Skier Balance posts

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.

The 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.

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.

 

THE MECHANICS OF PLATFORM ANGLE: PART 2

I believe the single most important factor affecting a skiers’ balance and directional control of a ski is the ability of the balance system to effect dynamic balance of the angle of the platform of an edged ski with the surface of the snow . So I am going to focus my efforts on explaining the mechanism of dynamic balance.

Skiing is an interaction of the skier with the snow. Since the interface of the interaction is the inside edge of the ski and the snow I’ll start here with an explanation of the principles of the associated mechanics.

Snow = Ground

Snow is an extension of ground. Hence we speak in terms of ground reaction force or GRF. In moving over the snow a skier is interacting through a layer of snow with the ground. In the name of consistency firm pistes will be the reference for the discussion of forces.

The interface with the ground is the base plane of a ski in particular the juncture of the transverse base and sidewall planes. In this interaction the angle of the base plane  with the surface plane of the snow is the plane of balance for the skier in terms of the management of angular forces.

Edging Forces are not Normal

In order for the platform of a ski to hold and not slip under the weight of a skier the edge and the adjacent portion of the base  must cut a step or ledge into the surface of the snow. But the portion of the ski that must cut a step into the surface of the snow is not a knife edge. It is more like a knife blade on the flat with the blade aligned perpendicular to the angle of attack of the force applied by the skier.

Typical force diagrams in technical discussions of skiing only show opposing angular forces with a platform perpendicular to the vector of the forces. The opposing angular forces R and GRF are said to cause the platform of the ski to cut a step or ledge into the surface of the snow as shown in the graphic below. One problem is that sketches such as the one below don’t show the perpendicular (Normal) or horizontal (Shear or Slip) components of the opposing angular forces R and GRF.

When an applied force is Normal to a surface (perpendicular) the penetration or cutting action is maximal.  But when a force applied to a surface is less than perpendicular it will have components of Normal and Shear or Slip forces such as shown in the graphic below. At an angle of attack of 45° the Normal and Shear components of the applied and GRF forces will be equal. But as the angle of attack decreases and becomes more aligned with the plane of the surface the Shear or slip components will increase in magnitude and the Normal components of the applied and GRF forces will decrease in magnitude.

As this happens the tendency of an applied force acting on a body like the platform of a ski that would cause it to penetrate into the surface of the snow and cut a step will decrease as the angle of the platform with the snow increases. As the platform angle increases so will the tendency of the ski to slip and not hold an edge. The components of opposing applied and GRF forces acting on a solid body or surface are determined mathematically by sine/cosine. They are not negotiable. Nor can their impact on ski platform mechanics be ignored.

 

In my next post I will discuss the real force that makes the platform of a ski cut a step or groove into the surface of the snow that the edge of the ski tracks in.

THE MECHANICS OF PLATFORM ANGLE: PART 1

In order to engage in an interactive productive dialog on issues pertaining to ski technique and related equipment a frame of reference based on validated, non-negotiable principles of physics, mechanics and (neuro)biomechanics as well as a schedule of defined reference terms such as exists in the sciences of mechanics, anatomy and physics is essential. Defined technical reference terms help ensure all participants in a discussion are on the same page.

I decided to start the new direction of The Skier’s Manifesto with a critical examination of the mechanics of platform angle starting with a schedule of the technical terms associated with platform angle and their definitions. Additional technical terms and their definitions will added in future posts according to the content of the discussion. The intent at this point is to start with a basic discussion of forces applied to a rigid body and/or surface (in this case, the surface of the snow) and then expand the scope of the discussion in future posts. Agreement on terms and definitions is important. So please comment if you feel one or more the following terms are inappropriate or inaccurate or should be expanded and/or refined.

Technical Terms associated with Platform Angle

  • Platform Angle: the angle of the transverse aspect of the body of the ski underfoot with the surface of the snow.
  • Edge Angle: the angle of the edge of the ski in relation to the plane of the transverse aspect of the body of the ski adjacent the edge.
  • Force: an unopposed interaction that will change the motion of an object. A force has both magnitude and direction, making it a vector quantity.
  • Force Vector: the magnitude and direction of a force.
  • Applied Force: a force applied to a rigid body or surface.
  • Reaction Force: a force that opposes a force applied to a rigid body or surface.
  • Normal Force: a force acting perpendicular to a rigid body or surface that is resisting a force applied to it.
  • Angular Force: a force applied to a rigid body or surface that is not normal (perpendicular) to the rigid body or surface to which the force is applied.
  • Angle of Attack: the angle an angular force forms with the rigid body or surface to which it is applied to.
  • Resultant Force: also known as Net Force, is a single force associated with torque obtained by combining a system of forces and torques acting on a rigid body.

Technical discussions of the forces associated with the angle of the platform with the snow typically show opposing resultant and ground reaction forces implying a state of balance of the forces acting on platform created by the outside ski underfoot.

Schematic diagrams showing forces acting on the platform created by the body of the ski underfoot often show two opposing forces in alignment with each other acting close to or at the axis point created by the inside edge of the outside ski. Or diagrams may simply show opposing forces aligned with each other implying the existence of a state of equilibrium.

In my next post I will discuss whether the above force diagrams accurately reflect a state of equilibrium of the forces acting on the platform of the outside ski. Please join the conversation.