ski plafform mechanics

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 8

A soon to be published study (1.) recognizes that recent advances in sensor-technology allow the kinematics and kinetics of skiing to be monitored and data collected during training and actual competitions. The data will generate detailed information about the biomechanical factors related to success in competition and used to individualize training and skiing and equipment for each unique skier and, most important, motivate innovative scientific research for years to come.

 Individualize equipment for each unique skier

I fervently hope that this marks the beginning of the realization of a vision I had 30 years ago; one that resulted in the 1992 on snow studies using a unique instrumented research vehicle I designed with a biomedical engineer. This research vehicle allowed data to be acquired from skiers ranging from World Cup and Olympic champions to neophytes during actual ski maneuvers and meaningful metrics generated with which to assess performance. The objective of the study was to validate my hypothetical model of the mechanics, neurobiomechanics and physics of platform balance and the mechanism of skier CNS mediated dynamic stability. A validated model is essential for the interpretation of performance extrapolated from data. The intent of the subsequent patents was to provide a knowledge base to serve as a foundation for a science that would eventually enable individual skier optimization of every aspect of equipment and make skiing as easy and intuitive as walking for the masses.

A major source of inspiration and direction for my work and especially for my persistence came from the medical text-book The Shoe and Sport, in particular, Part 6 The Ski Boot.

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). In this way, the skier can adapt to the requirements of the skiing surface and snow conditions. These conditions can be met if the height, stiffness, angle and functions (rotational axes, ankle joint (AJ)/shaft) of the shaft are adapted, as well as possible to the individual skier.

The modern ski boot must be designed from a functional point of view, i.e., the design must take into consideration the realities of functional anatomy (axes etc.).

It (the design) should not make compromises at the expense of other joints (length of shaft, flexibility and positioning).

It (the ski boot) must represent the ideal connecting link between man and ski (steering and feedback).

I am forever indebted to  Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland, Professor Dr. M. Pfeiffer of the Institute for the Athletic Science, University of Salzburg, Salzburg, Austria, Dr. med. H.W. Bar, Orthopedics-Sportsmedicine, member of GOTS, Murnau, West Germany and W Hauser and P. Schaff of the Technical Surveillance Association, Munich, West Germany and other pioneers who inspired my efforts and paved the way to the future of skiing.

In the words of W Hauser and P. Schaff:

In the future, ski boots will be designed rationally and according to the increasing requirements of the ski performance target groups.

I sincerely hope that the work of Supej Matej and H-C Holmberg (1.) ushers in the future of skiing.

The Platform Balance Solution

In previous posts I established that:

  • the axis of rotation of the foot and the ski (steering) resulting from rotation of the femur in the socket of its ball joint with the pelvis by what amounts to a muscle driven torque motor, occurs behind the center of the long running surface of the base of the ski.
  • the ball of the foot of a skier is located on the proximate center of the long running surface of the base of the ski.
  • edging and carving force require components of force with vectors aligned 180 and 90 degrees to the transverse aspect of the base plane of the outside ski.
  • the rotational or steering force (torque) is the source of the vector of the 180 degree force acting into the snow.
  • the point of application of the rotational cutting force when the axis of rotation of the ankle is oriented on the X-Y axis of the ski is mechanically ineffective. The monoplanar nature of the torque makes it less effective in terms of contributing to skier dynamic stability.
  • the effect of side-cut on platform angle mechanics must also be considered.

From a mechanical-neurobiomechanical perspective, the logical place to apply the center of force of the foot acting 90 degrees (or slightly less) to the transverse base plane is under the ball of the foot (i.e. the head of the first metatarsal).

After a thorough investigation and analysis of the forces associated with platform angle mechanics I reached the conclusion that given the robust structure and the degree of stability of the head of the first metatarsal and the fact that the 90 and 180 degree forces should be congruent it seemed logical to apply the force acting 180 degrees to the transverse base plane of the ski to the medial aspect of the head of the first metatarsal. The 1992 study was designed to confirm or disprove the validity of this conclusion.

he graphic below shows the application of the rotational (steering) force to the medial aspect of the head of the first metatarsal.The photo below shows the robust force transfer structures under and on the inner (media) aspect of the head of the first metatarsal. 

