INTEREST IN MY POST, ‘WHAT THE TWO HIGH COP PRESSURES IN THE OTTAWA STUDIES MEAN’



I started this blog with the objective of stimulating thinking and discussion among serious skiers, racers, ski pros and coaches about the mechanics, biomechanics and physics of skiing. Ingrained and accepted beliefs in skiing, that have little or no basis in science, have created resistance to ideas that question these beliefs .

The response to a post serves as a barometer of the type of audience my blog is connecting with. Since my post on excessive ramp angle as a stance killer stimulated interest in my post on the meaning of high COP pressures reported in University of Ottawa studies, I have decided to delay my post on my vision of a standard specification for boot boards and instead provide links to two papers below that are related to the Ottawa studies.

In the first paper (1), the authors correctly identify the moment of force created on the outside foot and ski of a turn by an offset in the alignment of the opposing force applied to the ski by the weight of a skier Fr with the ground reaction force F Ground as shown in Figure 2 below from the paper. The resulting drift angle creates medial compression of the associated knee joint.

BB MoF

In the second paper (2), the authors also correctly identify the moment of force created on the outside foot and ski of a turn by an offset in the alignment of the force applied by the weight of the skier with the ground reaction force, GRF as shown in Figure 1 below from the paper. Sketches a) and b) show the effect of greater width of the ski underfoot in increasing the length of the moment arm.

Waist 2

The authors of both of the papers recognize the problem; an offset in the alignment of opposing applied and ground reaction forces sets up a moment arm that causes the outside ski to rotate out of the turn. Each of the papers attempts to resolve the problem in a different manner.

Do either of the approaches presented in the 2  papers cited above address the problem?

…… to be continued.


1. An innovative ski-boot: Design, numerical simulations and testing

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3887325/

https://www.researchgate.net/publication/259878372_An_innovative_ski-boot_Design_numerical_simulations_and_testing

2. The Waist Width of Skis Influences the Kinematics of the Knee Joint in Alpine Skiing 

https://www.researchgate.net/profile/Matej_Supej/publication/281541462_The_Waist_Width_of_Skis_Influences_the_Kinematics_of_the_Knee_Joint_in_Alpine_Skiing/links/560cdf1e08aea68653d38633.pdf?inViewer=0&pdfJsDownload=0&origin=publication_detail

3 comments

  1. Re: paper #2 on Ski Width and Knee Kinematics. I commend the authors for recognizing the problem. I have two critical comments:

    The authors begin with a hypothesis based on visual observation. Unfortunately their ensuing objective measurements are marred by their concept of knee abduction. Flexion and internal rotation of the knee contribute to the APPEARANCE of increasing knee valgus, but no actual true abduction occurs. The surrounding medial and lateral collateral ligaments and both cruciate ligaments are all isometrically tensioned throughout knee flexion to 90º. There is no ligament “floppiness” or laxity allowing knee abduction.

    Secondly, the authors have ignored the role of wide ski leveraged inversion(supination) at the foot which causes external tibial(shank) rotation. They have interpreted movement from ski to knee, not ski to foot to knee in evaluating the entire leg response to the ski width variation. On wide skis, the leveraged foot inversion loses edge control and allows the ski to rotate out of the turn. Skier eversion(pronation) recovery attempts create an oscillating slip-catch pattern leading to medial outside knee stress. This inversion-eversion pattern of the outside foot is the basis for our previously described FAT SKI SYNDROME(Skier’s Manifesto, November 26, 2014 by Skikinetics).

    1. I agree on all points.

      Given the science background of the authors, I am both perplexed and disappointed that they seem to have embraced and accepted the knee angulation narrative of the ski industry; one that has no basis in science. The defining feature of the world’s elite skiers is in-phase internal rotation of the outside leg and thigh of a turn in conjunction with eversion of the outside foot against a ground reaction acting along the inside edge resulting in an over-center into the turn rotation of the outside ski controlled by eccentric contraction of the inverter muscles in the absence of any significant varus-valgus thrust.

      This subject will be given more scrutiny and discussion in future blog posts.

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