduced and stand height increased. When Ligey says he creates pressure, he is creating far more than just pressure.

While LeMaster appears to recognize the importance of a platform angle less than 90° for edge control and, to some degree, the effect of stand height, the explanation offered for superior edging is that this can be attributed to waist width and stand height making skis more like ice skates.In my next post, I will discuss the role of Turntable Rotation in setting up a platform under the body of the outside ski for a skier to stand and balance on while maintaining edge angle.


  1. http://eliteskiing.com/2017/03/31/from-psia-examining-transitions/
  2. WHAT THE TWO HIGH PRESSURE COPS IN THE UNIVERSITY OF OTTAWA STUDIES MEAN – https://wp.me/p3vZhu-1fV
  3. IMPLICATIONS OF THE UNIVERSITY OF OTTAWA PRESSURE STUDIES –https://wp.me/p3vZhu-1e2
  4. AN INDEPENDENT STUDY IN SUPPORT OF THE UNIVERSITY OF OTTAWA FINDINGS – https://wp.me/p3vZhu-1gR

 

EDGE CHANGE INERTIA: WHY THE TRANSITION PHASE MATTERS

One of the most important events in the turn sequence is edge change. Yet, it is rarely mentioned in technical discussions. One of the few references I was able to find on edge change is in the CSIA Technical Reference which states:

Edge Change = Balance Change: Changing edges requires a change of balance.

Edge change occurs during an unbalanced, controlled fall in the transition phase that leads to the development of a balanced position on the outside ski as it crosses the fall line in the bottom of a turn. Properly executed, edge change leads to the development of a platform under the outside ski for the skier to stand and balance on.

The edge change sequence starts in the transition phase when a skier begins to transfer weight from the outside (downhill) ski to the inside (uphill ski). At the start of the transition, the edges of the inside ski are uphill and on the lateral (little toe) side of the foot. From a perspective of the gait cycle, the base of the ski is inverted (turned inward towards the center of the body). This is the normal configuration when the foot is unweighted in the gait cycle. The foot strikes the ground on the lateral (little toe) side and rotates about it’s long axis in the direction of eversion to bring the three points of the tripod of the foot into contact with the ground. As the foot everts, the leg rotates internally through torque coupling in the subtalar joint. The normal kinetic flow from foot strike to the support phase in mid to late stance is one of inversion of the foot/external rotation of the leg to eversion of the foot/internal rotation of the leg. Put another way, the human lower limbs will naturally rotate into a turn so long as the biomechanics are not interfered with.

At the start of the transition leading up to ski flat between edge change, the center of pressure (COP) of the weight of the body applied by the sole of the inside foot will be under the heel where it is aligned on the proximate center of the ski.

The Eversion/Internal Rotation Cascade

Transferring the weight from the outside foot and ski to the inside foot and ski in the transition phase sets in motion what I call the  Eversion/Internal Rotation Cascade. When the cascade starts, the force F W applied to the ski by the foot  by the weight of the body will impart rotational inertia as the ski rotates about the pivot point formed by its inside edge.

For the sake of simplicity, the stack of equipment between the sole of the skier’s foot and the snow is represented by a rectangle in a 3:2 ratio where the stand height is 50% higher than the width (FIS maximum stand height = 93 mm – maximum profile width = 63 mm). Sidecut is also not shown.

The following graphics show the sequence of the Eversion Cascade. Note: Internal rotation of the leg is not shown in this sequence.

The first graphic below shows the moment or torque arm ma that is set up by the offset that exists between GRF from the firm piste acting at the inside edge and the point where the center of pressure of the weight of the body acts in the plane of the base of the ski. The large red arc shows the radius of rotation. The small red arc shows the radius of the moment of force. In this sequence, the ski is rotating downhill away from the pivot at the uphill edge.

When the base of the ski comes into full contact with the surface of the snow, rotational inertia, will make it want to continue rotating about the uphill edge and penetrate into the snow surface on the downhill aspect. If the force FW applied by the weight of the body is still aligned on the transverse center of the ski, it will oppose edge change.

In my next post I will discuss how the Second Rocker affects the mechanics of edge change at ski flat.

