Skier Balance posts

THE SKI BOOT FLEX INDEX INSTABILITY PROBLEM

It has been known for decades that an unbalanced moment of force or torque will be present on the outside ski when the center of pressure of the load applied to the ski by a skier is acting along the center of the transverse axis of the ski where it is offset from GRF acting along the inside edge. Ron LeMaster acknowledges the existence of an unbalanced moment of force on the ouside ski in both The Skier’s Edge and Ultimate Skiing (Edging the skis). LeMaster states in Ultimate Skiing;

The force on the snow is offset from the center of the skier’s and creates a torque on it that tries to flatten the ski.

Ron didn’t get the mechanics right. But he correctly shows the unbalanced torque acting on the ankle joint. LeMaster tries to rationalize that ice skates are easy to cut clean arcs into ice with because the blade is located under the center of the ankle. While this is correct, ice skaters and especially hockey players employ the Two Stage Heel-Forefoot Rocker to impulse load the skate for acceleration. Hockey players refer to this as kick.

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

…..In the absence of this internal rotation movement, the center of pressure remains somewhere in the middle of the forefoot, which is some distance from the medial edge of the ski, where it is needed.

The load or weight of COM is transferred to distal tibia that forms the ankle joint. This is the lower aspect of the central load-bearing axis that transfers the load W from COM to the foot. What happens after that depends on the biomechanics. But the force will tend to be applied on the proximate center of the stance foot. This is a significant problem in skiing, (one that LeMaster doesn’t offer a solution for) when the ski is on edge and there is air under the body of the ski. The unbalanced torques will move up the vertical column where they will manifest at the knee against a well stabilized femur.

But this unbalanced torque creates another problem, one that is described in a paper published in 2005 by two Italian engineers (1.) that describes how this load deforms the base of the boot shell.

The Italian study found large amounts of deformation at mean loads of up to 164% body weight were measured on the outer ski during turning. The paper suggests that the ski boot flex index is really a distortion index for the boot shell. The lower the flex index, the greater the distortion potential.

For the ski-boot – sole joint the main problem is not material failure, but large amounts of local deformation that can affect the efficiency of the locking system and the stiffness of the overall system.

Values of drift angle of some degree (>2-3°) cannot be accepted, even for a small period of time, because it results in a direct decrease of the incidence of the ski with the ground.

My post GS AND KNEE INJURIES – CONNECTING THE DOTS (2.) cites studies that found that knee injuries are highest in GS in the shortest radius turns where peak transient forces are highest.

As shown in Figure 2a FR (sum of centrifugal and weight forces) and F GROUND (ground reaction force) are not acting on the same axis thus generating a moment MGR that causes a deformation of the ski-boot-sole system (Figure 2b) leading to a rotation of the ground reaction force direction. The final effect is to reduce the centripetal reaction force of the ground, causing the skier to drift to the outside of the turn (R decreases, causing the drift event).

An imperfect condition of the ski slope will emphasize this problem, leading to difficulties maintaining constant turning radius and optimal trajectory. The use of SGS ski-boot in competitions requires a particular focus on this aspect due to the larger loads that can be produced during races.

I have added a sketch showing that the moment arm M R created by the offset between the F Ground and F R is in the plane of the base of the ski where it results in an Inversion-lateral rotation torque.

The importance of sole stiffness is demonstrated with a simplified skier model…..…ski boot torsional stiffness with respect to ski longitudinal axis in particular is very important as it deeply influences the performance of the skier during turning…. A passage over a bump or a hollow may generate a sudden change in ground reaction force that may lead to a rapid change in the drift angle delta. The ski boot must be as stiff as possible going from the lower part of the boot to the ski (i.e. lower shell-joint-sole system)

As explained in the method section using the simplified model, values of some degree cannot be accepted, even for a small period of time, because the skier stability and equilibrium could be seriously compromised especially when the radius of curvature is small. A non perfect condition of the ski slope will emphasize the problem, leading to big difficulties for maintaining constant turning radius and optimal trajectory.

