World Cup ski technique

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE – PART 12

At this point my discussion of the mechanics and biomechanics of platform angle is at what I can appropriately call the moment of truth. Moment in the context of the mechanics and biomechanics of platform angle means moment of force or torque; platform angle involves the ability of the CNS of a skier to control torques across the inside edge of the outside ski so the skier can stand and balance on the platform.

What is Balance?

That balance is the single most important factor in human movement, especially movement associated with athletic performance, is undisputed. In complex activities like skiing that involve movement in 3 dimensional space in a dynamic physical environment, optimal balance is critical. But what constitutes balance? In order to know if a skier is has optimal balance or is even in balance one has to know what balance is and what factors enable or compromise balance (i.e. postural) responses and  especially the factors that enable optimal balance.

The Balance Zone

A skier is in balance when the CNS is able to maintain the position of a skiers’ COG within the limits of a narrow band close to the inside edge of the outside ski during the load phase of a turn. The load phase of a turn occurs in the bottom of a turn when the force exerted on the platform by the COM of a skier must be balanced against the external resultant force of gravity and centrifugal force. In the load phase, the CNS must maintain COG within the forward limit of the Balance Zone within close proximity to the ball of the foot. When balance is challenged COG must not exceed the rearmost limit of the Balance Zone that lies just in front of the ankle joint. The Balance Zone and its limits are shown in the graphic below. If COG exceeds the limits of the Balance Zone shown in pink, the skier will lose their state of balance and with it dynamic control of the platform underfoot.  They will also suffer a lose of dynamic stability in the joint system of the lower limb.

The Balance Plane

In the ski system platform the plantar plane under the plantar aspect (sole) of the foot is the interface of CNS mediated balance activity. When the coordinated, concurrent forces are applied at the main force transfer point of the foot that I call the Center of Control, shown in the preceding graphic, the applied forces will manifest in more than one plane as shown in the graphic below.Force Fa applied under the head of the first metatarsal will be distributed over an area around its center.  When the force applied in the plantar plane is transferred through the structure of the platform to the base plane the center of force will maintain its position. But when the force area of distribution will increase as shown in the pink zones under the head of the first metataral and the base plane. In free rotation of the ski, resistance from the force of friction Ff will be minimal as will any force applied in the torque arm plane by the eccentric torque arm. Rotational force will be largely confined to the base plane.

The Missing Force Factor: Sidecut

In the free rotation, the effect of the sidecut of a ski is not a significant factor in terms of a source of resistance. But as the transverse aspect of the base plane of the ski acquires an angular relation with surface of the snow the resistance created by GRF acting at the  limit of sidecut at the shovel sets up an interaction between the rotational force applied to the inner wall of the boot shell adjacent the medial aspect of the head of the first metatarsal with the resistance created by GRF at the limit of sidecut at the shovel. In the graphic below I have connected the  2 dots of the platform ground effect problem with a line drawn between the two points.The graphic below shows a schematic of the mechanical aspects of the opposing moment or torque arms between the two dots that I connected in the preceding graphic. The inside edge below the head of the first metatarsal acts as a pivot in conjunction with the Center of Force applied 90 degrees to the transverse aspect of the base plane for the plaform to rotate about as the ski goes on edge.

As the base plane of a ski acquires an angular relationship with the snow the torque arm rotating the ski goes into what cane best be described as turbo torque boost. Whole leg rotational force continues to rotate the whole ski but the eccentric torque arm engages and applies a high torsional load that winds the body of the platform about the shovel. This mechanism has to be considered in the perspective of the of the inertia from the movement of the skier driving the cutting action of the shovel.  The graphic below shows the opposing how opposing torsional forces at the limit of sidecut and applied by the application of for by eccentic torque arm to the vertical shell wall by the medial aspect of the head of the first metarasal act to apply a upward force that extends to the outboard end of the plantar plane of the platform.  This is the mechanism that enables elite skiers to balance on their outside ski and initiate precise movement from from a dynamically stable platform.I first solved basic mechanics and biomechanics of the outside ski balance problem 30 years ago. The degree of difficulty was not great. Solving the problem took diligence and persistence in researching all the relevant aspects and identifying all significant forces and associated planes.