 

THE ORIGINS OF KNEE ANGULATION

A recent post on the Foot Collective Facebook page titled, Are you stable on 1 leg?, advises that if  you stand on one leg and look like the top row of pictures in the graphic below (red X), you have a foot & hip that are dysfunctional. This test is best done barefoot on a hard, flat, level surface.

Graphic with permission of Correct Toes

The lower photo (green checkmark) shows the alignment of a leg that is torsionally balanced (stiffened) in the ankle and knee joints. The foot and knee cap align straight ahead and square with the pelvis while the alignment of the knee with the foot, leg and thigh is substantially linear. If you can move to single limb support from two feet, easily achieve this alignment with minimal effort, sustain it for 30 seconds or more, and achieve similar alignment on both left and right legs, you probably have good stability in single limb support.

If you look like the upper photo (red x), it indicates dysfunction and especially a lack of torsional stability in the support limb. The problem is usually caused by constrictive, supportive, cushioned footwear and/or arch supports that, over time, deform feet and weaken the arches. Ski boots are one of the worst offenders in this regard.

If you and when you can achieve good stability in single limb support, you are ready to test the effect of footwear, especially your ski boots. Start by putting on your day to day footwear. Then do the same test on the same surface with each pair of shoes. Work your way up to your ski boots. Adjust the closures of your ski boots to the tension you normally set for skiing. If you are not able to quickly and easily assume the stable position shown in the lower photo (green checkmark), then you know that cause  is the footwear. You can then test the effects of insoles, including ski boot footbeds by removing them from the footwear, placing them on the test surface and moving to single leg support. While not perfect, these tests will help determine the cause of single support limb instability.

In skiing, an unstable outside support leg is characteristics of most skiers and even racers at the World Cup level. It is typically caused by ski boots interfering with the physiological processes that fascially tension the arches and forefoot that create the triplanar torsional stability of the ankle and knee joints of the biokinetic chain necessary to set up a platform under the outside ski to stand and balance on. But instead of addressing the underlying cause, the ski industry invented the term, knee angulation. Knee angulation is indicative of unbalanced torques acting about the uphill edges of the skis, especially the outside ski. When unbalanced torques are present about the edges of a skis or skis, unbalanced torques will also be present across the joints of the lower limb; not a good thing.

The alignment of the knee illustrated in the lower image (green checkmark) is seem as skier or racer enters the fall or rise line with outside leg extended, confirms the existence of a platform under the outside foot on which the skier or racer is balancing on with dynamic balance of torques across the joints of the ankle foot complex and knee. See my post MIKAELA SHIFFRIN AND THE SIDECUT FACTOR – http://wp.me/p3vZhu-1Uu

There is an abundance of information on programs to correct foot deformities,  muscle weakness and imbalances on web sites, YouTube and FaceBook groups such as The Foot Collective, Correct Toes, Feet Freex and the Evidence Based Fitness Academy – EBFA (Dr. Emily Splichal).

The Foot Collective web site has a series of posts on An Introduction to Feet and Footwear (1.) as well as a series of Foot-Casts (2.)

Meantime, a post on a web site called Rewire Me (3.) has an interview with Dr. Emily Splichal called No Shoes Allowed in which she discusses the importance of sensory information entering the body and the need to be able to process this information and handle the load and impact. Dr. Splichal suggests starting the process by getting the body and foot accustomed to sensory information without shoes acting as a barrier.

An excellent free paper with great graphics is The foot core system: a new paradigm for understanding intrinsic foot muscle function (4.)


  1. http://www.thefootcollective.com/an-introduction-to-feet-and-footwear/
  2. http://www.thefootcollective.com/footcast/
  3. https://www.rewireme.com/roses-blog/shoes-allowed/
  4. http://bjsm.bmj.com/content/49/5/290.full#xref-ref-39-1

THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: IMPULSE LOADING

In this post, I will discuss the role of impulse loading, in the perspective of phases of a turn cycle, in creating a platform under the body of the outside ski on which a skier can stand and balance on.