In my next post I will discuss the requirements of a ski boot necessary for the user to simultaneously apply plantar force to the platform and rotational force to the medial aspect of the head of the first metatarsal.


  1. Recent Kinematics and Kinetic Advances in Olympic Alpine Skiing: Pyeonchang and Beyond – Supej Matej and H-C Holmberg: Frontiers in Physiology

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE: PART 6


In my last post I identified whole leg rotation of the head of the femur at its joint in the pelvis as the source of the rotational force acting 180 degrees to the transverse plane of the platform. In the technical terms of skiing whole leg rotational force is called steering.

When I started skiing in 1970 the ability to hold an edge on hard pistes and especially ice was the exclusive domain of elite skiers. Back then, the majority of skiers and racers were still skiing in low cut leather or plastic boots with the shaft not much higher than the ankle bones.

Debates raged in ski magazines as to the reason why elite skiers were able to easily hold an edge on ice while the majority of skiers struggled. The question was posed as to which came first. Did elite skiers edge first and then turn their skis or did they turn their skis and then edge? The consensus was that the best way to hold an edge and not to slip was to establish edge grip early and not slip when the forces increased. Recovering an edge once a ski started to slip was next to impossible. 

Since holding an edge during a turn involves movement of the skier there was no static way to demonstrate how to hold an edge on ice. The only option was watch an elite skier and try and copy them. This was seldom successful because even elite skiers couldn’t describe what they were doing. Strength and athletic ability and/or level of fitness did not seem to be significant factors.  Even elite hockey players often struggled to hold an edge on skis. I had questions but few answers. Finally a female ski instructor gave me a valuable clue when she told me that she presses down hard on the ball of her outside foot to make her edges hold on hard snow.

Clues such as turning the skis and putting pressure on the ball of the outside foot pointed towards the mechanism of the mechanics of platform angle and dynamic balance. But before the mechanics could be explained the introduction of the high shaft rigid plastic ski boot distracted attention away from the problem. High stiff plastic ski boots made it easy for even a novice to stand, crank their knees into the hill and put their skis on edge. This turned out to be a good marketing tool because it made holding an edge appear easy even for a novice. But using the leg as a lever didn’t work except under ideal conditions.

When I tried using my leg to hold a ski on edge on ice I met with marginal success. Later, when I modelled the mechanics the combination of forces didn’t result in a mechanism that would enable a skier to cut a step into hard pistes so as to create a platform and control its angle.

But the crank the knee into the hill option prevailed and took root. It provided an easy way to demonstrate a complex issue. Once knee angulation became established the ski industry appeared to lose interest in trying to discover the real mechanism responsible for platform mechanics. In spite of a protracted effort I didn’t begin to understand the mechanism until about 1989 after getting some valuable clues from the chapter on the ski boot in the medical text, The Shoe In Sport (see my post – THE SHOCKING TRUTH ABOUT POWER STRAPS). But getting insights on the mechanism entailed making some significant discoveries that have only come to be recognized and studied in the l ast 10 years.

One discovery I made that was fundamental to understanding platform mechanics is that the Achilles tendon is capable of transferring large forces to forefoot as the pelvis moves forward in the stance phase of locomotion.

Steer onto the Platform

Although steering and edging are often discussed together they are typically considered different, but related, skills that are blended together. In fact, they are one and the same. Elite skiers steer their skis onto a platform but only if their equipment, in particular their ski boots, enables the requisite neurobiomechanics. 

The Center of Rotation of the Foot 

The turning effort from the pelvis is applied to the foot at the distal (farther end) of the tibia as shown in the graphic below. In terms of position on the running length of a ski this places the center of rotation on the rear half of the ski. The implications are that the forebody of a ski will rotate more across a skier’s line than the tail of the ski. In my foot, the center of rotation is approximately 12 cm behind the running center of the ski.
The femur has a typical range of rotation of 45 degrees in each direction (total ROM 90 degrees); 45 degrees medial (towards the transverse center of the body) and 45 degrees lateral (away from the transverse center of the body). 

If rotational effort is applied to the foot against a firm vertical surface the rear foot will be forced away from the surface.

