pronation

THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: BALANCE PLATFORM MECHANICS

Turntable rotation generated by the powerful internal rotators of the pelvis (the gluteus medius and minimus) in combination with second rocker mechanics can create a platform under the body of the outside ski and foot that a skier can stand and balance on using the same processes to balance on solid ground. The associated mechanics creates a platform under the body of the outside ski by extending  ground reaction force acting along the portion of the inside edge in contact with the snow, out under the body of the ski.

In order to understand the mechanics, we need to start with a profile through the section of the body of the ski, binding and boot sole under the ball of the foot. The graphic below is a schematic representation of a ski with a 70 mm waist and 100 mm shovel and tail with an arbitrary length of 165 mm. The total stack or stand height from the base of the ski to the surface of the boot that supports the foot is 80 mm. The uppermost portion of the schematic shows the shell sidewalls of a 335 boot in relation to the 70 mm width of the stack. A ski with a 70 mm waist will place the center ball of the foot of skiers with US Men’s 10 to 12 feet close to over the inside edge. The heavy black line at the bottom of the stack shows the projection of the sidecut width beyond the waist.The schematic serves as a base on which to overlay a free body diagram showing the forces acting across the interface of the inside edge with the snow. This is where the rubber meets the road.

There are two possible scenarios in terms of the axis on which the center of pressure W of the skier will act. Unless the foot can sufficiently pronate and especially generate impulse second rocker loading, W will lie on the proximate anatomic center of the foot and transverse center of the body of the ski as shown in the graphic below. In this location, W will create a moment arm due to the offset with the GRF Pivot under the inside edge at the waist. The resulting moment of force will externally rotate the ski and foot under load out of the turn while simultaneously rotating the leg externally.The graphic below shows the second scenario where the center of pressure W lies directly over the GRF Pivot under the inside edge. In this position, W will load the inside edge under the ball of the foot and assist edge grip. But in this configuration, rotating the ski onto its inside edge necessitates overcoming the moment of force created by the moment arm resulting from the offset between the GRF Pivot and GRF acting at the limits of the sidecut. This requires a source of torque that acts to rotate the ski into the turn about the pivot acting at the inside edge at the waist of the ski.An obvious source of torque is to use the leg to apply force to the inner aspect of the shaft of the foot; aka knee angulation. But this will not create a platform under the body of the outside ski. Applying a load to the vertical wall of the shell opposite the ball of the foot will apply torque load to center at the GRF pivot as shown in the graphic below. The moment arm is formed by the point at which the Turntable Torque is applied to the boot sidewall (green arrow) to the center of rotation at the GRF Pivot.

 

The torque applied to the vertical sidewall of the boot shell is the Effort. The sidecut of the ski is the resistance. What effect will this have on the body of the ski under the foot? There is a lot more to this subject that I will begin to expand on in my next post.

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

 

 

 

 

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.

OVERPRONATION? GET OVER IT

Over the past few decades pronation has been aggressively vilified as evil and dangerous. Any position in support of  pronation draws accusations of a wilful endorsement of over-pronation inferring that pronation is the product of an error of evolution. The reality is that without the ability of the foot to pronate (hands also pronate), humans would be rendered immobile.

In his book, Biomechanics of Sports Shoes (NiggShoeBook@kin.ucalgary.ca), Benno M. Nigg states:
“Pronation and supination have long been the “danger variables” hanging over the sport shoe community, but their time as the most important aspects of sports shoes is over. Pronation is a natural movement of the foot and “excessive pronation” is a very rare phenomenon (my emphasis added). Shoe developers, shoe stores and medical centres should not be too concerned about “pronation” and “overpronation”. pp 122-123

 

 

 

COUPLED FORCE WHOLE LEG ROTATION

The most important event in a turn is whole leg internal rotation (Event 7) following ski flat (Event 3). But the mechanism by which whole leg internal rotation is applied to the ski is as important, if not more important, than the actual whole leg rotation.

As the outside ski changes to its new inside edge, the racer rotates the whole leg internally using top down rotation from the pelvis. The purpose of ski flat at the conclusion of the transition (Event 1) phase, is to neutralize torsion across the pelvis so it is square to the trajectory of the racer. In order to use whole leg internal rotation, the COM of the racer must be positioned on the new outside foot at ski flat in what I call monopedal stance. Monopedal stance (aka monopedal function) is a physiologic state wherein balance is achieved with the weight of the body borne on the medial plantar aspect of a fully pronated foot.

The graphic below is Figure 23 from my US Patent No. 5,265,350.

Bi-MonopedalFigure 23 A depicts bipedal stance. The points of the central load-bearing axis are stacked vertically over top of each other in the frontal plane. The load W from COM is centred between the feet with each foot carrying half the load (W2).