Impulse Loading

Impulse loading is crucial to the ability to establishing a platform under the body of the outside ski by cantilivering GRF, acting along the running surface of the inside edge, out under the body of the ski to create a stable platform for the skier to stand and balance on.

Maximization of dynamic stability while skating is crucial to achieve high (vertical) plantar force and impulse. (1)

Impulse in particular has been identified as an important performance parameter in sprinting sports as skating. (1)

The preceding statements apply equally to skiing.

The most important aspect of alternating single limb support locomotion is the ability to rapidly develop a stable base of support on the stance or support leg from which to initiate precise movement. Dr. Emily Splichal refers to this process as Time to Stabilization. The ability to balance on the outside ski of a turn is unquestionably the single most important aspect of skiing. Time to Stabilization, especially in GS and SL , is where races are won or lost. Here, the time in which to maximize dynamic stability on the outside foot and leg on the outside ski is in the order of 20 milliseconds (2 one-hundredths of a second); less than a rapid blink of the eye.

The Mid Stance, Ski Stance Theory

The predominant position within the ranks of ski industry is that skiing is a mid stance activity in terms of the stance phases of the gait cycle. In the mid stance phase of the gait cycle, tension in the longitudinal arch (LA) resulting from passive tensioning of the plantar ligaments is minimal and the foot is continuing to pronate. Mid stance, as the assumed basis for ski stance, appears to have served as the rational for the assumed need to support the LA with a custom footbed or orthotic (usually in neutral STJ) and immobilize the joints of the foot with a custom fit liner. Hence, the theory that the foot functions best in skiing when its joints are immobilized. I am not aware of any studies, let alone explanations based on principles of applied science, that supports this theory. To the contrary, the available evidence suggests that immobilizing the joints of the foot, far from making it function best in skiing, has the exact opposite effect.

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 problem with the mid stance, ski stance theory, is that impulse loading cannot not occur until late stance when arch compression, fascial stiffening of the forefoot and torsional stiffening of the subtalar and knee joints, is maximal.

One factor that has been shown to reduce arch compression is arch supportive insoles and orthotics. A study done in 2016 (1.) compared the effect of half (HAI) and full insoles (FAI) on compression loading of the arch to compression loading of the arch that occured in a standardized shoe (Shoe-only). Two separate custom insoles were designed for each participant. The first insole was designed to restrict arch compression near-maximally compared to that during shod running (Full Arch Insole; FAI) and the second was designed to restrict compression by approximately 50% during stance (Half Arch Insole; HAI). The Full Insole (black) most closely resembles the type of arch support used in ski boots to support the foot. The bar graph below shows the resulting reduction compression. I have overlain the FAI bar to illustrate how it compares to Shoe Only compression. This kind of study can now be done and should be done in vivo in skiing – during actual ski maneuvers where the effect of insoles and custom fit liners on the physiologic function of the foot and lower limb as a whole can be studied and assessed.

Two pressure studies done in 1998 by a team from the University of Ottawa (2, 3), that used elite skiers as test subjects, found large variations in pressures applied to the ball of the foot observed in the data that suggested some factor, or combination of factors, was limiting the peak force and impulse in terms of the vertical force that skiers were able to apply to the sole of the boot and ski. The researchers suggested a number of potential factors but did not investigate them.

These highest pressures reach up to 30 newtons per square centimetre. Force-time histories reveal that forces of up to 3 times body weight can be attained during high performance recreational skiing (my emphasis added).

Conclusions/Discussion:

It is quite likely that the type of equipment (skis and boots) worn by the subjects had an effect on the values obtained (my emphasis added).

A factor that was not controlled during data collection was the equipment worn by the subjects. The skiers wore different boots, and used different skis, although two of them had the same brand and model of skis and boots. It still has yet to be determined if that factor had any effect on the results. A point that all the skis that the subjects used had in common is that the skis were all sharp side-cut skis (also called shaped skis). Another equipment variation which may have affected in-boot measurements, is that some subjects (n=5) wore custom designed footbeds, while the other did not (my emphasis added).

