In THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE: PART 8, I stated that after a thorough investigation and analysis of the forces associated with platform angle mechanics I reached the conclusion that rotational (steering) force should be applied to an isolated area of the inner shell wall of the ski boot by the medial aspect of the head of the first metatarsal as shown in the graphic below.Applying rotational or steering force to the medial (inner) aspect of the head of the first metatarsal requires the application of an effort by the skier that attempts to rotate the foot inside the confines of the ski boot. The application of rotational effort to the inner aspect of the vertical wall of the boot shell opposite the head of the first metatarsal will result in a reaction force that pushes the lateral (outside) aspect of the heel bone against the outer corner of the vertical shell wall as shown in the graphic below. The robust structure of the bones of the first metatarsal, midfoot and heel bone serve as a structural element in transferring rotational force to opposing aspects of the shell walls in an eccentric torque couple.The outline of the boot shell in the above graphic was generated from a vertical plane photo of an actual ski boot. The interference created by the inner wall with the localized application of rotational force to the shell wall by the medial aspect of the head of the first metatarsal should be obvious.
The radius of the moment arm acting on the outer aspect of the heel area of the shell is much smaller than the radius of the moment arm acting on the inner aspect of the shell opposite the head of the first metatarsal and many times shorter than the length of the moment arm acting at the shovel of the ski. The result is that rotational force applied to the eccentric torque arm couple by rotation applied to the ankle will attempt to rotate the torque arm and the axis of rotation at the ankle joint about an axis of rotation at the lateral aspect of the heel as shown in the graphic below. This mechanism enables a skier to apply much greater rotational force into a turn at the center of the ski than can be applied at the shovel. This has signficant implications for platform angle mechanics. In addition to the above, the plane of the rotational force applied by the medial aspect of the head of the first metatarsal and lateral aspect of the heel bone to the shell wall is elevated above the plane of the rotational force at base of the ski below.
In my next post I will discuss what happens when the reaction force from the snow that opposes the 180 degree force applied to the base plane of the ski becomes sufficient to arrest rotation of the ski about its axis of rotation at the ankle joint.