# LIGETY’S ROLL OVER EXPLAINED: THE SIMPLE VERSION – PART 1

In order to make it easy to understand the basic mechanism of Roll Over, I am going to use an overly simplistic model for illustrative purposes. The actual mechanics and biomechanics of Roll Over are much more complex. But I’ll start with simple principles and build on them.

The starting point to understand the mechanism of Roll Over is knowing in general terms the forces acting on the skis, especially across the edges. I’m using a graphic of Anna Fenninger because the head-on camera angle lends itself to showing the forces acting on her skis. The fact that Fenningers’ outside ski is holding and carving at a high base angle with the snow is typically explained as shown in the graphic below where R is the Resultant Force acting on COM and S is the Snow Reaction Force. According to the Critical Edge Angle theory, the angle of the base of the ski with the snow is irrelevant. So long as R aligns with S at 90 degrees to the base of the ski, the edge will hold.

(Click on graphics to enlarge them)

The fact of the matter is that no one has any way of determining exactly where Fenningers’ or any skiers’ Centre of Mass is in relation to the inside edge of the outside ski. That R is an attractive force, not a physical force, and sidecut is not even considered, further invalidates the Critical Edge Angle theory which really doesn’t explain anything. What we need to know is the forces acting across the edges of the ski. At this point in the left hand turn, Fenninger is about to start her transition by stepping on her inside (left) ski while it is still on its current (uphill) edge.

In the graphic below, the portion of the weight of her body W, that is acting on the inside ski, is under her heel, on the proximate center of her foot and ski. In this location W is offset to the outside or downhill aspect of the uphill edge.  Unless opposed, W will cause the ski to rotate and flatten on the slope of the hill. The insert in the upper left hand portion of the graphic shows the moment of force that acts to flatten the ski.

What is stopping Fenninger’s inside ski from flattening? An opposing moment of force created by Fenninger abducting her left leg. The closed kinetic chain created by the inside ski locked on its uphill edge causes abduction of her left leg to move her thigh into the hill. As it abducts, it exerts a force against the outer aspect of the shank of her boot as seen in the graphic below. The large angle at her knee, makes it appear as if the movement is originating at her knee instead of at the pelvis.

Stepping on the inside ski while it is still on its current edge and extending, primarily at the knee, releases the force applied to the shaft of the boot and causes the ski to start flattening on the slope of the hill. As the ski rotates, the body rotates in space with the ski about the pivot created by the current (uphill) edge. In the New York Times video, Ted Ligety on GS, Ligety talks about ‘creating pressure’. By extending his knee as he rotates in space, Ligety uses momentum and gravity to exert a force against the snow at ski flat that mimics the force of gravity by applying a force that is normal or perpendicular to the slope of the hill. It is like the trick of running against a vertical surface with sufficient speed so that if you throw yourself sideways at the surface, you can momentarily defy gravity and run horizontally along it.

The graphic below shows the rotation in space of COM stacked over the ski as it rotates about its current edge. The small inset to the right shows this occurring  on the slope of a hill. However, it is easier to view this movement horizontally. When the ski flattens on the snow, the rotational momentum will tend to maintain its rotation. In order for this to happen, the aspect of the ski opposite the pivot created by the current edge has to penetrate into the snow surface. On hard pistes, any penetration will be minimal. What needs to happen for the ski and skier to keep rotating? The pivot has to move to the opposite (inside) edge of the new turn and the aspect where the current edge is will have to …… rise up in relation to the pivot.

There’s a problem. If the Pressure on the ski is in the center it will act to flatten the ski on the snow surface no matter which edge the ski is pivoting about.

The Pressure exerted on the ski will oppose the ski from Rolling Over. Even the New York Times video shows the Pressure in the proximate center of the ski. What has to happen to enable the ski to Roll Over? That is the subject of my next post.

# LIGETY’S ROLL OVER EXPLAINED

In order to maintain the continuity of the ongoing discussion of Ligety’s Moment of Truth, I have changed the title of the series of posts on ROLL OVER to LIGETY’S ROLL OVER EXPLAINED. Although Roll Over is not exclusive to Ligety’s technique, the association will enable search engines to find and link the two series of posts.

