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

6 comments

  1. When you talk about applying pressure to the medial aspect of the first metatarsal head, am I correct in assuming that you are describing aligning yourself over it and letting the rest happen, as opposed to the forceful application of pressure advocated here?:

    (If you turn on the captions, there are English subtitles).

    1. The neurobiomechanics and especially the sensorimotor aspects associated with what a amounts a coordinated, synergistic application of vertical and horizontal components of force by the head of the first metatarsal are extremely complex. As such they are a long way from being fully understood if this is even ever possible. But based on my present level of knowledge and understanding of the mechanism, the application of force to the medial aspect occurs at a threshold isometric level regulated by a spinal reflex.

      There is a good reason why no one other than myself that I am aware has solved the platform mechanics problem – it is and was not easy.

      I have seen the video. I follow La GrandeNeve Check Point as I follow many other sources to help me understand how technical issues are perceived. With all due respect to Malfatto, his explanation and especially the demonstration, doesn’t reflect reality. At least he is trying to come to recognize and understand the issues.

  2. Hi! I’m a PSIA Level II Alpine instructor named Brad. I’ve been following this blog for a couple years now after Google led me with dismay to fact that this is the ONLY place on the internet where skiing physics were being discussed rationally.

    This matter of the femur being behind the center of the long running surface has vexed me to no end.

    Though it does not appear in what our textbooks have to say on steering, I often hear, and have myself instructed beginners to ”make bowtie shapes in the snow with your boot” as a way of demonstrating steering. Then showing how to rotate the boot around a point roughly beneath their arch to trace a “bowtie” shape in the snow. I have never understood where the forces were supposedly coming from that afforded the skier the leverage to make that move.

    Also, I know I know: static drill. But we all gotta start somewhere.

    If you’re still with me :), what I’m reading here is: the *ski* makes a bowtie shape, but the boot swipes out a much more oblong, non symmetrical path with axis of rotation being under the head of the 1st metatarsal (ball of the foot), NOT under the arch.

    All those instructors telling 5 year olds to ”squish the bug under your big toe” had it right all along?

    Help me understand!

    Thanks,
    Brad

    1. Hi Brad,

      Thank you for your comments. The fact that you found I was the ONLY place on the internet where skiing physics were being discussed rationally is both a compliment and a tragedy.

      I am retired now. But most of my careers involved my problem-solving skills. In many cases I was sought out for my skills at solving complex problems. In problem solving the only thing that matters or counts is the ability to solve every aspect of a problem so it never returns. No one cares about the details or what people think they know or don’t know. It comes to “Show me the solution. Make it work.” If you can’t solve the problem you don’t know what you’re doing. Simple.

      After some seriouysly flawed assumptions about skiing I realized that the issues are extremely complex and that I had a lot to learn. After a protracted effort I can claim I know a little bit. It turns out that a little bit is a whole lot more than most of the authorities in skiing know. When people don’t know something making up a techy term and flouting authority usually stops challenges from those with rational, reasonable questions.

      The mechanism of platform mechanics and balance on the outside ski are prime examples. For decades it has been acknowledged that the world’s best skiers can stand and balance on their outside ski using the same balance responses we all use to stand and balance on one leg on solid ground. But a ski on edge is not solid ground. But like you finding a rational discussion of physics I could never find any explanation of the mechanism that made the platform of an outside ski on edge into the equivalent of solid ground. Why?? I finally solved the problem. But it took over 10 years because there were few clues to help.

      Now to your questions:

      This matter of the femur being behind the center of the long running surface has vexed me to no end

      Careful. The femur rotates the tibia. The lower end of the tibia that forms the ankle joint with the talus (tibial talar joint) is rotated and this rotates the foot. The only reasonably accurate way I have found to show this is to place your foot on a smooth floor with reference lines like junctions of tile or panel flooring and rotate your foot with light weight on it with the ankle turning about the center of the reference axis under the foot. It helps to wear a sock.

      A fancier way is to mark an X on a smooth panel of plywood or MDF about 2 ft x 2ft, put this on the floor and try and rotate your foot about the center of the ankle.

      If you’re still with me :), what I’m reading here is: the *ski* makes a bowtie shape, but the boot swipes out a much more oblong, non symmetrical path with axis of rotation being under the head of the 1st metatarsal (ball of the foot), NOT under the arch.

      No. The ski doesn’t make a bow tie shape. The ball of the foot is on the running center of the ski. But the axis of rotation is behind directly under the bottom surface of the tibia.

      With my size 12 US men’s foot (335 cm ski boot) the axis of rotation of my foot is about 5.5 inches behind the ball of the foot/running center of a ski. I’ll post on this. It’s important to get this right and correct a long standing serious error.

      1. Evidence in support of what? That the lower end of the tibia rotates the foot? Or that rotating the femur rotates the foot? Or….?

        Although I don’t agree with a lot of things Ron LeMaster says, my experience agrees with his statement on page viii of the Preface of Ultimate Skiing when he says, “As coaches and instructors, we often confuse what we teach with how we teach it. And as skiers, we often confuse what we feel with what we actually do”. I would take it a step (or a turn) further.

        It has been my experience over many years that what elite ski pros and even World Cup coaches say they are doing or what a skier or even a World Cup Racer is doing bears little resemblance to what they are actually doing. The consequences of uninformed observation leads to conclusions like the earth is the center of the universe or the earth is flat or…… Conclusions ski pros and coaches take considerable liberties based on uninformed observation by making up stuff like “think of snuffing out a cigarette under the ball of your foot to get the feeling of how to steer your skis”. Wrong information like this teaches wrong movements like keeping the weight on the heels and pushing the tails of the skis out to steer them in a turn.

        It is difficult, if not impossible, to replicate the mechanics of steering and/or the mechanism platform angle in a static environment because it lacks the momentum associated with movement in skiing even if it is given in the name of simplicity. Simple is good but only if it is right.

        Let me know what you trying to find evidence in support of and I will assist you.

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