In 2013 (4), a study presented at the European Congress of Sports Science in Barcelona, Spain that used special hockey skates that I prepared to maximize peak force and impulse using principles described in my blog compared peak and impulse forces of elite skaters in the skates I prepared (NS) to peak and impulse forces seen in their own skates (OS). The skates I prepared were used as a standardized reference similar to the protocols where baseline data obtained barefoot is used to assess the effect of specific footwear on physiologic function. The bar graphs below compare NS (the skates I prepared) to OS (the subjects own skates).

The researchers noted:

Thus, the results of this study show that direct measurement of these dynamic variables may be important indicators in evaluating skating performance in ice hockey as it relates to skate design or skill development.

Peak force and impulse are associated with high peak tension in the LA created by Achilles to forefoot load transfer.

I expect that similar results would be seen in ski boots.

The Phases of a Ski Turn Cycle

In order to appreciate the dynamics of impulse loading in skiing, I have modelled the phases of a turn cycle into 2 main phases with associated sub phases. The graphic below shows the Loading (1 – yellow) and Stance (2 – red) Phases of the outside (left) foot in a turn cycle with sub phases. The actual turn phase starts at the juncture of the traverse and from fall line and ends when the skier starts to extend the inside (right) knee. I will discuss the turn cycle in detail in a future post. My long-held theory, which was partially validated with the 1991 Birdcage studies, is that ski movements should employ the same hard-wired patterns as walking and running and that skiing should as instinctive and transparent.

Locomotion results from intricate dynamic interactions between a central program and feedback mechanisms. The central program relies fundamentally on a genetically determined spinal circuitry (central pattern generator) capable of generating the basic locomotor pattern and on various descending pathways that can trigger, stop, and steer locomotion. (5)

The feedback originates from muscles and skin afferents as well as from special senses (vision, audition, vestibular) and dynamically adapts the locomotor pattern to the requirements of the environment. (5)

 

Peak Force and impulse loading occurs at ski flat between edge change (red circle). This is what I refer to as the Moment of Truth. Moment, in this context, being a moment of force or torque. The manner in which the torque acts in the sequence of events surrounding edge change determines whether GRF is cantilevered under the base of the ski or whether it acts to rotate the ski (invert) it out of the turn.

 

 

In my next post, I will discuss the 2-step rocker impulse mechanism that cantilevers GRF acting along the running inside edge of the outside ski out under the body of the ski.


  1. The Foot’s Arch and the Energetics of Human Locomotion: Sarah M. Stearne, Kirsty A. McDonald, Jacqueline A. Alderson, Ian North, Charles E. Oxnard & Jonas Rubenson
  2. ANALYSIS OF THE DISTRIBUTION OF PRESSURES UNDER THE FEET OF ELITE ALPINE SKI INSTRUCTORS: Dany Lafontaine, M.Sc., Mario Lamontagne, Ph.D., Daniel Dupuis, M.Sc., Binta Diallo, B.Sc.. Faculty of Health Sciences1, School of Human Kinetics, Department of Cellular and Molecular Medicine, Anatomy program, University of Ottawa, Ottawa, Ontario, Canada. 1998
  3. ANALYSIS OF THE DISTRIBUTION OF PRESSURE UNDER THE FEET OF ELITE ALPINE SKI INSTRUCTORS: Dany Lafontaine, Mario Lamontagne, Daniel Dupuis & Binta Diallo, Laboratory for Research on the Biomechanics of Hockey, University of Ottawa, Canada – Proceedings of the XVI International Symposium on Biomechanics in Sports (1998), Konstanz, Germany, p.485.
  4. A Novel Protocol for Assessing Skating Performance in Ice Hockey: Kendall M, Zanetti K, & Hoshizaki TB School of Human Kinetics, University of Ottawa. Ottawa, Canada – European College of Sports Science
  5. Dynamic Sensorimotor Interactions in Locomotion: SERGE ROSSIGNOL, RE´ JEAN DUBUC, AND JEAN-PIERRE GOSSARD Centre for Research in Neurological Sciences, CIHR Group in Neurological Sciences, Department of Physiology, Universite´ de Montre´al, Montreal, Canada – 2006 the American Physiological Society

 

 

THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: WHERE IS GROUND?