I’ll let the readers ponder the informaton in this for a while after which I will be happy to respond to questions and comments.

COMMENTS ON SUPER PETRA VLHOVA

With the Mens’ and Ladies’ GS and SL World Championship events starting tomorrow I am republishing my March 28, 2017 post on Petra’s performance in the Aspen Slalom where I said,

This was not the same Vlhova I had analyzed earlier in the season. Vlhova has definitely changed and it is for the better.

The degree of neurobiomechanical function permitted within the structures of a ski boot, more than any other factor, can literally determine who stands on the podium. The GS and SL World Championship events will show which racer has the functional edge.


COMMENTS ON SUPER PETRA VLHOVA

As time permits, I analyze the movement and loading patterns of elite skiers such as Mikaela Shiffrin, Lindsey Vonn, Ted Ligety, Tessa Worley and others. Occasionally, a source sends me video of these racers training.

I have identified a specific movement and loading sequence pattern that I use to analyze technique. This requires decent quality video and specific camera angles. In a future post, I will describe the process, the key metrics I look for and what they indicate.

Up until I saw the video of Vlhova, that is the subject of my post, SUPER PETRA VLHOVA’S EXPLOSIVE IMPULSE LOADING IN ASPEN SLALOM, I rated her as one of the better technical racers on the World Cup circuit. But I did not consider Vlhova to be in the same class as a Shiffrin or a Worley.

When someone posted a link on FaceBook to Vlhova’s winning run in the Aspen slalom, I was stunned by what I saw in the first few gates. This was not the same Vlhova I had analyzed earlier in the season. Vlhova has definitely changed and it is for the better.

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE: PART 10 – SUPPLEMENTAL INFORMATION


Because of the complex issues I am about to start discussing in the next series of posts I am providing supplemental reference information to assist the reader in understanding the issues associated with platform angle mechanics and biomechanics and underlying process of dynamic stability.

Background of events leading up to the outside ski platform ground balance solution

In late 1989, after gaining valuable insights from the medical textbook, The Shoe In Sport, I had formulated a hypothetical model that explained the macro details of the mechanics and biomechanics of platform angle and the mechanism of user CNS postural balance control.

Insights from The Shoe in Sport:

Correct positioning of the foot is more important than forced constraint and “squeezing” the foot.

Forward sliding of the foot should not be possible. 

From a technical (skiing) point of view, the ski boot must represent an interface between the human body and the ski. This implies first of all an exchange of steering function, i.e., the skier must be able to steer as well as possible, but must also have a direct (neural) feedback from the ski and from the ground (snow). 

The comment about the importance of correct positioning of the foot and the ski boot  representing an interface between the human body and the ski gave me insights that led to the discovery of key mechanical of the foot whose position in relation to the inside edge and X-Y axes of the ski affects the transfer and control of steering and platform forces to the ski and control.

When I wrote the application for US Patent No 5,265,350 in late 1991 and early 1992 I described the mechanics and biomechanics of plantar angle in great detail knowing this information would be freely available to the entire world to use once the patent was published. The only exception was the information covered by claims. Known mechanics and biomechanics are not in themselves patentable.

Patents and Research

It is important to note that patents, even when granted, do not apply to the use of a patented device for the purpose of pure research. Knowing this at the time I wrote the patent, I described the Birdcage research vehicle in sufficient detail with many figures to enable the device to be constructed at minimal cost so research could be conducted by others as soon as possible for the purpose of advancing the knowledge base and science of alpine skiing.

The following unedited text is excerpted from the patent.

……. the teaching of this (patent) application is that force must be applied and maintained only to specific areas of the foot and leg of the user while allowing for unrestricted movement of other areas.

The performance of such mediums (skate blades and skis) is largely dependent on the ability of the user to accurately and consistently apply forces to them as required to produce the desired effect.

In addition, in situations where the user must interact with external forces, for example gravity, the footwear must restrain movements of the user’s foot and leg in a manner which maintains the biomechanical references with the medium with which it is interacting.