Figure 23 A depicts monopedal stance. In monopedal stance, the load W from COM is aligned over the proximate centre of the head of the first metatarsal in both the frontal plane (across the racer) and saggital plane (front to back). Monopedal stance at ski flat is eloquently demonstrated by Bridget Currier  in the Burke Mountain Academy YouTube video, Get Over It with commentary by Mikaela Shiffrin – (http://youtu.be/Bh7KF49GzOc).

The opening graphics advise the racer to Get Over It and Stay Over It, meaning maintain the alignment of W from COM, over the proximate centre of the head of the first metatarsal in the frontal and saggital planes throughout the entire turn. But few racers can Get Over It, let alone Stay Over It, because the structures of their ski boots prevent them from assuming monopedal stance. This is especially true of racers whose boots are closely formed to the shape of their foot and leg in what amounts to perfect envelopment.

The graphic below is a re-creation of the stick person in Figure 23 above. The notations have been revised to reflect the terminology used in blog posts. The left stick person is depicted in bipedal stance. The centre stick person is depicted in monopedal stance. The right stick person is depicted in fixed neutral stance. When the foot is fixed in neutral, pronation is not possible and the foot is prevented from everting (the sole turns outward). In order for W emanating from COM to be positioned over the proximate centre of the head of the first metatarsal, the foot must evert approximately 7 to 8 degrees as depicted in the centre stick person.

The graphic below shows the effect of fixing the foot in neutral. When a racer attempts to balance on the new outside limb at ski flat, the inability to align W with GRF at the inside edge of the outside ski will cause the racer to fall into the new turn or consciously move away from the outside ski. .

Falls into turnPreventing the foot from pronating within a ski boot causes other problems. When the leg is rotated internally relative to the foot by the hip rotators, a torsional load is applied to the foot. Conventional ski boots do not provide surfaces for the foot to transfer biomechanically generated forces such as torque to. In addition, the structures of conventional ski boots present sources of resistance which interfere with the movements necessary to establish force transfer connections of discrete aspects of the foot with the boot shell.

Figures 22 A through 22 D below are from US350. Figures 22 A and 22 B depict the architecture of a foot in bipedal stance. Figures 22 C and 22 D depict the architecture of a foot in monopedal stance. Changes in the length of the foot in bipedal and monopedal stances are annotated as  L1 (bipedal) and L2 (monopedal). Changes in the angle of dorsiflexion of the ankle joint in bipedal and monopedal stances are annotated as  A1 (bipedal) and A2 (monopedal). Changes in the height of the arch in bipedal and monopedal stances are annotated as H1 (bipedal) and H2 (monopedal). Internal rotation of the leg in monopedal stance is annotated at 6. Changes in the length of the foot in bipedal and monopedal stances are annotated as  L1 (bipedal) and L2 (monopedal).  Changes in the position of the head of the first metatarsal in bipedal and monopedal stances are annotated as  2. Changes in the position of the medial tarsal bone in bipedal and monopedal stances are annotated as  3. Changes in the width across the heads of the metatarsals in bipedal and monopedal stances are annotated as  4. Shear forces, which will be the subject of a future post, are shown in Figure 22 D.

Screen Shot 2015-01-08 at 2.12.34 PMIn order to apply top down internal rotation, the racer has to move the load W to the ball of the foot as shown in the graphic below.

IdealThe short video clip below shows how the foot must be able to pronate within the confines of the ski boot without interference in order to set up the force couple required to transfer whole leg internal rotation to the new outside ski. The typical most significant source of interference is the structures of the ski boot in front of the ankle joint on the inner aspect of the boot.

 

The red bars in the BIPEDAL foot define common sources of interference created by structures of the ski boot that prevent the foot from pronating and establishing force transfer connections with the shell as shown in the MONOPEDAL foot. While the connection of the two transfer points suggests that the centre of rotation lies within the confines of the foot its true centre is not intuitive. This will be the subject of the next post.

BIO-MEDICAL PERSPECTIVE

Normal medial STJ movement of the talus is followed by a mandatory normal 1:1 coupling of the tibia to encourage normal internal leg rotation and normal dorsiflexion of the ankle. This normal coupling mechanism produces a synergistic postural response enhancing internal rotation of the entire leg.  Pelvic counter ensures hip capsule tightening which stabilizes the hip joint during the turn.

Screen Shot 2015-01-28 at 1.09.11 PM

Dr.Kim Hewson is an Orthopaedic Surgeon and former Director of Orthopaedic Sports Medicine  at the University of Arizona.  He is currently a veteran Telluride Ski School Alpine Instructor and Staff Trainer in the Biomechanics of Alpine Skiing.

 

SKI LEVERS

In a simplistic schematic form, the transverse aspect of a decambered ski acts in the capacity of a dual-pivot, offset lever. The model I will use to illustrate this assumes that the load W is transferred by the central load-bearing axis (LOAD TRANSFER) to an axis lying on the transverse center of the portion of the ski underfoot. It also assumes that foot is in neutral and that this has extended the central load-bearing axis from the lower aspect of the tibia to the base of the heel and from there to the plane of the base of the ski.