This excellent paper by the two Italian engineers concludes with the following statements:

Authors pushed forward the integration of experiments and modeling on ski-boots that will lead to a design environment in which the optimal compromise between stiffness and comfort can be reached.

The possibility of measuring accurately the skier kinematics on the ski slope, not addressed in the presented study, could represent a further step in the understanding of skiing dynamics and thus could provide even more insightful ideas for the ski-boot design process.

I first recognized the shell deformation, boot board instability issue in 1980, at which time I started integrating rigid structural boot boots into the bases of boot shells I prepared for racers. The improvement in ski control and balance was significant. The instability of  boot boards associated with shell/sole deformation with 2 to 3 degrees of drift at modest loads of up to 164% body weight has significant implications for footbeds.


  1. AN INNOVATIVE SKI-BOOT: DESIGN, NUMERICAL SIMULATIONS AND TESTING – Stefano Corazza 􀀍 and Claudio Cobelli Department of Information Engineering – University of Padova, Italy – Published (online): 01 September 2005 – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3887325/
  2. http://wp.me/p3vZhu-zx

THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: THE ROCKER/TURNTABLE EFFECT

The Two Phase Second Rocker (Heel to Ball of Foot) described in the previous post is dependent on inertia impulse loading. A good discussion of the basics of inertia and momentum is found in Inertia, Momentum, Impulse and Kinetic Energy (1.)

Limitations of Pressure Insoles used in Skiing

A paper published on May 4, 2017 called Pressure Influence of slope steepness, foot position and turn phase on plantar pressure distribution during giant slalom alpine ski racing by Falda-Buscaiot T, Hintzy F, Rougier P, Lacouture P, Coulmy N. while noting that:

Pressure insoles are a useful measurement system to assess kinetic parameters during posture, gait or dynamic activities in field situations, since they have a minimal influence on the subject’s skill.

acknowledge limitations in pressure insoles:

However, several limitations should be pointed out. The compressive force is underestimated from 21% to 54% compared to a force platform, and this underestimation varies depending on the phase of the turn, the skier’s skill level, the pitch of the slope and the skiing mode.

It has been stated this underestimation originates from a significant part of the force actually being transferred through the ski boot’s cuff. As a result, the CoP trajectory also tends to be underestimated along both the anterior-posterior (A-P) and medial-lateral (M-L) axes compared to force platforms.

Forces transferred through the cuff of a ski boot to the ski can limit or even prevent the inertia impulse loading associated with the Two Phase Second Rocker/Turntable Effect. In addition, forces transferred through the cuff of a ski boot to the ski intercept forces that would otherwise be transferred to a supportive footbed or orthotic.

Rocker Roll Over

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

In the absence of this internal rotation movement, the center of pressure remains somewhere in the middle of the forefoot, which is some distance from the medial edge of the ski, where it is needed.

Rock n’ Roll

To show how the Two Phase Second Rocker rocks and then rolls the inside ski onto its inside edge at ski flat during edge change, I constructed a simple simulator. The simulator is hinged so as to tip inward when the Two Phase Second Rocker shifts the center of pressure (COP) from under the heel, on the proximate center of a ski, diagonally, to the ball of the foot.

The red ball in the photo below indicates the center of gravity (COG) of the subject. When COP shifts from the proximate center to the inside edge aspect, the platform will tilt and the point of COP will drop with the COG in an over-center mechanism.


A sideways (medial) translation of the structures of the foot away from the COG will also occur as shown in the graphic below. The black lines indicate the COP center configuration of the foot. The medial translation of the foot imparts rotational inertia on the platform under the foot.

Two Phase Second Rocker: The Movie

The video below shows the Two Phase Second Rocker.

Click on the X on the right side of the lower menu bar of the video to enter full screen.

The graphic below shows to Dual Plane Turntable Effect that initiates whole leg rotation from the pelvis applying multi-plane torque to the ski platform cantilevering reaction force acting along the running edge of the outside ski out under the body of the ski. A combination of over-center mechanics and internal (medial or into the turn) application of rotation of the leg from the pelvis, counters torques resulting from external forces.


