In order to be able to develop a dynamically stable base of support on the outside ski of a turn, one that supports the processes that use external forces to drive the ski into the turn, the ski must have a waist that is close to the centre-to-centre dimension ‘X’ between the balls of the great and 2nd toes. In addition, the anatomical centreline of skier’s foot must be aligned with the running centre of the ski. The graphic below shows what it looks like when the foot is correctly positioned on a ski with the appropriate width underfoot. In reality, there is a stack of equipment between the sole of the skier’s foot and the snow surface. F.I.S. rules allow up to 100 mm of stack height between the sole of a racer’s foot and the snow surface. So the graphic below does not reflect reality. The reason I am starting with the skier’s foot directly on the ski top plate is to demonstrate that an applied vertical force acting against an opposing snow reaction force (SRF) is insufficient to explain the edging mechanics that skiers like Ligety and Shiffrin are able to develop. There are other factors at play that I will introduce in future posts.
US Patent No 5,265,350 – MacPhail: November 30, 1993 – “The prior art refers to the importance of a “neutral sub-talar joint”. The sub-talar joint is a joint with rotational capability which underlies and supports the ankle joint. The sub-talar joint is substantially “neutral” in bipedal function. That is to say that the foot is neither rolled inward or rolled outward. If the foot can be substantially maintained in a neutral position with the arch supported and with a broad area of the inner aspect of the foot well padded, there will exist a good degree of comfort. Such a state of comfort exists because the foot is not able to roll inward (pronate) to a degree where significant mechanical forces can be set up which would allow it to bear against the inner surface of the boot shell. In effect, this means that initiation of the transition from a state of bipedal to a state of monopedal function, is prevented. This transition would normally be precipitated by an attempt to balance on one foot. If the foot is contained in a neutral position, traditional supportive footbeds (arch supports) are quite compatible with the mechanisms and philosophies of the prior art.”
Here is what the inside and outside feet of a skier in a turn look like when the feet are in neutral.
Since there are offsets or moment arm between CoP and SRF on each foot, the sole of the inside foot of the turn will tend to roll away from the centreline between the feet (ergo, it will tend to evert) while the sole of the outside foot of the turn will tend to roll towards the centreline between the feet (ergo, it will tend to invert). Under specific conditions the external forces acting on the skier will tend to make the outside foot of the turn rotate into the turn (ergo, it will tend to evert). But, for reasons that will be provided in a future post it is not possible to create conditions under which the external forces acting on the skier will tend to make the inside foot of the turn rotate into the turn. For this reason the force applied to the snow by the skier must be directed to the inside edge of the outside ski of the turn. The inside foot and leg are used to help direct the force to the outside ski. This what Ligety and Shiffrin do so well.