It was my intent to discuss the key move in the First Step to Balance on the Outside Ski; Impulse Loading of the Forefoot. However, it has become apparent that it is necessary to preface this subject with a discussion on the source of ground in relation to the outside foot in order to impart an appreciation of why a mechanism is required to extend ground from the running edges of the ski in order to create a platform for a skier to stand and balance on when the outside ski is on its inside edge.

In typical discussions of ski technique and the mechanics, biomechanics and physics of skiing, the prevailing mental model assumes that a skier is in balance (see REVISION TO FEATURE POST: CLARIFICATION OF DEFINITION OF SKIER BALANCE) if they are able to stand upright and exercise a degree of control over their skis. In studies of balance performed in gait labs, ground reaction force in the form of stable surface for subjects to balance on is assumed.

Mental Models

Mental models are a form of cognitive blindness. Once people assume they know something, they not only don’t question what they believe, they filter out information that conflicts with their mental model. And they typically fail to see the real issue even when it is in plain sight.

A man should look for what is, and not for what he thinks should be.

                                                                                                                 –  Albert Einstein

The Skier Balance Paradox

Even though I quickly became a competent skier soon after I started skiing,  I struggled to hold an edge on firm pistes and especially glare ice. It was disconcerting to see elite skiers hold an edge on ice with minimal effort while making controlled turns. When I sought the advice of the experts, they claimed that holding an edge on ice was matter of sharp edges and/or driving the knees into the hill. When I protested that after trying both and found it harder to hold an edge, the experts claimed that the ability of some skiers to do what I couldn’t was due to superior technique. They were just better skiers. No further explanation was needed.

The inability of experts to explain why a small number of skiers seemed able to balance on their outside ski and hold an edge even on ice provided me with the impetus to look critically at this issue with the objective of formulating an explanation based on principles of applied science.

The only plausible explanation for the ability of a skier to be able to stand and balance on their outside ski when it is on its inside edge is that some source ground (reaction force) must be present under ski that they are using to stand and balance on. Hence, the question, Where is the Ground?

On very hard pistes, ground as a source of reaction force, is limited to the running portion of the inside edge of the outside ski and the small portion of the base adjacent the edge with the edge and base supported on a small shelf cut into the surface of the snow/

In Figure 2.11 on page 26 of his book, Ultimate Skiing, LeMaster explains how the sidecut of a ski creates a smaller radius turn as the edge angle increases.

In Figure 2.12 on the following page, LeMaster shows misaligned applied (green arrow) and ground reaction (purple arrow) forces creating an unbalanced moment of force (yellow counter-clockwise rotation arrow) that  rotates the ski down hill (out of the turn). LeMaster goes on to state that as the skier edges the ski more, the ski bites better. But he fails to offer an explanation as to how the skier can edge a ski more against an unbalanced moment of force acting to reduce the edge angle.

The mechanism that generates a moment of force that opposes the moment force shown by LeMaster in Figure 2.12 and has the effect of extending ground (reaction force) acting along the running length of the edge of the ski  is the subject of this series of posts.

Edge Angle Sidecut FXs

A simple way to acquire an appreciation for the location of ground relative to the outside ski on edge is to make a simple model out of flexible piece of sheet plastic material a few mm thick.

The photo below shows a model I made from a piece of sheet plastic about 8 inches long. The upper portion of the plastic piece has a shorter sidecut with less depth than the sidecut in lower portion of the piece of plastic piece. Both the model and sketches that follow are for illustrative purposes to demonstrate the effects of sidecut geometry on edge angle and a source of ground. Although the basic principles are the same, it is not intended that they be viewed as an accurate representation of actual ski geometries  The symmetrical geometry is for the benefit of the simplifying what is already a complex issue.

sidecut-1

There is a relationship between the depth and length of a sidecut in that the greater the ratio of the depth to the length of a sidecut, the lower will be the edge angle it forms with the surface in relation to the camber radius. In the sketch below, the upper rectangular ski shape will maintain a vertical relationship with a surface regardless of the camber radius.