# 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.

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.

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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.

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 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.

# WHY LIGETY AND SHIFFRIN’S SECRET MOVES REMAIN SECRET

The reality is that neither Ligety or Shiffrin appear to be making any attempt to keep their unique techniques a secret. To the contrary, of all the racers on the World Cup circuit, no one has been more open and forthcoming about their techniques than Ted Ligety and Mikaela Shiffrin.

In a series of Burke Mountain Academy YouTube videos posted in the summer of 2014, Shiffrin narrates a series of drills that she uses to hone her technique. She continues to comment on her technique in videos on a regular basis. Still, views of Burke videos are 5,000 to 10,000 compared to 500,000 to a million views for videos that advocate using the legs like 2″ x 4″s to hold both skis on edge to make pure carved turns.(https://www.youtube.com/user/burkemtnacademy)

In the New York Times video, ‘On Giant Slalom: Ted Ligety’ (http://www.nytimes.com/video/sports/olympics/100000002705897/on-giant-slalom-ted-ligety.html), the commentator states that for 8 years Ligety has been perfecting a new style of turning. As someone who is passionate about skiing and committed to advancing the sport as a science, that statement instantly got my attention. The commentator goes on to say, “the most noticeable element of Ligety’s style is seen mid turn where his body is horizontal to the hill”. The narrative on the video states, “Ligety’s path is smoother than that of his foes, who ski in violent fits and starts, making adjustments that spray snow”. Ligety, “I’m starting my turn earlier (while its still on its uphill edge) and finishing it later than the other guys”. The clean, low profile snow pattern off Ligety’s outside ski tells the story. The edges are slicing a clean path into the snow with no sideslipping.

Says Ligety, “I always been known for my big edge angles even on the old skis”. The angle of Ligety’s outside ski is shown in the NY Times video. I measured it at approximately 80 degrees to the snow surface.

The NY Times video compares Ligety to Bode Miller at the same point in a turn. Miller is spraying a large amount of snow off his outside ski, Ligety? Almost none.

Obviously, Ligety is putting very different forces on his outside ski than Miller. But in what way? And how is it that Ligety is able to make the edges of his outside ski hold at such extreme angles? That’s a question I put to members of a FaceBook group for serious skiers and ski professionals and coaches.

When force is applied to the outside with the base at an angler greater than 45 degrees to the snow, the component of sheer force that makes the edge slip is greater than the component of vertical force that makes the edge grip. Since as far as I know, Ted Ligety is bound by the immutable laws of the universe, there has to be a logical explanation for why he appears to be defying the laws of physics. Cracking the Ligety Code holds the promise to make skiing better for all skiers, not just World Cup racers. But interest from the members of the FaceBook group from those who should be motivated was the equivalent of a yawn and a “Why bother?”. The explanations from those who did respond were surprising, but not shocking, “It’s magic”, “Ligety is different” or “This is a dumb question”.

Although some critical issues are left out of their discussions, probably because Shiffrin and Ligety are not aware of these omissions, the superiority of their techniques, as evidenced by visits to the podium should serve as the impetus for those who are in the ski teaching and coaching professions to put Ligety and Shiffrin under a microscope, analyze their movements and integrate them into ski teaching methodologies and coaching world wide. The main reason this isn’t happening appears to be that those who should be motivated aren’t or, perhaps just not interested in putting in the effort. This group represents an influential camp that’s slowing killing skiing by discouraging the development and analysis of ski technique based on sound principles of science. Instead, they default to the scripted narrative that every skier and every country is different, that there are an unlimited number of ways to ski and that no one way is better than another. Every country has its own unique technique and they all work yada, yada, yada.

Ted and Mikaela don’t have secret techniques. They are ‘just different’.

# LIGETY’S ROLL OVER: HOW TED GETS EXTREME ANGLES

Here’s a highly edited video clip of Ligety that I created to show how Ted uses Roll Over.

The technique of the world’s best skier has always been about torsion, not simple leverage.

