The short video clip below shows Mikaela Shiffrin in 2 different left hand turns. The video has been slowed and split into segments to show the different phases of the turn.
In the first segment Shiffrin is completing a left hand turn. She has a dynamically tensioned base of support on her right foot and leg that extends her foot core to her pelvis. Her right leg is extended. Centre of Pressure or CoP is under the ball of her foot. External forces acting on Shiffrin are tending to cause her ski to rotate about its inside edge into the turn. An associated torque coupled through the subtalar joint is tending to cause her outside leg to rotate on a vertical axis into the turn on a 1:1 ratio as a horizontal force. This is imparting a horizontal twisting force or torque (aka moment of force) that is coupled between the outside or lateral aspect of her heel and the inside or medial aspect of the ball of her foot. In addition, there is a force acting on a vector aligned through the heel and the ball of the foot that is aligned on an axis across the ski that falls on the outside turn aspect of the tail of the ski and on the inside turn aspect of the forebody of the ski.
The Centre of Pressure or CoP of Shiffrin’s inside foot is under her heel. Because CoP is towards the outside turn aspect of the inside or uphill edge, Shiffrin’s ski is tending to flatten on the snow; ergo her left foot is everting, seeking ground. This tendency is being resisted by her inside leg which is being used through the pelvis as a lever acting on her boot cuff to control the edge angle of her inside ski.
For reasons that will be explained in future posts, turns start with the outside leg extended and the ski base flat on the snow surface. Balance is a bottom-up process. In skiing, establishing a dynamically tensioned base of support requires contact of the tripod-like load points of the sole of the foot with the supporting surface; ergo the base of the ski must be flat or plantigrade with the snow surface.
As Shiffrin’s turn nears completion, she makes a transition to move her CoM forward and over the ball of the inside foot of the new turn. This is the equivalent of taking a step forward in walking. To prepare to step forward, Shiffrin squares her hips and upper body and arms with the current inside ski. Once her hips are squared, her skis must run straight until she has established a dynamically tensioned base of support on her left foot. As Shiffrin moves forward in the hips and extends her left leg, this action draws her right ski forward with her hips and eventually off the snow. While it may appear that she is pushing off her right foot this is an illusion. Unfortunately the coverage switched to another camera before Shiffrin could complete her extension. The same sequence follows with a sequence of stills.
The second left hand turn sequence shows the completion of Shiffrin’s transition to the point where she is fully extended on her left leg. In the freeze frame her ski is flat on the snow and she has established a dynamically tensioned base of support. She rotates her left leg internally from the hip. This sets up an over-centre mechanism that causes the external forces acting on Shiffrin to help to rotate her ski into the turn in multiple planes. In the next moment Shiffrin rapidly pivots her skis across her trajectory amplifying these forces.