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.
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.
- 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.
- WHAT THE TWO HIGH PRESSURE COPS IN THE UNIVERSITY OF OTTAWA STUDIES MEAN – https://wp.me/p3vZhu-1fV
- IMPLICATIONS OF THE UNIVERSITY OF OTTAWA PRESSURE STUDIES – https://wp.me/p3vZhu-1e2
- AN INDEPENDENT STUDY IN SUPPORT OF THE UNIVERSITY OF OTTAWA FINDINGS – https://wp.me/p3vZhu-1gR