Precise coupling of the foot to the footwear is possible because the foot, in weight bearing states, but especially in monopedal function, becomes structurally competent to exert forces in the horizontal plane relative relative to the sole of the footwear at the points of a triangle formed by the posterior aspect and oblique posterior angles of the heel, the head of the first metatarsal and the head of the fifth metatarsal. In terms of transferring horizontal torsional and vertical forces relative to the sole of the footwear, these points of the triangle become the principal points of contact with the bearing surfaces of the footwear. 

The most important source of rotational power with which to apply torque to the footwear is the adductor/rotator muscle groups of the hip joint. In order to optimally link this capability to the footwear, there must be a mechanically stable and competent connection originating at the plantar processes of the foot and extending to the hip joint. Further, the balanced position of the skier’s centre of mass, relative to the ski edge, must be maintained during the application of both turning and edging forces applied to the ski. Monopedal function accommodates both these processes. 

Yet a further problem relates to the efficient transfer of torque from the lower leg and foot to the footwear. When the leg is rotated inwardly relative to the foot by muscular effort a torsional load is applied to the foot. Present footwear does not adequately provide support or surfaces on and against which the wearer can transfer biomechanically generated forces such as torque to the footwear. Alternatively, the footwear presents sources of resistance which interfere with the movements necessary to initiate such transfer. It is desirable to provide for appropriate movement and such sources of resistance in order to increase the efficiency of this torque transfer and, in so doing, enhance the turning response of the ski.

In skiing, the mechanics of monopedal function provide a down force acting predominantly through the ball of the foot (which is normally almost centred directly over the ski edge). In concert with transverse torque (pronation) arising from weight bearing on the medial aspect of the foot which torque is stabilized by the obligatory internal rotation of the tibia, the combination of these forces results in control of the edge angle of the ski purely as a result of achieving a position of monopedal stance on the outside foot of the turn. 

The edge angle can be either increased or decreased in monopedal function by increasing or decreasing the pressure made to bear on the medial aspect of the foot through the main contact points at the heel and ball of the foot via the mechanism of pronation. As medial pressure increases, horizontal torque (relative to the ski) increases through an obligatory increase in the intensity of internal rotation of the tibia. Thus, increasing medial pressure on the plantar aspect of the foot tends to render the edge-set more stable.

There are many figures that illustrate the concepts expressed in the above text which I will include in future posts.

The photo below shows the strain gauges (black disks) fit to the 1991 research vehicle. These gauges recorded first metatarsal forces under and to its inner or medial aspect and the outer and rearmost aspects of the heel bone.

I’ve learned a lot since the above information was made public after the patent was issued on November 30, 1993.

In Part 10, I will discuss the mechanism by which forces applied by the ball of the foot to what I call the Control Center of the platform provide quasi ground under the outside foot and leg in the load phase of a turn for a skier to stand and balance on.

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE: PART 9

In my previous post I the described a mechanism by which whole leg rotational or steering force can be applied to a vertical extension of the platform by inner (medial) aspect of the head of the first metatarsal. The associated user biomechanics have a number of requirements the structures of a ski boot must meet in order to apply this force. These include, but are not limited to the following:

  • in the load phase in what is called ‘the bottom of the turn’ the foot must be able to rapidly pronate with minimal interference from the structures of the ski boot.
  • force applied to the vertical extension of the platform must be localized on (medial) aspect of the head of the first metatarsal and not from other structures of the foot, including the inner (i.e. medial) aspect of the big toe and the medial boney structures of the ankle and midfoot.
  • the big toe must be able to be aligned straight ahead on the anatomical center of the long axis of the foot without significant interference from the structures of the ski boot including structures of the liner.

In addition to the above, there must also be minimal interference with the ability of the Achilles tendon to transfer high loads to the head of the first metatarsal (i.e. ball of the foot) to the platform underneath as the 90 degree component of edge cutting force. The magnitude of force, especially peak impulse force, that a skier can apply to the head of the first metatarsal has a direct effect on the degree of force that can be applied to the medial aspect the head of the first metatarsal.

