Sports Science posts

THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: CLOSED CHAIN OUTSIDE LEG ROTATION

A recently published study on foot pressure data acquired during skiing (1.) recognized that compressive force pressure data acquired in skiing is underestimated by 21% to 54% compared to pressure data acquired on a force platform in a controlled environment.  The underestimation varies depending on the phase of the turn, the skier’s skill level, the pitch of the slope and the skiing mode. The paper states that other studies have stated that this underestimation originates from a significant part of the force actually being transferred through the ski boot’s cuff (to the ski). 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.

In conclusion, these studies have highlighted a major contribution of different factors to the nGRF applied throughout a turn, such as the foot’s position during a turn (inside vs. outside), the CoP A-P (front to back) displacement, or precise loading of different foot sole regions.  Unfortunately, these results have been studied separately.

There is a lack of continuity across the various positions in skiing and, in particular, a lack of a model with which to explain mechanisms such as balance on the outside ski and open and closed chain internal rotation of the leg and foot in conjunction with progressive inclination and G force loading on it as the skier crosses the fall line in the bottom of a turn. The associated mechanics and biomechanics represent a new paradigm requiring new thinking and new insights. Existing text-book explanations are not sufficient to explain these mechanisms.

Open Chain Whole Leg Rotation vs. Closed Chain Rotation

Rotation of an unloaded (non-weight bearing) lower limb is relatively straight forward when there is a small angle at the knee. As resistance to rotation of the foot is progressively introduced with increasing weight imposed on it, the kinetic chain begins to close. As it closes, the points at which the foot transfers torque to the walls of rigid shell footwear such as ice skates and ski boots starts to emerge as an issue as does the loading of the foot created by the weight of the body imposed on it and the position of COM in relation to the foot.

In order to tension the biokinetic chain and trigger the two-phase Second Rocker, COM must be aligned over the foot as shown in the grahic below.  This alignment requires that the leg adduct (move towards the center of the body) approximately 6.5 degrees. To bring the 3 points of the tripod of the foot into contact with the ground, the foot must evert (sole turn outward) the same amount. Eversion is accompanied by a corresponding torque coupled 6.5 degrees of internal rotation of the leg as shown in the left hand figure in the graphic below (see my post – OUTSIDE SKI BALANCE BASICS: STEP-BY-STEP). The bipedal figure on the right shows adduction, eversion and internal rotation as 0.0 – 0.0 – 0.0 for reference. The monopedal figure on the left shows the changes in adduction, eversion and internal rotation as 6.5 – 6.5 – 6.5.

 

The alignment of COM with the foot can be achieved by moving COM laterally as shown by the arrow emanating from COM in the Monopedal figure or by moving the foot medially as shown by the white arrow or through a combination of the two movements.  The act of positioning COM over the outside foot (Getting Over It), sets in motion internal rotation of the outside leg and eversion of foot into the turn. This engages an over-centre mechanism between the platform of the ski and the inside edge underfoot.

The over-centre mechanism results in an alignment of the resultant force R forming an angle with the transverse aspect of base of the ski that is slightly less than 90 degrees. In order to Get (COM) Over It (the foot), it is essential that the outside leg is not only able to adduct and rotate internally as the foot everts, but to achieve this configuration without delay in order to set up the over-center mechanism. The problem for the majority of skiers is that the objective of most boot fit systems and boot-fitting procedures is to support the foot in a neutral configuration. Eversion of the foot is a component of pronation. Impeding or preventing pronation, restricts or even prevents the required amount of eversion.

Closing the Kinetic Chain on Whole Leg Rotation

Open kinetic chain leg/foot rotation with the foot unloaded (not bearing weight) is relatively simple. But the mechanics and biomechanics begin to get complicated when resistance is progressively introduced that starts to close the kinetic chain as happens when the outside ski is rotated across the path of the skier in the fall line in the bottom of a turn.
The graphic below shows a foot supported on a platform with 2 points of resistance (FR) applied to the platform opposite the 2 points of application of the moments of force, ML (green) and MM (red). The forces tangent to the arc of the moments of rotation are shown as FT.
When the weight of the body is progressively shifted to one foot (i.e. Monopedal Stance) and the foot everts, the talus (shown in gray in the graphic above) moves inward towards the center of the body and shifts slightly rearward as evidenced by the corresponding movement of the inside ankle bone.  This is easily seen when moving from bipedal to monopedal stance on a hard, flat surface while barefoot.In order to effectively transfer torque from the foot to the platform, the forefoot and ankle and knee joints must be fascially tensioned. This requires that the big toe (Hallux) be aligned on the anatomical axis (dashed line) and the forefoot fully splayed. This stabilizes the heel and head of the 1st metatarsal (ball of the foot).  Torque from internal rotation of the leg will be transferred to two discrete points adjacent the Load Counters mounted on the resistance platform.

Removing the resistance force FR from the inner (big toe) aspect of the platform provides insights to what I refer to as the Turntable Effect that is associated with internal rotation of the leg and eversion of the foot that creates an over-center mechanism. The turntable rotation is shown in light yellow. The effect will vary for different structures of the foot depending on the location of the center of rotation of the platform under the foot.

The location of the blade of an ice skate on the anatomical center of the foot has been used to explain why it is easier to cut into a hard ice surface with a skate compared to the edges of a ski. But the real reason it is easier is because ice skaters use the Second Rocker, Over-Center, Turn Table Mechanisms as shown in the graphic below. The skate is positioned under COM. It can be readily seen that the skater is not using the inner aspect of the shaft of the skate to hold the skate on edge.

In my next post, I will discuss the progress of emerging CARV and NABOSO technologies after which I will continue with my discussion of the Mechanics of Balance on the Outside Ski.


  1. Influence of slope steepness, foot position and turn phase on plantar pressure distribution during giant slalom alpine ski racing: Published: May 4, 2017  – Thomas Falda-Buscaiot, Frédérique Hintzy, Patrice Rougier, Patrick Lacouture, Nicolas Coulmy
  2. http://wp.me/p3vZhu-29n

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

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

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

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

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

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

The Three Rockers

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

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

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

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

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

Two Phase Second Rocker

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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.

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