There is also a relationship between the edge angle a ski with sidecut will form with a uniform surface and the radius of the camber with the edge angle formed with a uniform surface. The edge angle will increase (become more vertical) with a decrease in the radius of the camber. This explains why GS skis that are longer and have less sidecut depth than SL skis can attain much higher edge angles.

side-cut-factor

The photo below shows how the aspect of the model I made with the smallest sidecut ratio forms a steep angle with a uniform surface when bent to sufficient camber radius to allow the sidecut to become compliant with a uniform surface.

sidecut-2

When viewed from the rear of the model, the location of ground in relation to the structure of a ski with sidecut and camber should become readily apparent.

The graphic below shows what a photo taken at a low enough vantage point to the snow would capture looking straight on at a ski carving a turn with its edge compliant with the surface of the snow. This may seem foreign, even extreme to some. But when the edge of a ski is compliant with a uniform surface, the curve of the sidecut becomes linear.

The left image below depicts the schematic model of the ski shown in the second graphic with the camber angle sufficient to make the edge in contact with the uniform surface compliant with it. The angled line represents the surface of the snow. The schematic model of the ski represents the proximate end profile associated with a high load GS turn. A photograph in Figure 1.18 on page 17 of the Skier’s Edge  shows a similar profile in Hermann Maier’s outside (left) ski which is at a very high edge angle.

The graphic on the right shows some penetration of the running surface of the edge of the ski in conjunction with the forces commonly shown in the prevailing mental model that are used to explain how forces acting on the outside are balanced.

 

side-cut-angle

The reality is of the applied forces acting on the ski are shown in the vertical profiles in the graphic below as captured by digitized force plate data. Once the foot is loaded on a surface there is what is called a Center of Pressure as shown by the peaks in all 3 graphs. But when the foot is in compliance with a uniform surface, some pressure is expressed by the entire contact surface of the foot. So, the point application of applied force in opposition to a point application of GRF as depicted in the right hand graphic above is a physical impossibility.

Screen Shot 2015-12-20 at 9.59.06 PM

Viewing a transverse vertical profile of a ski on edge from the perspective of ground as a source of GRF for a skier to stand and balance on puts the issue of skier balance in a whole new, albeit unfamiliar, perspective. But it is a reality that must be dealt with in order to engage in realistic narratives on the subject. Overly simplistic explanations of skier balance attributed to a basic alignment of opposing forces do not serve to advance the sport of skiing as a credible science.

I concur with LeMaster’s position that the platform angle a ski forms with resultant and GR forces must be at 90 degrees or slightly less in order for the edges to grip. In my next post, I’ll start to introduce mechanical principles that explain how this can be accomplished.

It is the ability of racers like Mikaela Shiffrin to stand and balance on their outside that enables them to consistently dominate World Cup competition.

THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: FIRST STEP

In this post, I am going to begin the first of what I expect to be a series of posts on the Two Step Process to Balance on the Outside Ski.

Prelude

Before I start, I am going to caution the reader that they should not expect that the ability to learn and engage the processes responsible for balance on the outside ski to be easy to understand or quick to learn.  Many obstacles stand in the way of the ability to balance on the outside ski. As Benno Nigg’s experiments in the early ’90s at the Human Performance Laboratory at the University of Calgary demonstrated, the human body is highly adaptable. If a person puts their feet in footwear that prevents natural barefoot function, the body will find a best case work around compromise.

This is what happens to skiers when they put their feet in ski boots. As the Polish study showed, over time, the body will adapt. But adaptation always comes at a price.  Some skiers may adapt to constraints of a ski boot to the point where they are considered expert skiers by the prevailing standards. But they typically reach a point where they can no longer advance. Given same ability, the least compromised skiers become the best.

The problem faced by skiers who wish to learn balance on their outside ski (foot) is that the ingrained motor patterns their brain has created as a work around to address the limitations caused by their ski boots can be exceedingly difficult to erase. A skier will typically make some progress only to have their brain revert to motor patterns that have worked in the past when it senses danger. When this happens, the odds are great that even the most athletically gifted skier may have to relearn skiing to some extent. I have seen many graphic examples of this pattern over the past several years in skiers and racers I have worked with.

