Centre of Mass

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: 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

 

 

 

 

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

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.

GROUND SUPPORT

In this post, I will discuss where the source of ground or GRF lies in relation to the forces applied by the outside or stance foot of a skier.

Explanatory diagrams of the forces of skiing typically show a resultant force (R) of the components of gravity (G) and centrifugal force (C) acting on the centre of mass (COM) of a skier. The vector of the resultant force (R) emanating from COM is shown acting at a point in the vicinity of the inside edge of the outside ski. If a force diagram is sophisticated, it might show a ground reaction force (GRF) acting at the inside edge of the outside ski or even the centripetal component of centrifugal force.

Regular

The overly simplistic nature of such force diagrams infers that control of the edge angle of the outside ski requires nothing more than an alignment of R with inside edge. In this context, the effect of the width of the ski under foot on the skier as shown in the image below is of no consequence.

Fats

The influence of the mechanics of both of the above ski configurations on the skier are inferred to be the same even though they are dramatically different as shown in the overlay below.

Overlay

The reality of such force diagrams is that they represent a static moment in time; a snapshot that tells nothing of the dynamic nature of the forces at play. In order to accurately represent the dynamic forces, force diagrams must include the load from the weight W of COM and especially how and where the load is transferred to the outside foot of a turn, to the ski and from there to the point that I call Ground Zero. Ground Zero is where the forces of the snow and skier meet. The forces at the skier/ski equipment/snow interface should be the subject of intense discussion and debate. More than simply being important, understanding and managing these forces is fundamental to skier balance and the global control of COM essential to a biomechanically sound technique.

Force diagrams like the one shown below should be the minimal starting point for meaningful dialogues on the forces of skiing.

With Central Axis

 

 

 

 

 

GOOD COP, BAD COP

The science of the study of human balance is well established. Studies of balance use two key metrics; COM (Centre of Mass) and COP (Centre of Pressure). The following text is excerpted from Human balance and posture control during standing and walking – D A Winter PhD, P. Eng. – Gait & Posture: 1995; Vol 3: 193-214, December. (1)

Centre of Pressure (COP) is the point location of the vertical ground reaction force vector. It presents a weighted average of all pressures over the surface of the foot that is in contact with the ground. It is totally independent of COM. If one foot is on the ground, the net COP lies within that foot. If both feet are in contact with the ground, net COP lies somewhere between the two feet depending on the relative weight taken by each foot.

The location of COP under each foot is a direct reflection of the neural control of the ankle muscles (my emphasis added).

Increasing plantarflexion activity moves COP posteriorly (ergo, toward the back of the foot). Increasing inverter activity moves COP laterally (ergo, towards the outside of the foot). COP is often mistakenly equated with COG (Centre of Gravity). COP is calculated with software from pressure data obtained from a force plate or in-shoe pressure insole. (my emphasis added)

Because it is calculated COP can reside in the arch of the foot even though it may not be in contact with the ground.  – my comment

“Centre of Mass (COM) is a point equivalent of the total body mass in the global reference system (GRS). It is the weighted average of the COM of each body segment in 3-dimensional space. It is a passive variable controlled by the balance control system. The vertical projection of COM onto the ground is often called the Centre of Gravity (COG).

“Balance is a generic term describing the dynamics of body posture to prevent falling. It is related to the inertial forces acting on the body and the inertial characteristics of body segments.  The CNS is totally aware of the problems of controlling a multisegment system and interlimb coupling that can facilitate balance control.

“In the literature there is a major misuse of the COP when it is referred to as ‘sway’, thereby inferring that it is the same as the COG. Unfortunately some researchers even refer to the COP directly as the COG.”

In the mechanism of balance control, COP is the equivalent of the Balance Police. It keeps COM from breaching the limits of there base of support by outpacing COM in the race to the limits of the base of support within the foot or feet. In quiet standing, the force of gravity disturbs equilibrium by pulling COM forward. This causes the ankle to dorsiflex. As COM moves forward, it starts to overtake COP. In order to prevent a forward fall, the CNS signals muscles that plantarflex the ankle to increase their contraction. This increases the force of COP and pushes COM rearward. As COP shifts rearward, the CNS reduces the contractive force of plantarflexion so that COP passes COM in the race to the rear of the foot.

A similar process is employed by the CNS to prevent a sideways fall. Here, the force of gravity disturbs equilibrium towards inner or medial aspect of the foot. This causes the foot to pronate. To oppose the disturbing force, the CNS signals muscles to contract that invert the foot.

It is important to recognize that it is the external forces that disturb equilibrium  that cause the foot to pronate.