  1. http://learn.parallax.com/tutorials/robot/elev-8/understanding-physics-multirotor-flight/inertia-momentum-impulse-and-kinetic
  2. http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0176975

 

 

 

 

WHO NEEDS FOOTBEDS? NO ONE

There are some who can benefit from footbeds or orthotics and some who do actually need them. But these groups are the rare exception. And they are unlikely to be skiers.

Orthotics. The pros / cons of orthotics in today’s society!

In a recent YouTube video (1.), Podiatrist & Human Movement Specialist, Dr Emily Splichal, explores the concept of orthotics and their role in today’s society. Dr. Splichal doesn’t pull any punches when she says:

“…..I have been through the conventional podiatric school and been fed pretty much the bullshit from podiatry of how every single person needs to be in orthotics, that our foot is not able to support itself without orthotics……if we do not use orthotics our foot is going to completely collapse  and you are going to lose your arch…….”

“……Our foot is designed to support itself. If we actually needed orthotics, we would be born…..we would come out of the womb, with orthotics on our feet.”

Meantime, The Foot Collective  asks (2.) Are you promoting weak feet?

  • Anything you use for artificial support at the feet (footwear with arch support & orthotics) your brain takes into account and accommodates for it.
  • That means if you provide your foot support your brain shuts down the natural arch supporters to reduce un-necessary energy expenditure.
  • Stop using support to help with pronation and understand why your feet pronate in the first place – because they are weak.
  • Strong feet = strong foundation = strong body.

The Real Source of Support for the Arch

Ray McClanahan, D.P.M. offers a perspective on the issue of Arch Support in his post on the CorrectToes blog (3.)

Are Custom Footbeds and Orthotics better than stock insoles?

In his post of August 20, 2017, Custom Foot Orthotics; No Better Than Stock Insoles (4.), Rick Merriam, of Engaging Muscles, explores the issue of orthotics in depth.

Prior to being told that supportive insoles are the way to go, I think it’s safe to say that all of those people didn’t know what they didn’t know.

The erroneous assumption that every skier needs footbeds or orthotics was made at a time when little  was known about the function of the foot and lower limb, especially in late stance. I was one of those who didn’t know what I didn’t know when initially when down the ‘the foot needs to be supported in skiing’ road up until I realized what I didn’t know and took steps to acquire the requisite knowledge.

Footbeds; is anyone checking what they do?

In 2000, I formed a company called Synergy Sports Performance Consultants (5). Synergys’ product was high quality information. One of my partners, UK Podiatrist, Sophie Cox, was trained by Novel of Germany and was one of the few experts in the world at that time on the Pedar system. Synergy did not make and/or sell footbeds or orthotics. Instead, we checked the effect of footbeds on skier performance. We performed a quick footbed check for a minimal fee of $20 using the sophisticated Novel Pedar pressure analysis technology.

Synergy was one of the first companies in the world to use the Novel Pedar pressure analysis system synchronized to video to acquire data on skier performance and analyze the captured data.  The Synergy team with diverse expertise studied the effect of ski boots and custom insoles on skier performance and identified functional issues in the body that needed to be addressed. It was a common finding that custom footbeds were significantly compromising skier performance, especially the ability to create the necessary platform under the foot on which to stand and balance on the outside ski.

Synergy offered a comprehensive 5 Step Performance Program that started with a footbed check. A key component was item 2., the Biomechanical Check.

With increasing recognition of the negative effect of most footwear on the user and criticism of the unproven claims made for footbeds and orthotics coming hard and fast, credibility in skiing is rapidly going downhill. It is time for proponents of custom insoles for ski boots to support their claims with solid evidence, especially evidence supported with data acquired during actual ski maneuvers. The technology to do this has existed since at least the year 2000.