Data from the 1998 University of Ottawa study of pressures under the feet of elite skiers (1.) found that maximal forces ranged from a low of 522 N to a high of 1454 N; a difference of 279%. The data also found significant differences in the maximal forces recorded between the left and right feet of all elite skier test subjects for all turn types except dynamic parallel.

Table 1 below from the shows the forces generated from the pressure data acquired in University of Ottawa study.

The large differences seen between a range of elite skiers and especially between left and right feet of the same skier has significant implications for the ability to apply force to a vertical structure with the head of the first metatarsal, a force not considered in the University of Ottawa pressure study or any study I am aware of.

To the best of my knowledge my 1992 skier force study that used a research vehicle called The Birdcage is the only study even today that examined force applied by the medial aspect of the head of the first metatarsal to a vertical structure of the platform of a ski boot/ski. The Birdcage studies also examined the interaction and effect of vertical plantar forces applied to the platform in conjunction with horizontal force applied to a vertical extension of the platform.

Center of Force

Sometimes call Center of Pressure in gait/balance studies, Center of Force (COF) or Center of Pressure (COP) do not represent a point application of a force vector. COF and COP are point centers of force applied to an area of a surface or body. (2.)(3.)(4.)

In platform mechanics, the sole of the foot applies force to a large area of the platform. The closest point to the inside edge of the outside ski where the Center of Force can act is under the head of the first metatarsal. Force applied to the platform of the ski will always apply a force to the running surface of the inside edge. Even if CoF is aligned over one aspect of the GRF acting on the inside edge of the outside ski it is impossible for COF of the outside foot to be aligned over the entire sidecut arc of the inside edge in contact with the snow. Since the foot cannot access GRF (i.e. ground) under the entire length of the inside edge of the outside ski, ground needs to be brought out under the platform.

In order to successfully solve a problem all aspects of a problem must be identified and their implications understood. The solution to the platform/ground problem is finding a way to extend the ground under the entire running surface of the inside edge of the ski out under the platform. In my next post I will begin to explain how this is tied to the ability to apply robust force with the head of the first metatarsal to a vertical extension of the inner aspect of the platform.


  1. 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.
  2. WHAT THE TWO HIGH PRESSURE COPS IN THE UNIVERSITY OF OTTAWA STUDIES MEAN – https://wp.me/p3vZhu-1fV
  3. IMPLICATIONS OF THE UNIVERSITY OF OTTAWA PRESSURE STUDIES – https://wp.me/p3vZhu-1e2
  4. AN INDEPENDENT STUDY IN SUPPORT OF THE UNIVERSITY OF OTTAWA FINDINGS – https://wp.me/p3vZhu-1gR

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE: PART 7

On January 12 of this year I started a new direction for The Skier’s Manifesto with a critical examination of the mechanics of platform angle after concluding that this issue and its effect on dynamic stability is the single most important factor in skiing. The platform is the portion of the stack of equipment between the sole of a skiers foot and the base of a ski. I started my discussion with a review of some of the typical technical terms associated with platform angle mechanics.

In my last post, I examined rotational force applied to a ski. I noted that in the technical terminology of skiing this is referred to as steering. I identified a number of inconsistencies, ommissions and errors pertaining to steering that I will expand on in this post.

Platform Paradoxes

Technical discussions on steering typically show a ski rotating like a propeller about the center of its long axis. In my last post I demonstrated that the source of the rotational force or steering is the femur rotating in its joint with the pelvis and applying rotational force to the foot its lower (distal) end at the tibia.

The graphic below shows the axes of rotational force (steering) applied to a ski through the foot/ski boot interface by the leg. I’ve used a large ski boot and a short ski to illustrate the effect of the location of the axis of rotation.

Technical discussions of steering don’t always mention the source of steering force let alone show its location. In addition, no explanation is offered that would explain how a ski can rotate about its center like a propeller.

The graphic below shows a ski with the running center of the long axis with approximate location of the axis of rotation indicated. In this example the axis of rotation is approximately 11.5 cm behind the running center (C). On my own skis, the axis of rotation is approximately 13.5 cm behind the running center for my 335 mm ski boot.

