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

6 comments

  1. David, thanks for replying. A little more feedback. Skied the same slope, similar or more slick snow, boots buckled but loosely. Focused up and over and letting the ski edge get a purchase before diving down the hill. Not rushing really helps engagement. At completion of the turn letting the skis transition across the hill before starting the sequence over again. also added forward lean to the boots with a wedge. Much better balanced and control. I’m going to try a little more forward lean and see what happens. Thanks again!

    1. Thanks for the good report.

      Focused up and over and letting the ski edge get a purchase before diving down the hill. Not rushing really helps engagement.
      REPLY: Two good moves. Getting over the outside ski and settling onto the ball of the foot (I think of this is connecting or rooting with the snow) and not rushing really helps. In one of the Burke Mountain videos narrated by Shiffrin she stresses the importance of Patience.

      A caution. The priority of our brain is survival. As exclusive bipeds we are vulnerable if our ability to move is compromised or our brain interprets our movements as a threat. So if we compromise our processes of balance and control on skis our brain will implement what I call a survival technique. This is so common these days that I see it when analyzing video of World Cup racers.

      So go slowly with changes and make sure they are in the right direction. It also helps to do drills to confirm stance and getting over the new outside ski before starting to ski each time. A few years ago I saw video of Vonn skiing skiing very slowly hunched over like a gorilla. She was connecting with her stance. Shiffrin does a lot of drills with her mom who is very smart.

      It’s not a good marketing story but if you want to ski your best you have to invest in your skiing.

      Keep going.

  2. I’ve been following your blog for years and appreciate your systematic and scientific approach. I’ve been playing with my boots for years and have tried many of the things you have recommend with varying results. That’s not saying they don’t work just relating to the many variables that each of out bodies, feet, etc. plus tecq. bring to the equation. I am getting to a question. The other day skiing a 35-40 degree slope with my 13 meter slalom skis on carvy snow, ie hard but not icy. I noticed when I didn’t buckle my boots (they fit really well) I had very smooth turns, when buckled I chattered horrible, almost losing control. Any thoughts??? I really appreciate your thoughts boots??? Tecq.?? Cuffs are aligned, canted and fitted well with a flexible foot bed. Any thoughts would be greatly appreciated! Thanks!!

    1. Unbuckled boots – smoother turns? Buckled boots – chattered horribly? A good example of what I call the Perfect Fit Paradox.

      As a sidebar, decades ago when Franz the Kaiser (the king of downhill) Klammer said the reason he was so fast was that he had the buckles on his boots very loose. He told the skiing world what you just experienced and he was written off as a paradox, a bad joke.

      Thoughts? Yes. I think the most probable explanation is that you were able to apply force to your skis with an asymmetric force couple. What? You have probably never heard of it. I’m about to start discussing this in my next post. In the meantime, you will find that if you buckle your boots really, really tight and try to ski on the same slope your skis will chatter terribly and you will have almost zero control. Don’t worry. You’re OK. You’re just an example of the Perfect Fit Paradox.

      1. If you believe, as I do, that ski boots are often a primary cause of skier balance issues (as in compromised balance) then the problem is in the ski boot and skiers need to look to the makers for solutions.

        Thirty years ago when I solved the problem and explained how elite skiers can stand and balance on their outside foot and ski as if the ski were supported by ground I was unable to elicit any interest let alone initiate a discussion of what I had found despite a concerted and protracted effort. The argument I was consistently confronted with is words to the effect that, ‘balance problems are the problem of the skier, not the equipment. Skier problems are not an area of interest or concern’.

        When the comments made back in 1987 by international authorities such as Dr. E. Stussi, Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland, that “Improvements in the load acting on the ankle make it biomechanically

          very likely

        that the problems arising in the rather delicate knee joint will increase.” [problems mean injury to the knee] accurately predicted knee injuries would increase because of boot fitting practices are dismissed or ignored it makes telling statement that should concern all skiers. I certainly paid attention to Stussi and others with international expertise and credentials.

        If you believe ski boots are a problem for you I suggest you seek solutions from the boot makers.

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