WARNING: The Mechanics of Balance on the Outside Ski is Not Simple

About the simplest way I can describe the mechanics is that the First Step involves a heel to 1st MPJ rocker loading mechanism while and the Second Step involves an intertia-driven turntable, over-centre mechanism. The mechanics is unified sequence of events. The reason I have broken it two steps is to make it easy to understand the critical nature of the first part of the sequence.

More than 25 years ago, I tried to make the First Step simple and easy to understand with the model I fabricated shown in the photo below and that graphic illustration that follows that shows how the Achilles tendon tensions the Plantar Aponeurosis (aka the Plantar Fascia) and enables foot to pelvic core sequencing. Note the annotation in graphic to Late Stance and (SR) Ski Stance Zone.

In my demonsrations, I  would drop the model on a table from a height of a few inches.  The rotation of the leg of the model would be quickly arrested by simulated isometric contraction of the Achiles. The model and the demonstration failed to garner attention or interest because the importance of the forefoot to foot function was not on the radar screen. Instead, the focus was on the hindfoot and addressing the known looseness of the forefoot associated with the mid stance phase of gait. A late stance phase was not yet part of the gait cycle narrative. The importance of late stance and fascial tensioning of the forefoot to foot function and foot to core sequencing has only recently been recognized.

sr-tripod-demo

Plantar Apo Dynamics

First Step

The First Step is to tension the biokinetic chain that extends from the MPJs of the foot to the pelvis. The timing of this event, which is critical, will be discussed in a later post.

The key move is the loading of the outside foot. This should happen in the top of the turn as the fall line is approached. This is the point where a skier should become the tallest in relation to the snow. At the end of a turn (in the bottom) is where a skier should be lowest.

It is not possible to replicate the loading move except when skiing because of the dynamic nature of the 3-dimensional forces associated with ski maneuvers. But the forefoot loading move that creates fascial tension the forefoot is essentially the same move we make when we move forward on the stance foot in walking in preparation to take a step. Once the foot has adapted to the ground, forward rotation of the shank (ankle flexion) is arrested by isometric contraction of the calf muscle. At this point, further forward movement of the torso occurs through knee extension in what amounts to a heel to ball of the foot rocker mechanism; i.e. a forward and downward action that applies force to the ground to prime the energy return foot spring in preparation to propel the body forward.
One way to get a feel for this mechanism is to stand sideways across the bottom of a stair and place one foot on the first tread about a whole foot length ahead of the foot on the floor. The knee of the leg on the floor should have slight bend so the calf muscle is in isometric contraction (SR Stance). The angle of the shank of the foot on the tread should be a little less than 90 degrees in terms of dorsiflexion. From this base position, the torso is projected forward in order to achieve a position of balance over the foot on the first tread. This is roughly what the loading move should feel like in skiing that is made as the fall line is approached. Once a feel for this has been acquired I can discuss how this integrates with rotation of the leg.
It is important to not have the ankle flexed for the above exercise because the ski boot limits ankle flexion. At the start of the transition at the end of a turn, the weight will be under the heel of the inside (uphill) foot. It is also important that the calf muscle of the foot on the stair tread go into isometric contraction so that further forward movement of the torso occurs through knee extension.
In a ski turn, the forefoot loading move is one of a quick heel to 1st MPJ forward rocker knee extension pulse that loads the ball of the foot (1st MPJ). Loading of the 1st MPJ (ball of the foot) is caused by forward movement of the torso (CoM), not plantarflexion. This loading move is made in the top of a turn as the fall line (aka rise line) is approached. The window in which to make this move is narrow and the time required  to complete the move, brief.
If you watch video of Shiffrin slowed to 0.25 normal speed or step the video in frame-by-frame, you will clearly see her make this loading pulse which usually involves a lifting of the fore-body of the old outside ski due to swing leg reaction force.
In my next post, I will discuss Step Two.

 

 

 

 

 

 

 

 

 

 

 

INFORMATION FOR ACADEMICS AND TECHNICAL AND MEDICAL PROFESSIONALS

With the increasing evolution and availability of microsensor technologies, some which can work with smart phones, there has been a corresponding increase in both the level of interest and studies on the science aspect of skiing.

