In SHIFFRIN DEMYSTIFIED I annotated an image of Shiffrin with nine metrics that are markers of her superior technique.
1. Shiffrin’s right leg is extended with her soleus, gastrocnemius and hamstrings active in eccentric contraction.
2. Shiffrin’s Center of Mass is aligned in the same vector with the Centre of Pressure in her foot which is located under the head of the first metatarsal (ball of the foot).
3. Shiffrin’s right foot is tending to evert (ergo; the transverse plane of the sole of her foot is tending to rotate into the turn) resulting in the transverse base plane of the ski being at an angle with the vector of the Centre of Mass and Centre of Pressure that slightly less than 90 degrees.
4. Shiffrin’s right leg is rotated into the turn (internal rotation) as a result of the coupled Degree of Freedom with eversion that is tending to rotate the transverse aspect of the base of the ski into the turn. The combination of the coupled movements eversion and vertical axial internal rotation of the leg is called pronation.
5. The Centre of Pressure on Shiffrin’s left foot is under the heel with the result that the transverse aspect of the ski is tending to rotate away from the turn (ergo; Shiffrin’s left foot is seeking ground or snow contact with the tripod base of the sole of her foot).
6. The outer or lateral aspect of the top of the cuff of Shiffrin’s ski boot is applying a corresponding varus force that is tending to push Shiffrin’s knee towards the outside of the turn.
7. Shiffrin is using the rotator muscles of her pelvis to control the transverse position of her knee to oppose the Centre of Pressure and Varus loads that are tending to rotate her left ski away from the turn.
8. Shiffrin’s pelvis is rotated about her right leg to the outside of the turn.
9. In order to establish a dynamically balanced base of support on her left foot and set up the over-centre mechanism that uses the external forces acting on her to drive torque forces into the turn in multiple planes Shiffrin needs to move the Centre of Mass and Centre of Pressure from the head of the first metatarsal (ball of the foot) of her right foot to the head of the first metatarsal (ball of the foot) of her left foot. In other words, Shiffrin must apply force to her foot against a plantigrade reaction force. She only has a brief opportunity lasting a fraction of a second when her ski is momentarily flat on the snow in which to pronate her left foot sufficiently to move the Centre of Pressure to the ball of the foot. This means more than pronating the foot. It means maximally pronating the foot. It is not possible for the foot to pronate while a ski is on edge. Hence the sage advice through the years, “establish an edge before you turn (steer) the outside ski”.