Watch carefully how Ligety starts to apply rotation to his uphill as soon as he starts the transition phase. As Ted crosses the fall line, his inclination reaches a point where the inside edge locks up with the snow and the shovel engages creating what I call a ‘comma hook’ that redirects his line while loading his outside ski like a springboard. The actual ‘loaded turn phase’ is a mere fraction of second after which Ligety is into the transition phase, off the edge of his old outside ski and accelerating forward towards the next gate. Anyone who thinks they can beat Ted or any racer using this technique with a pure carved turn needs to see a really good therapist.

Ligety’s use of Roll Over and torsion explains how he can reach extreme edge angles on his outside ski.

# WINNING BY A FOOT

Numerous studies have attempted to find a correlation between foot size and height. I believe there is a reasonable correlation between foot size and height if stature is considered. Ski racers tend to have moderate body masses. When I worked on racer’s boots, most female racers had a US ladies size 7 foot (US men’s 6). Male racers tended to have a US  8 or 9 foot size. Ski boot makers usually make prototypes in a US men’s size 9 then scale the shell up or down to create other sizes.

Lange recently introduced a size 4 US men’s race shell. Perhaps it is more accurate to state that Lange reintroduced a size 4 race shell.  After I learned how to read the shell mold codes forty years ago, I knew that Lange made size 4 shells. However, they only sold them in softer children’s boots in North America. The XLR Race boot was only sold in Japan. After I got the Canadian distributor to bring in a supply of size 4 XLR Race boots, I was inundated with calls from all over Canada from female racers with small feet who heard that a size 4 XLR was available.

In my experience, I was usually able to estimate the size of a racer’s foot by looking at their stature and height. Here is a list of 6 top World Cup racers in order of height.

Ted Ligety 1.80 m
Lindsey Vonn 1.78 m
Marcel Hirscher 1.73 m
Tina Maze 1.72 m
Mikaela Shiffrin 1.70 m
Anna Fenninger 1.66 m

Based on Lindsey Vonn’s stature, I estimate that she has bigger feet than both Tina Maze and Mikaela Shiffrin who are very close to the same height. Vonn is  close to the height of Ted Ligety. I estimate that she has at least a US ladies Size 8 to 8.5 while Ted Ligety’s foot is in the order of a US men’s 9. Anna Fenninger is 1.66 m and small boned. She probably has small feet, smaller than Shiffrin who probably has a size 7 ladies (6 men’s). Anna Fenninger’s foot will be no bigger than a size 6 ladies (US men’s size 5). Marcel Hirscher at 1.73 m probably has a size 7.5 to 8 US men’s foot.

With a ski that is 65 mm in Minimum Profile Width who has the winning feet? Ted Ligety and Lindsey Vonn.

# WORLD CUP TECHNICAL ANALYSIS

Now that the 2012-2015 World Cup season is underway, I plan to start interspersing analyses of the technique of racers such as Ligety, Maze, Vonn, Shiffrin, Hosp and Velez Zuzzuolva. There is an emerging trend towards the extension-pendulum effect technique that is the signature of Ligety and Shiffrin.

After indicating before the start of the season that she would make her debut in World Cup Super-G this year,  Shiffrin  recently announced that she would not compete in the Super-G in Val d’sere next week but would instead focus on GS. This came after Shiffrin’s worst drought since her the first of her 10 World Cup victories two years ago. I suspect that Shiffrin’s difficulties can be attributed to a change in some aspect of her equipment, most likely her boots. She looked much closer to her usual form in Are last Saturday. If the source of her problems was, in fact, in her boots and this has been corrected, I am confident that she will regain her form in the new year. Focussing on GS is a smart move by Shiffrin. If Shiffrin can dominate in GS she has an excellent chance of winning in Super-G.

One of the challenges in analyzing technique using video clips is that the camera angles and image quality are typically far from ideal. In addition, cameras often switch at key moments in a sequence, usually part way through the transition phase that starts at the end of a turn. Finding a sequence that illustrates the mechanics, biomechanics and physics of skiing can take hours of frame by frame analysis followed by additional hours of work annotating and animating the key images. More interesting than the racers who are very close to getting every aspect of a technique right are those who make the essentially the right moves at initiation only to have to resort to survival tactics usually due to interference from their ski boots.

Since extension is a hot topic on the internet, mainly due to Ligety’s very effective use of it, extension will be the subject of  my next post.