base of support

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 FGROUND (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 198o, 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

 

 

 

 

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.

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 MECHANICS OF BALANCE ON THE OUTSIDE SKI: IMPULSE LOADING

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

Impulse Loading

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

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

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

The preceding statements apply equally to skiing.

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

The Mid Stance, Ski Stance Theory

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

Wearing ski boots for a few hours can lead to a weakening of the muscles that operate within the ankle joint. This works as though one joint was excluded from the locomotive function.

………. according to Caplan et al. [3], the muscle groups that determine strength and are responsible for the function of stability in the ankle joint are very sensitive to changes caused by immobilisation. They found that immediately after immobilising the ankle joint for a week, the balance parameters were 50% lower than before the immobilisation.

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

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

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

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

Conclusions/Discussion:

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

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

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

The researchers noted:

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

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

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

The Phases of a Ski Turn Cycle

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

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

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

 

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

 

 

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


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

 

 

THE MECHANICS OF BALANCE ON THE OUTSIDE SKI

In the next series of posts, I am going to focus on the single most important, but least understood, aspect of skiing; skier balance, in particular, the ability to balance perfectly on the outside ski. Given its univerally recognized importance in the ski culture, it is both perplexing and disconcerting that little attention appears to be given to the study and analysis of the mechanics associated with balance on the outside ski.

For decades, the worlds greatest skiers, including Patrick Russell and Marc Giardelli, have stressed the importance of standing on the downhill (outside) ski. Giardelli said that once you can balance perfectly on the outside ski, everything else follows. The ability to stand on the outside ski and balance perfectly on it, implies the same mechanics of balance we engage in when we balance perfectly on one leg when we take a step to move forward in locomotion. Balancing perfectly on one leg requires a stable surface under the entire plantar aspect of the foot to provide a source of GRF. The reason why the ankle-foot complex has a triplanar joint system is so the tripod-like structure of the foot can seek stable ground. This is the classic text book definition of one-footed or monopedal balance and the standard for studies on balance performed on one foot.

The problem is that there is no ground or any form of stable GRF under the outside foot of a turn when the ski is on its inside edge other than the GRF acting along the portion of the edge in contact with the snow surface and a small portion of the base of the ski adjacent the edge. If elite skiers such as Russell and Giardelli really can stand on their outside ski and balance perfectly on it the question is where is the source of GRF coming from that acts to support weight of the body expressed on the plantar foot?

By 1990, I had an explanation in a hypothesis I had articulated. According to my hypothesis, elite skiers extend GRF acting along the portion of the inside edge of their outside ski from the snow to the base of the ski by rotating their outside leg and foot into the turn. This action causes the base of the ski on the outboard side of the inside edge to pivot upward about the portion of inside edge underfoot with sufficient force to support the weight of the body. The Birdcage studies done in 1991 were designed to find out if my hypothesis were right.

Balance on the outside ski is a Two-Step process

Having seen great skiers like Nancy Greene Raine and Toni Sailor ski with ease on pistes that would be difficult, if not impossible, for most skiers to hold an edge on, I was convinced that some skiers really could balance perfectly on their outside ski when it was on its inside edge, the same way that every skier could easily balance on one ski when the base of the ski was fully supported on a firm, stable surface.

I set out to try and figure out how this was possible. It took me about 10 years between 1980 and 1990, to formulate a hypothesis that explained the mechanics. Once I had an explanation, I understood why no one else had been able to figure it out.

Balancing on the outside ski does not adhere to the text book descriptions of single leg balance where a stable source of GRF under the plantar foot is assumed. The ability to stand on the outside ski when on its inside edge and balance perfectly on it, is a Two-Step Process. The key is that the Second Step is dependent on the First Step.  The First Step makes the Second Step possible. Without getting the First Step right within a very short window of opportunity, the Second Step is not possible.

Since my hypothesis predicted that sequence and timing is the critical, it was quite simple to prove my hypothesis with strategically placed strain gauges mounted in the Birdcage on discrete force plates positioned opposite the predicted force transfer points of the foot. The critical nature of the sequence was easily confirmed by preventing the First Step from occurring.

In my next post, I will discuss the Two Steps of the balance process and provide examples using screen shots and video clips from recent World Cup races showing the sequence in a turn where racers such as Mikaela Shiffrin make the two steps to balance on the outside ski.