  1. https://youtu.be/CIRf9WHmMXI
  2. http://www.thefootcollective.com
  3. https://www.correcttoes.com/foot-help/articles-studies/arch-support/
  4. http://www.engagingmuscles.com/2017/08/20/custom-foot-orthotics/
  5. DIGITAL SALVATION FOR THE SOLE [BACK TO THE FUTURE] –  http://wp.me/p3vZhu-24g

THE MECHANICS OF EDGE CHANGE

Comments made by followers of my blog suggest that significant confusion exists 0n the meaning of terms and representations of mechanics, biomechanics and physics used in typical explanations of ski technique and ski mechanics. In particular, there appears to be confusion between pressure and the representation of point forces.

Pressure is a physical force applied to an object that is distributed over the surface of the object.

Center of Pressure or COP is the point center of ground reaction force opposing a corresponding center of applied force acting on a object supported on the ground or a stable surface that acts in the capacity of ground in terms of providing a source of reaction force.

Torque or Moment of Force results from an offset between the centers of opposing physical forces acting on either side of an object.  This offset results in a torque or moment arm that tends ti create rotation about a center. When one force has a greater magnitude than the other force, rotation of the object will occur around the point of rotation.

Why typical balance explanations of skier balance are wrong

Balance in skiing is often depicted as a simple alignment of opposing point forces, usually a resultant force R acting in opposition to a snow reaction force S. The mechanics that make the edges of a ski grip are often shown as a simple alignment of opposing forces acting a single point on the edge. Explanations of this nature are physically impossible. What the authorities in skiing seem to conveniently be ignoring is the fact that pressure is applied by the snow along the entire running surface of the edge in contact with the the snow while an opposing area of pressure applied by the weight of the skier is acting on the body of the ski with an offset between the two centers of pressure. The authorities in skiing also seem to conveniently ignore what is arguably the key even in establishing a platform under the outside ski for the skier to stand and balance on, edge change.

Mikaela Shiffrin’s Get Over It drill on the Burke Mountain YouTube site makes a good segue to an explanation of the Mechanics of Edge change in the my next post – https://youtu.be/Bh7KF49GzOc

Bridget Currier is the model every skier should aspire to. She perfectly executes what I call the skimove. The skimove engages the external forces at ski-flat/edge-change to drive multi-plane torques acting about her outside ski into the turn while setting up a solid platform under her outside foot for her to stand on. Magnificent! This video should have at least a million views.

My comment from 2 years ago

Note carefully Currier’s stance in balance on her new outside ski, in particular, the angle of her torso with the snow. This is key to loading the ball of her outside foot.

Note carefully Shiffrin’s comment to move forward onto her new ski and how she used to think the movement was a lateral (sideways) move.

Most important of all – Patient Initiation. The reason? Shiffrin and Currier, don’t tip their outside ski on edge. They rock it on edge with a rocker impulse loading mechanism. The sequence is Rock, Roll n’ Rotate then Rotate the outside leg.

ERROR IN LAST POST ON EVERSION

In my last post, I erroneously stated that the sole turns inward, towards the center of the body, in eversion. I meant to state that the sole turns outward, away from the center of the body, in eversion.

I have revised the paragraph in my post so it reads correctly.

In order for the torso and Center of Mass to stack vertically over the ball of the foot, the sole of the foot must turn outward, away from the center the the body. This is called eversion. It is enabled by the joint that lies below the ankle called the sub-talar joint. The sub-talar joint is tied to the tibia where it acts as a torque converter. When the foot everts or inverts, the sub-talar joint translates this on an approximately 1:1 ratio into internal or external vertical axial rotation of the leg.

I apologize for any confusion this may have caused.

OUTSIDE SKI BALANCE BASICS: STEP-BY-STEP

In view of the positive response to my recent posts and comments I have received, I have decided to take a step-by-step approach to explaining the mechanics and biomechanics of balance on the outside ski.

I am going to start the process by comparing balance on one foot to balance on two feet. I refer to balance on one foot as monopedal stance (one foot) and balance on two feet as bipedal stance (two foot). The graphics are for illustrating general principles only.

The graphic below shows monopedal stance on the left and bipedal stance on the right. Orange hash marks delineate the alignment of major body segments. Black reference lines on the right leg of both figures show the angle of the leg in relation to the ground.