When the ball of the foot is located on or close to the transverse center of the long axis of the running surface of a ski the axis of rotation will move progressively towards the shovel as a foot gets shorter and move progressively towards the tail as a foot gets longer. No one seems to mention this even though it raises a number of signficant issues, among them the effect on the edge hold and carving characteristics associated with platform dynamics.

Where is the Force Applied?

Technical discussions of platform mechanics typically don’t show or even mention the location of the force applied to a ski by the weight of a skier. Since the weight of the body is transferred to the foot from the lower end of the tibia the weight tends to be transferred to the foot close to the heel.

Some discussions of platform and steering mechanics even suggest that a skier should feel their weight under their heel when steering the skis. This would place the applied force on the transverse center of a ski, behind the center of the long axis and offset from the inside edge where it will create a torque or moment arm that will degrade platform mechanics.An analogy of the mechanics of rotational force applied to a ski by rotation of the leg is a vertical shaft (leg) rotated by a force with an arm (ski) projecting outward from the shaft.

As the arm gets longer the distance the end of the arm travels for every degree of rotation of the shaft will increase.

  1. How will increasing the length of the arm effect the application of force applied to an object by the end of the arm distant from the shaft given a rotational force (torque) of a fixed magnitude applied to the shaft?
  2. How would reducing the effective length of the arm acting on a ski affect platform mechanics, in particular edge hold and carving characteristics?

There is a way to reduce the effective length of the arm acting on the ski. Elite skiers can do it. This will be the subject of my next post.

THE MECHANICS OF PLATFORM ANGLE: PART 3

For the sake of simplicity I have started the discussion of the mechanics of platform angle with opposing static forces acting across the platform edge/snow surface (i.e. ground) interface. The use of static forces and drills to illustrate platform mechanics is not realistic because skiing involves the acceleration and deceleration of a body (i.e. mass). A realistic discussion must consider all significant external and internal forces and the effects of momentum and inertia. A key component of any discussion of this nature is the orientation of the platform or transverse base angle of the outside ski in relation to the vector of opposing applied and reaction forces and the angle of the vector with the plane of the surface of the snow. The mechanism of control of the platform angle must also be considered.

The objective of the initial posts on the mechanics of platform angle is to create a set basic principles to serve as a frame of reference for multi disciplinary dialogs on the mechanics, neurobiomechanics and physics of skiing.

In my last post I discussed how the shear or slip component of an applied angular force acting on a surface or body will increase in magnitude as the angle of attack decreases and becomes more aligned with the plane of the surface while the normal component of the applied and force will decrease in magnitude.  As this happens the tendency of the force applied to the snow that would cause it to penetrate into the surface and cut a step will decrease. As the platform angle with the snow becomes increasingly more perpendicular and the vector of the applied force becomes more aligned with the plane of the surface of the snow the component of shear force will increase and the ski will slip regardless of a perpendicular orientation of the platform with the applied force R.

 Platform Forces: A different perspective

The force diagram below shows how the angle of the point of application of force applied to the inside edge of the platform that would cause it to cut a step into the surface of the snow becomes progressively less aggressive as the vector of the opposing forces becomes more aligned with the plane of the surface of the snow.The graphic below shows another way looking at angular forces acting on a surface. This graphic only shows the components of the applied and reaction forces. The advantage of showing the components is the magnitude of the normal and shear or slipping forces can be shown in relation to each other. I’ve taken some liberties in showing the normal GRF force as having greater potential magnitude than any force applied by the platform of the ski.

As the angle of the platform with the surface of the snow increases (becomes closer to perpendicular) the magnitude of the normal force will decrease. As it does the magnitude of the shear (slipping) force will increase in lock step. As the magnitude of the shear (slipping) force  increases, the potential magnitude of the GRF shear component will decrease and the platform will tend to slip and not cut a step into the surface of the snow.


Since we know that elite skiers and racers can carve a step or ledge into the surface of very hard pistes at high platform angles it is reasonable to assume that some unidentified force or forces are at work that are altering the angle of attack of the applied force R so that it is more aggressive in terms of carving a step into the surface of the snow as shown in the graphic below. What would the components of the applied and reaction forces look like?As always, comments, suggestions and objective criticisms are welcome. In Part 4 we will look for the elusive forces that make skis carve at high platform angles.