 

 

THE EFFECT OF LIFT PLATES ON SKI MANEUVERS

In THE EFFECTS OF LIFT PLATES – CONTINUED, I used a simple model to show how the presence of joints with rotational capability below the lower end of the mechanical line at the tibia affect the initial length of the torque arm acting on the outside foot stabilized in a neutral position and the force vector of the mechanical line as the foot rotates about its long axis about a pivot under the inner aspect of the foot. In this post, I am going to set this basic model in the context of the forces acting on a skier in the arc of a turn.

When it comes to discussions of the forces involved in ski maneuvers, most of the force diagrams I have been able to find by others are those that show components of gravity (G) and centrifugal forces (C) with a resultant force (R) acting at the inside edge of the outside ski. If a force diagram is really sophisticated, it might show a ground reaction force (GRF) acting in opposition to the resultant force (R) similar to what is shown in the annotated photo below.

Forces on skier

The inference of such simplistic explanations is that, far from being complicated, the forces involved in skiing are really quite simple.

Gravity (G) and centrifugal force (C) are components of a resultant force (R). The resultant force merely has to be shown aligned in opposition to a ground reaction force (GRF) at the inside edge of the outside ski in order to satisfy an explanation of the mechanics of  edge hold. Forces applied by the foot? No need to complicate things. Keep it simple. Ignore them. If other forces are ignored they aren’t important.

Riser plates? Torques? Ignore them too. It would be nice if things were that simple. But when things like centre of pressure (CoP) and the torsional effects of lift plates are added to the discussion, it quickly begins to become obvious that the forces in skiing are anything but simple. Significant forces other than gravity and centrifugal force are present. And they do affect the skier.

In THE EFFECTS OF LIFT PLATES – CONTINUED, I used a simple model to show how the presence of joints with rotational capability below the lower end of the mechanical line at the tibia affect the initial length of the torque arm acting on the outside foot stabilized in a neutral position and the force vector of the mechanical line as the foot rotates about its long axis about a pivot under the inner aspect of the foot. In this post, I am going to set this basic model in the context of the forces acting on a skier in the arc of a turn as shown in the above sketch.

The sketch below shows the forces applied to the outside foot and ski of a turn by the foot through the mechanical line in conjunction with a resultant force acting at the inside edge before a load is applied. This situation would exist if a skier were to get caught inside and lose contact of the outside ski with the snow. The tendency of a limb that unloads from a force applied to it is to release muscle tension and unwind into a supinated position. In the situation described in the sketches the foot has been stabilized in a neutral configuration with arch supports or custom insoles and/or a form fitted liner or possibly a form fitted shell. When the foot is in a neutral position the force applied to the foot will act on the proximate centre of a line that runs through the ball of the 2nd toe and the heel.

1

The sketch below shows force applied to the outside foot causing the rotation described in THE EFFECTS OF LIFT PLATES – CONTINUED. There is way more going on than shown in the sketch. But I will get to the other issues in future posts.

2

The sketch below shows and overlay of the first and second sketches to show the changes. As rotation occurs in the subtalar joint the force vector of the mechanical line shifts towards the outside of the turn. As it does the transverse angle of the ski base flattens. These changes will tend to cause the ski to slip out of the turn forcing the skier to increase the angle of the resultant force R by increasing the angle of inclination.

3

Increasing the angle of inclination makes a bad situation worse because the forces become more aligned with the slope of the hill thus increasing the magnitude of the forces that tend to make the ski slip out of the turn.

20 degreesThe NY Times video – Ligety on GS commented, “The trace of his (Ligety’s) path is smoother than that of his foes, who ski in somewhat violent fits and starts, making adjustments that spray snow.” What are Ligety’s foes doing that is different from what Ligety is doing? The forces on Ligety’s outside ski are consistently rotating into the turn. The forces on the outside ski of his foes are rotating out of the turn. But changes in the consistency of the snow texture and the forces acting on the outside ski cause changes in the edge angle. The animated video clip below show this effect.

Changes in edge angle cause the outside ski to oscillate into and out of the turn. Ligety’s foes, which includes the majority of World Cup racers, are unable to develop a dynamically tensioned base of support on their outside foot and ski because the forces they are applying are on the wrong side on the inside edge and they are unable to apply a countering torque with internal rotation of the outside leg from the pelvis. The result is the outside ski is unstable. It makes small oscillations in response to perturbations in ground reaction force necessitating a corresponding series of small adjustments by the racer. Edge angle oscillation can also cause the ski to suddenly hook into turn without warning causing a fall. In a future post I will include some video clips showing edge angle oscillation.