In order to transition from a balanced position in bipedal stance to a balanced position in monopedal stance, either the foot must move towards the L-R center of the torso or the torso must move towards the foot that will become the stance foot, or a combination of the 2 movements must occur. The central issue is the amount of inertia acting on the torso. In skiing, due to the degree of inertia, the new outside foot of a turn is normally guided into position under the torso as the skier or racer approaches the fall line in the top of a turn.

Moving the foot into position under the Centre of Mass so it stacks in line with the ball of the foot usually takes an inward movement (adduction) of the leg from the pelvis of 6 to 7 degrees. In the upper left figure in monopedal stance, the leg is adducted 6.5 degrees and has formed a varus or outward leaning angle with the ground.

If the leg only adducted, then the sole of the foot would end up at an angle of 6.5 degrees with the ground and the figure would end up on the outer edge of the foot; on the little toe side. In order for the torso and Center of Mass to stack vertically over the ball of the foot, the sole of the foot must turn outward, away from the center the the body. This is called eversion. It is enabled by the joint that lies below the ankle called the sub-talar joint. The sub-talar joint is tied to the tibia where it acts as a torque converter. When the foot everts or inverts, the sub-talar joint translates this on an approximately 1:1 ratio into internal or external vertical axial rotation of the leg.

When the foot everts, the subtalar joint rotates the vertical axis of the leg towards the center of the body an equivalent amount; in the subject case, 6.5 degrees.

The combination of eversion/internal vertical axial rotation of the leg is called pronation. If either of these actions is interfered with, or worse, prevented, it is impossible to create the alignment necessary to stack the torso and Center of Mass over the ball of the support foot.

The consistently stated objective of footbeds is either to limit or even prevent pronation. Put another way, the whole idea of footbeds is to make it difficult or even impossible to balance on the outside foot and ski.

If this issue is not crystal clear, please post comments as to what is needed.

ADDENDUM TO THE ORIGINS OF KNEE ANGULATION

The intent of my last post was to create an awareness of the lower limb alignment indicative of stability and how a lack of stability, whether intrinsic or caused by footwear, especially ski boots, will cause a skier to default to the use of knee angulation in what will be a failed attempt to hold the edge of the outside ski.

A skier will be unable to develop the requisite biomechanics to balance on their outside ski if they lack stability in barefoot monopedal stance under the minimal challenges associated with a flat, level unperturbed surface. If they lack lower limb/pelvic stability, there could endless combinations of causes which is why I listed a number of resources to help address this deficiency.

If a skier/racer exhibits good to excellent  stability under this basic test and they become unstable with the addition of any form of footwear, it suggests, but does not unequivocally prove, that the footwear is the cause. In more 4 decades of working with skiers and racers at all levels, I have consistently found that I can turn monopedal stability off and on at will. That I can do this without limitation, is indicative of cause and effect. In the 2 world class racers I am presently working with, even a small change in a liner or the over-tensioning of a shaft buckle or power strap has an immediate and noticeable effect on outside limb/pelvic stability and balance.

A key exercise I like to use with racers and elite skies I am working with is the vertical stacking exercise shown in the graphic below. This exercise is performed by starting from bipedal stance with the feet stacked under the heads of the femurs and the head and torso vertical and then making fluid arcing movement of the COM over the ball of the big toe while keeping the torso and head stacked vertically and the pelvis and shoulders horizontal as indicated by orange vertical and horizontal references in the graphic below. The torso should be aligned with the transverse or frontal plane, square with the foot.

A lack of stability in the biokinetic chain is typically evidenced by a drop of the opposite side of the pelvis and a leaning in the opposite direction of the torso and/or the head or both. While this reduces the load on the pelvis side of the  leg it creates a myriad of issues. Inside hip drop will cause the inside leg of a turn to assume the load as the skier inclines thus creating further instability on the outside leg.

Elite skiers and racers like Shiffrin are able to get over it (find stability on their outside foot and ski) in milliseconds. This enables them to retract the inside foot and ski with knee flexion as they incline into a turn similar to the mechanics cyclists use when they corner; outside leg extends, inside leg retracts.

The vertical stacking exercise is best performed in front of a mirror.