PRELUDE TO LIGETY’S ROLL OVER EXPLAINED


In a series of posts, starting after this one, I am going to describe the mechanism of what I call Roll Over. This next series of posts will be the culmination of a series of posts on the mechanics, biomechanics and physics of Ted Ligety’s technique that started on February 19, 2014 with a post called LIGETY’S MOMENT OF TRUTH – STEP 1. The last post called LIGETY’S MOMENT OF TRUTH – STEP 5 – LIGETY STEPS UP THE PRESSURE was made on March 3, 2014. I first described the basic mechanism that I later called Roll Over in US Patent No. 5,25,350 published on February 3, 1993.

The difficulty in finding an easy way to communicate this action is the reason I did not take the LIGETY’S MOMENT OF TRUTH posts to a conclusion after the last post on March 3, 2014. In order to make Roll Over easier to appreciate and understand, I have designed a device that will enable a user to experience the feel of the events that are set in motion when the new outside ski changes edges and translates the foot towards the inside of the new turn as it everts the foot. As a preface to my explanation of Roll Over, I have provided excerpts below from my five posts on LIGETY’S MOMENT OF TRUTH. In order to acquire a complete understanding of the events surrounding Roll Over, the full content of all 5 posts should be read. If for some reason the links to the full posts don’t work, please copy and paste them into your browser.

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LIGETY’S MOMENT OF TRUTH – STEP 1

https://skimoves.me/2014/02/19/ligetys-moment-of-truth-step-1/

If indeed no other skier in the world carves the way Ligety does then I say everyone else is skidding, not really carving at all. If Ligety is carving the right way, and we want to ski like him, we need to know what he is doing and why he is doing what he does. Stepping on the uphill ski in the transition when it is still on its uphill edge is the first step towards better carving. If it were that simple, everyone would be skiing like Ligety within 5 minutes of viewing the NY Times video. But what Ligety is doing is anything but simple. Stepping on the uphill ski is just the first step. Several more steps are needed before Ligety gets to the point that I like to call The Moment of Truth.

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LIGETY’S MOMENT OF TRUTH – STEP 2

https://skimoves.me/2014/02/24/ligetys-moment-of-truth-step-2/

Given that Ligety’s inside turn (left) foot is ahead of his outside turn (right) foot and the angle of the left boot shaft is controlling the angle of flexion of his ankle, Ligety’s Centre or Pressure on his left foot will be under the heel and on the proximate centre line of the ski. It will stay there until his foot is flat or (i.e. – plantigrade) on the snow surface. It is at this point that what I refer to as the Moment of Truth occurs. The animation in the NY Times video got the part right that shows pressure (Centre of Pressure) acting on the proximate transverse centre line of Ligety’s left foot. What the NY Times didn’t show, and what is the most critical part of Ligety and Shiffrin’s technique, is what happens (or should happen) in the very brief interval during which Ligety’s foot is flat on the snow surface between edge changes and, thanks to Newton’s Laws, he is standing at a right angle to the slope. The NY Times also did not show the geometry of Ligety’s GS ski and its relationship with the key mechanical references of the foot. Twenty-seven years ago, in 1987, Professor Dr. M. Pfeiffer at The Institute for Athletic Sciences at the University of Salzburg in Salzburg, Austria said, “Correct positioning of the foot is more important than forced constraint and “squeezing” the foot. Pfeiffer also said, “If muscle function is inhibited in the ankle area (which is the role of the modern form-fitted ski boot), greater loads will be placed on the knee”. It appears that Dr. Pfeiffer’s words fell on deaf ears.

“The most important source of rotational power with which to apply torque to the footwear is the adductor/rotator muscle groups of the hip joint. In order to optimally link this capability to the footwear, there must be a mechanically stable and competent connection originating at the plantar processes of the foot and extending to the hip joint. Further, the balanced position of the skier’s centre of mass, relative to the ski edge, must be maintained during the application of both turning and edging forces applied to the ski. Monopedal function accommodates both these processes

“In skiing, the mechanics of monopedal function provide a down force acting predominantly through the ball of the foot (which is normally almost centred directly over the ski edge). In concert with transverse torque (pronation) arising from weight-bearing on the medial aspect of the foot which torque is stabilized by the obligatory internal rotation of the tibia, the combination of these forces results in control of the edge angle of the ski purely as a result of achieving a position of monopedal stance on the outside foot of the turn. (COMMENT – These biomechanics can only be set up when the new outside ski of a turn is flat on the snow surface).

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LIGETY’S MOMENT OF TRUTH – STEP 3

https://skimoves.me/2014/02/26/ligetys-moment-of-truth-step-3/

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LIGETY’S MOMENT OF TRUTH – STEP 4

https://skimoves.me/2014/02/27/ligetys-moment-of-truth-step-4/

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.

L foot on ski

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.

Neut edge

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LIGETY’S MOMENT OF TRUTH – STEP 5 – LIGETY STEPS UP THE PRESSURE

https://skimoves.me/2014/03/02/ligetys-moment…p-the-pressure/

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 (i.e., 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 (i.e., 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.

Here is a step by step photo sequence that shows these movements.

Edge Change

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.

 

Ligety

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 unbalanced 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.