One of the best studies I have found to date is the study (1) by Michał Staniszewski, Przemysław Zybko and  Ida Wiszomirska of the Józef Piłsudski University in Warsaw, Poland. As a preliminary step to the design of a study, researchers typically conduct a search of the literature for existing papers in the field. Researchers who design studies for such things as skier balance are typically surprised when results of searches find that few, if any studies exist.

Thus, the authors of the Polish study (1) 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) on materials, designs and standards for the ski boot made a similar comment:

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

If not many scientific papers have been published on issues pertaining to skiing, the question arises as to what, if any, process was used to influence the design of the modern ski boot. The authors of the paper (3) comment:

The development of alpine ski equipment has been done, in the beginning, mainly by trial and error, using on-snow tests.

In other words, the process appears to have been largely subjective and artisan driven as suggested by the comment:

The first skis were only plank of woods with laces to link the traditional leather boots used in the Nordic and Alpine regions. At that time skiing was mainly performed in flat areas to help the transportation of loads and for this reason the first recreational activity with skis was cross-country skiing. Alpine skiing was born only afterwards and, therefore, the first equipment used to ski down the steep terrains of the Alps was developed starting from those already used for skiing in flat snow-fields (thin skis, leather boots and heel free bindings).

This comment suggests that the modern plastic ski boot was an outgrowth of the artisan leather hiking boot with advancements being mainly in the form of new plastic materials in place of leather.

While the authors acknowledge that research in the field of plastic materials and the optimization of new software for the design of sport equipment have permitted the development of new materials and designs that have increased the level of performance, security and comfort of ski-boots, no mention is made of advancements in the understanding of the physiologic aspects of alpine skiing. The lack of existing studies places researchers who wish to investigate this issue in the awkward position of having to rely to a significant extent on established ski industry positions on such crucial aspects as skier balance as the authors of the 2014 Polish study did when they cited LeMaster’s definition of balance:

He or she is in balance and will not fall as long as the total of all of the forces acting on the center of gravity passes through the body’s base of support.

Not falling does not conform to the text-book postural response paradigm wherein the opposing forces associated with single balance align across the plantar/ground interface with CoM maintained within defined limits of the foot.

Although studies in the field of the mechanics, biomechanics and physics of alpine skiing are rare, the number of studies in related fields that are directly applicable to skiing is steadily increasing.

Recent studies (4) on forefoot loading (fascial tensioning) found that peak plantar aponeurosis tension occurs in late stance, suggesting that it was an error to not include the late stance phase in gait and balance studies and positions in general on the phases of gait.

The plantar aponeurosis transmits large forces between the hindfoot and forefoot during the stance phase of gait. The varying pattern of plantar aponeurosis force and its relationship to Achilles tendon force demonstrates the importance of analyzing the function of the plantar aponeurosis throughout the stance phase of the gait cycle rather than in a static standing position. 

This and similar studies have significant implications for skiing because the late stance phase of the gait cycle stabilizes the subtalar and knee joints in preparation for the application of high ground forces for propulsion. The implications of such findings  to alpine skiing is that the late stance phase (SR Stance) allows high loads to be applied by the 1st and 2nd MPJs and the application of reverse subtalar joint translated vertical axial rotational torque generated from the pelvis to the ski.

Understanding and appreciating these mechanisms opens the door to new opportunities for research projects.


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

4. Dynamic Loading of the Plantar Aponeurosis in Walking: Ahmet Erdemir, PhD; Andrew J. Hamel, PhD; Andrew R. Fauth, MSc; Stephen J. Piazza, PhD; Neil A. Sharkey, PhD – J Bone Joint Surg Am, 2004 Mar; 86 (3): 546 -552 . http://dx.doi.org/

Intrinsic foot muscles have the capacity to control deformation of the longitudinal arch – Luke A. Kelly1,2, Andrew G. Cresswell1, Sebastien Racinais1,2, Rodney Whiteley2 and Glen Lichtwark1⇑