The same process is at play in skiing. However, since the sideways balance strategy involves inverter muscles, it is only possible to establish a balance platform (DOT 4: PLATFORM) on the outside foot of a turn and only then under specific conditions. In the skier/ski equipment system, COP is the point where the Resultant Force acting on a skier at ski flat that pulls COM downward towards the snow is opposed by muscles that the CNS recruits to oppose the pending collapse of the skeletal system and prevent a fall.

COP is calculated from pressure data obtained from a force plate or in-shoe pressure insole such as  the Novel Pedar system or Tekscan. Since COP reflects neural control of ankle muscles when a foot (the whole foot) is in contact with the ground or a stable source of (ground) reaction force, the use of the term COP is not technically correct in a situation where a ski is on edge unless a platform exists as described in DOT 4: PLATFORM. Until the ski lies flat on the snow between edge changes and there is full foot contact ground reaction force the appropriate term to describe the force applied by the foot to snow through the stack of ski equipment is centre of force or COF.

In a turn, COP is a good COP when it is on the right side of the law: ergo, when COP lies under the head of the 1st metatarsal and R is aligned between the inside edge underfoot and the limits of sidecut. The sketches below show the progression of COF at ski flat that moves COP to the head of the 1st metatarsal. If COP arrives at the head of the 1st metatarsal before the outside ski has attained a significant edge angle and COP remains in this position through the turn COP is a good COP.

Sketch 1 below shows the 2 key mechanical points in skiing (red cross)

Centres of key pts

Sketch 2 below shows the Centre of Force (COF) under the heel of the inside foot at the start of the transition between turns. The red dashed line shows the approximate trajectory of COF as it moves forward and becomes COP at ski (foot) flat between turns as the external forces cause the foot to pronate.

 

COP 1

Sketch 3 below shows the forward progression of COP towards the head of the 1st metatarsal.

COP 2

Sketch 4 below shows the successful transition of COP to the head of the 1st metatarsal where it lies over top of the inside edge of a ski of appropriate width.

COP 3

 

Sketch 5 below shows axis on which COP and R must align in order to engage the external force R to drive edging and turning mechanics.

COP 4

Sketch 6 below shows R on the same axis as COP.  In this configuration the alignment of R described under DOT 4: PLATFORM will enable multiplane torques generated by pronation to be directed into the turn.

COP 5

In sketch 7 below COP has failed to make a transition to the head of the 1st metatarsal. When COP fails to make the transition to the head of the 1st metatarsal at ski flat between edge change before the new outside ski attains a significant edge angle, a moment arm will be setup between the inside edge and COP that will create an inversion moment of force or torque with an associated external vertical axial rotation of the whole leg.

COP 6

 

In sketch 8 below COP has reversed direction. Once an inversion moment arm has been set up on the outside ski there is no way to undo it. The odds are great that COP will revert to its default position under the heel because it is under the mechanical line of the lower limb.

COP 7

When this happens COP becomes a bad COP.


1. You can obtain a copy of David Winter’s paper at the following link:

SHIFFRIN’S SKI MOVE

It is hard to find a movement sequence  in video footage that shows what I call the Ski Move from the optimal angle. In order to clearly see how a skier’s centre of mass rotates about the inside (uphill) edge of the inside ski and changes its position in relation to the inside ski when Shiffrin or Ligety start to step on it, the camera has to be looking at the racer head on. Since this sequence is almost impossible to find, I am going to try and create it in Poser.

The marked up sequence in the photo below shows how Shiffrin’s center of mass starts to rotate about the inside edge of her inside ski and into her new turn as she progressively steps on her new ski during the transition phase. As she does this, a point will be reached where her inside ski flattens on the snow. But it does not stop rotating at this point. Shiffrin’s momentum and the pressure she is applying to the ski cause it to change edges and begin to rotate into the new turn. As this is happening, Shiffrin is extending and moving forward in the hips. This drives the centre of pressure to the ball of her foot. The foot pronates and reinforces the rotation of the ski into the new turn. The circled images show where this happens. As Shiffrin’s foot pronates and the forces she is applying rotate her left ski into the new turn, she applies another layer of in-phase rotation with her hip rotators. These synchronized actions produce an over-centre mechanism that rotates her ski in multiple planes into the new turn.

There is a common perception that racers let their outside ski create all the turning effects. But the pivoting Shiffrin and Ligety apply at this key moment does much more than simply rotate the ski into the turn. It creates powerful forces that engage the external forces to drive the outside ski into the turn while creating a platform to balance on.

 

Shiffrin