 

WHAT DOES HIRSCHER HAVE IN COMMON WITH BRIGNONE, WORLEY AND SHIFFRIN?

The short answer to this question is that the 4 racers share a stance with the muscles of the biokinetic chain in isometric contraction during what I term the Load Phase of a turn sequence and the ability to use the elastic recoil energy created during the Load Phase for acceleration.

One of the key visual cues of an isometric stance is an extended outside leg with small angles at the knee and ankle and a forward position of the pelvis. Another key visual cue is high hands with arms reaching forward as if the racer is trying to reach forward and hug a large barrel.

The screen shot below is of Marcel Hirscher in the December 16, 2018 Alta Bada GS that he won by 2.53 seconds.

The screen shot below is of Tessa Worley in the 2018-19 Soelden GS.

Reductionist Anatomy

A longer answer to the question posed by the title of this post, one that I will expand on in future posts, is that Hirscher, Brignone, Worely and Shiffrin are examples of the application of the principles of an emerging paradigm that is challenging the fundamental way in which muscular anatomy has long perceived muscles as separate systems with specific functions. In the new paradigm that has arisen out of recent discoveries muscles function in conjunction with the myofascial network as a wholly integrated system; one that responds and adapts to the stresses imposed on it. Since these discoveries are almost ten years old the odds are that the dominant technique of Hirscher, Brignone, Worely and Shiffrin is not by chance.

In previous posts, I described a stance based on isometric contraction as the SR Stance. SR is an abbreviation for the Stretch Reflex. Technically, a better term for the stretch reflex is the stretch-shorten cycle

The reason I chose SR for the name of the stance is that isometric contraction and the stretch reflex are not part of the narrative of ski technique. I discuss the three forms of muscle contraction in my post I-C-E: SR (2.) which I have recently updated.

The reason a ski stance based on isometric contraction provides a huge competitive advantage has to do with recent finding discussed in a 2009 article (1.) in which ultrasound imaging that allowed for quantitative assessment of the mechanisms for elastic energy storage and return at the ankle joint during human walking found that the Achilles tendon stores elastic energy as the mid stance phase progresses until the energy peaks in late midstance and is released to produce a rapid recoil with very high peak power output. The researchers named this the Catapult Mechanism (3.).

An important feature of the ankle ‘catapult mechanism’ is that the stretch and recoil of the Achilles tendon allows muscle fibers to remain nearly isometric producing high forces with very little mechanical work. In the isometric state, muscles expend much less metabolic energy to produce force when compared to muscles shortening in concentric (positive work) contractions.

Recent research has also found that during explosive movements, the contractile elements of a muscle remain in an isometric state to increase tension in the non-contractile components in an effort to produce higher levels of force. The enhanced stiffness from the contractile component can help the connective tissue rapidly store mechanical energy during the lengthening (recoil) phase delivering greater power output during the shortening phase. (4.), (5.)

What all this means is that the power advantage seen in racers like Hirscher, Brignone, Worely and Shiffrin results from an integrated system. But the human body can only function as an integrated system under conditions which allow multi-plane movement, something conventional ski boots intentionally interfere with.

In my next post I will start from what I see as the fundamental element of a ski stance based on isometric contraction and progress upward from there.


  1. It Pays to Have a Spring in Your Step – 2009 Gregory S. Sawicki1, Cara L. Lewis2, and Daniel P. Ferris2 – 1. Department of Ecology and Evolutionary Biology, Brown University, Providence, RI; and 2. School of Kinesiology, University of Michigan, Ann Arbor, MI
  2. https://wp.me/p3vZhu-1wT
  3. Fascial Fitness: Fascia oriented training for bodywork and movement therapies – Divo G. Muller, Robert Schleip 
  4. Cutting Edge: Training the Fascial Network (Part 1) by Pete McCall M.S.
  5. Cutting Edge: Training the Fascial Network (Part 2) by Pete McCall M.S.