From a practical perspective, it is exceedingly difficult to immobilize the joints of the foot, let alone immobilize the subtalar joint in a neutral configuration with any degree of precision. But since the predominant view of the experts in skiing is that the foot functions best when it’s joints are immobilized in neutral, my schematic model  assumes this is actually possible in the name of simplicity. Since a vertical alignment of the heel bone with the supporting surface is often cited as an objective in the fabrication of insoles, boot fitting and lower limb alignment, it is reasonable to assume that neutral means subtalar joint or STJ neutral.

The graphic below depicts the ski as a dual-pivot, offset lever. For the sake of simplicity, the effective length of the lever offset create by the width of the sidecut at the tail and fore body of the ski are represented as being equal.

Screen Shot 2014-11-18 at 11.10.41 AM(Click on graphics to enlarge the image)

The graphic below shows a section through the ski where W would be transferred to when the foot is immobilized in neutral.

Ski Lever SectionIn the above schematic, W is the load transferred to the ski by the COM of the skier. C is the central axis of the ski. P is the pivot created by the edge of the ski that is engaged with the snow.  R is the resistance of the snow acting on the portion of the base of the ski within the sidecut area. In the configuration shown in the above graphic, the length of the lever is measured from the point of the transfer of W to the pivot P created by the active inside edge (aka – Ground Zero).

The graphic below depicts the 3 classes of levers. The First Class Lever is the classic teeter-totter mechanism, one that most children experience at an early age.

Lever ClassesOf the 3 classes of levers, only First and Second Class levers have practical application to the ski as a dual pivot, offset lever.

The center pivot format of the First Class Lever represents the configuration that exists when W is transferred to the center of transverse center axis of the ski as it will be when the foot cannot pronate at ski flat between edge changes.

Ski Lever leftThe first problem that arises is that R is in phase with W. In this configuration, both forces are cooperating to rotate (invert) the ski towards the outside of the turn. Without an opposing force or effort, W and R will invert the ski and foot as a unit as shown in the graphic below. This will occur even in the absence of resistance R that is created by the sidecut of the ski.

Inversion lever 2

In addition to the preceding, no vertical force is present acting in opposition to the GRF at P. So the edge will not grip. In the mechanics of sidecut, the inside edge of the outside ski is Ground Zero. Opposing forces W and GRF must be aligned at 90 degrees to the transverse plane of the base of the ski in order to enable the sidecut to cut into the snow as the ski rotates into the snow about P. This alignment must be maintained at all times during the turning phase.

The central load-bearing axis (DOT 9: LOAD TRANSFER) is a straight line between head of the femur and the lower aspect of the tibia. A neutral configuration of the subtalar joint extends the lower aspect of the central load-bearing axis to the base of the heel. An unbalanced moment of force results when W is offset with P on the inner (inside turn) aspect of the outside foot.  The top down load transfer of  W through the central load-bearing axis induces a state of inversion stress as unit inversion of the element underfoot and the foot are translated through subtalar joint coupling to external vertical axial rotation of the leg as a whole.

The graphic below shows the 3 degrees of freedom of the foot/ankle complex of the foot. The line joining the arcs of Inversion-Eversion and Lateral-Medial Axial Rotation indicates coupling through the subtalar joint which acts in the capacity of a torque converter.

3 degrees of freedom r1Inversion Stress

A state of inversion stress exists when the centre of the load W is transferred to the centre axis of outside ski when it is on its inside edge and W is offset laterally from the P. This creates a moment arm with the inside edge acting in the capacity of a pivot P. The load W applied by the foot to the moment arm causes the ski to invert. Inversion occurs either as a unified movement of the ski equipment stack/foot-leg system or as a combination of ski equipment stack inversion in conjunction with a degree of subtalar joint inversion of the foot and leg within the confines of the ski boot. In either case, inversion of the foot involves translation through the subtalar joint of an inversion moment of force into a lateral axial moment of force of the outside leg. This is accompanied by a degree of hip adduction of the leg against the closed kinetic chain created by the edged ski. This combination creates medial compression of the knee joint in combination with a degree of lateral axial rotation of the tibia against a well stabilized femur.

A state of inversion stress results from the inability of the central load-bearing axis to complete the transfer of load W through the load transfer elements of the foot to a source of contiguous GRF.

In the First Class ski lever format that exists when W is on the outside turn aspect of the moment arm created by an offset with P, it is virtually impossible for pronation of the outside foot of a turn to be induced by the load transfer of W to the foot, let alone for pronation to be prevented or precisely controlled as is implied by some authorities in skiing. To suggest that it can, is absurd.

In order for the inside edge to grip and serve as a pivot for the sidecut to rotate about and cut into the snow surface, the transfer of the load W must be completed and an opposing force or effort provided by the skier to restrain inversion of the outside ski and foot.