The implications for skiing are that as the platform angle of a ski with the plane of the snow increases towards perpendicular (normal) to the slope the turning effort applied to the feet will direct the forebody into the surface of the snow. As a reader commented on a previous post on platform angle mechanics the tips (shovel or forebody) of the ski leads the charge. A carved turn starts at the tip with the edges engaging and cutting a step into the snow for the portion of the edge that follow to track in. The shovel leads the charge and starts the carving action. 

Mechanical Points of Force 

A final point for this post is the two key mechanical points where loads on the foot apply high force to the platform; one under the ball of the great toe (i.e. head of the first metatarsal) and the other under the heel in an area called the tuber calcaneum. These are the primary centres of force in skiing. 

The effect of any rotational force or steering to a ski is significantly affected in the carving or loading phase by where the center of force is located. This will be the subject of my next post.

THE MECHANICS OF PLATFORM ANGLE: PART 5

In my initial posts on the mechanics of platform angle I demonstrated the physical impossibility of making a ski carve an edge into hard pistes at high platform angles with the snow by a skier aligning opposing applied and reaction forces with the vector perpendicular to the transverse plane of the platform of the outside ski. The reason for this is that the component of shear or slipping force will progressively increase as the angle of the applied force Fa becomes increasingly aligned with the plane of the surface of the snow as shown in the examples in the graphic below.

In my previous post I said that a reader who commented on Part 3 correctly stated for a ski to hold and carve at high platform angles required two separate forces acting on the transverse plane of the platform; one force oriented at 90 degrees to the plane and a second force oriented parallel or 180 degrees to the transverse plane with the vector acting into the surface of the snow. I ended my post by asking the reader what the source of the 180 degree force was.

The graphic below shows the answer. Elite skiers can make the outside ski of a turn hold and carve at very high platform angles because they are able to apply two separate forces in a coordinated manner. The reason I say ‘able to apply’ is that many factors can severely limit or even prevent the coordinated application of these two forces; the most significant factor being interference from the structures of the ski boot with the associated coordinated joint actions of the foot and leg.The graphic above is for the purpose of illustrating the source of the 180 degree force acting on the transverse plane of the platform. As such, the graphic  is not accurate because it shows the plantar (sole) plane of the foot oriented on the transverse plane of the platform. The actual mechanics and biomechanics are much more involved. I’ll start to explore the various factors in my next post.

THE MECHANICS OF PLATFORM ANGLE: PART 4

In Part 3 of the mechanics of platform angle I suggested that some unidentified force or forces are at work that enable elite skiers to alter the angle of attack of the applied force R so that it is more aggressive in terms of cutting (carving) a step into the surface of the snow. I asked the reader what the components of the applied and reaction forces would look like.

One reader correctly identified two separate forces acting on the transverse plane of the platform of the outside ski; one oriented vertically at 90 degrees to the plane and a second force oriented parallel or 180 degrees to the transverse plane with the vector aligned into the snow.

The right hand graphic below shows the 90 and 180 degree components of the angular force acting on the platform in the left hand graphic.

The right hand graphic below is the same as the graphic above but with the angular force superimposed over the 90 and 180 degree components.

I am taking the discussion of platform mechanics in small steps in order to provide the reader with a chance to assimilate the issues and ask questions if my discussion is not clear.

THE SHOCKING TRUTH ABOUT POWER STRAPS AS A REFERENCE

Most of the views of the series on the Mechanics of Platform Angle are accompanied by views of The Shocking Truth About Power Straps which contains quotes from the medical textbook The Shoe in Sport (published in German in 1977 as Der Schu im Sport). This medical textbook has been invaluable to my efforts.

Here are some pertinent quotes by Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland

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). In this way, the skier can adapt to the requirements of the skiing surface and snow conditions.

These conditions can be met if the height, stiffness, angle and functions (rotational axes, ankle joint (AJ)/shaft) of the shaft are adapted, as well as possible to the individual skier.

The modern ski boot must be designed from a functional point of view, i.e., the design must take into consideration the realities of functional anatomy (axes etc.).

It (the design) should not make compromises at the expense of other joints (length of shaft, flexibility and positioning).It (the ski boot) must represent the ideal connecting link between man and ski (steering and feedback).

 Biomechanical Considerations of the Ski Boot (Alpine)

The question for this post is what is the source of the 180 degree force? Please consider Dr. E. Stussi’s comments above when contemplating an answer to this question.