Right up until the point where Ligety begins to step on his inside ski while it is still on its current edge, he is applying very little pressure to his inside ski. Instead, he is using the stability of his pelvis arising from the dynamically balanced base of support on his outside ski to use his inside thigh like a lever to control the angle of his inside leg which amounts a dead stick at this point pressed against the inside turn aspect of his boot cuff. Because the physiological tightening mechanism of his foot associated with postural responses is not active, the pressure is under the heel, acting on the transverse running centre of the ski . This where the pressure will always be in situations where the physiological tightening mechanism of the foot associated with postural responses is inactive. In this state, the only cohesive structure in the foot resides in the hind foot. The pressure Ligety is applying to the heel of his inside foot allows him to use his inside ski like an outrigger to help him direct C0M to the inside edge of his outside ski.
Ligety’s outside ski is the source of resistance that is opposing the external forces that are tending to pull his CoM to the outside of the turn. As Ligety begins to extend on his inside ski while it is still on its current (inside) edge he destabilizes the mechanics that are driving his outside ski into the turn. At the same time, Ligety releases the muscle action of his inside thigh that is controlling the edge angle of his inside ski which is tending to rotate out of the turn (ergo – downhill). As these two things happen, Ligety’s Centre of Mass begins to move down hill and down towards the centre of the earth where it is drawn by the force of gravity. As Ligety’s inside ski rotates out of the turn in the transition phase, his Centre of Mass follows. Here is a video sequence that shows the approximates movement.
Here is a step by step photo sequence that shows these movements.
Frame 1 – This is the point just before Ligety begins to step on the current edge of the inside ski. Due the offset between the pressure applied on the centre of Ligety’s inside ski and its inside edge his ski is tending to rotate out of the turn. This force is opposed by the action of Ligety’s left thigh which is using his leg like a lever to press against the uphill aspect of the cuff of his ski boot.
Frame 2 – Ligety has started to step on the uphill ski while it is still on his current edge. As he extends his inside leg, he releases the pressure on his outside ski and the muscle force in his thigh that has been controlling the edge angle of his inside ski. The pressure under the heel of Ligety’s left foot starts his ski rotating out of the turn. From a biomechanical perspective, his left foot is ‘seeking 3-point ground contact’.
Frame 3 – As Ligety continues to progressively extend his left leg, he is simultaneously moving forward in the hips, something most racers fail to do.
Frame 4 – Ligety’s foot has flattened on the snow and his Centre of Mass is stacked on top of it. This is the point where the edge change occurs. Ligety will transition from the inside edge of his old inside ski to the inside edge of his new outside ski. This point marks the end of the transition phase and the start of the new turn. What happens next is critical. Depending on what he does next, the forces will either rotate Ligety’s out of the turn or into the turn. Since they are actually moments of force or torques, I call this point the Moment of Truth. It was at this point that the NY Times video: Ligety on GS, showed the pressure Ligety is applying on the proximate centre of his ski.
Frame 5 – If the pressure Ligety is applying stays under his heel or even on the centre line of his ski it will cause his ski to rotate out of the turn.
Frame 6 – The tendency of Ligety’s ski to rotate out of the turn will depend on the magnitude of forces opposing this movement. The only option Ligety or any skier has to counter the tendency of the external forces acting on him to cause the outside ski to slip is to increase the edge angle, usually by increasing the angle of inclination. The problem with this option is that a point of diminishing returns is reached where further inclination has the opposite of the intended effect.
At the point where Ligety’s ski flattens on the snow between edge changes there are two options in terms of which side of the inside edge of the ski the pressure applied by Ligety will end up. Put another way, there are two possible moment of force outcomes. Given the vagaries of human nature, there will be those who will take the position that each option offers advantages. Because of this they are, in effect, equal. In fact, I have already had this argument put to me. This is like positing that walking sideways or backwards, or crawling on all fours is equal to doing what we are designed to naturally do, walking forward using alternating single limb support. Being unstable and therefor balanced on 2 feet, is not the same as, let alone superior to, being dynamically balanced on a base of support on one foot. In my next post I will explain why racers like Ligety and Shiffrin prefer the latter option.