# EDGE CHANGE INERTIA + ROCKER ROTATION INERTIA

As I was in the process of writing this post, a FaceBook group on skiing posted a link to an article From PSIA: Examining Transitions. The article is based on a presentation last fall by US Ski Team Head Men’s Coach, Sasha Rearick, in which he shed new light on transitions (1.).  While Rearick did shed light on some events associated with transitions, as with previous efforts by others on this subject, Rearick failed to shed light on the mechanics and physics associated with edge change.

As I explained in my last post, transferring the weight from the outside foot and ski of a turn to the inside foot and ski in the transition phase sets in motion what I call the Eversion/Internal Rotation Cascade that rotates the base of the ski into a transient moment of full contact with the surface of the snow between changing to the new (downhill) edge.

At the start of the transition leading up to ski flat between edge change, the center of pressure (COP) of the weight of the body applied by the sole of the inside foot will be under the heel where it is aligned on the proximate center of the ski. In this configuration, the force applied to the ski by the skier is working with gravity to rotate the ski.

The post left off by showing how rotational inertia will tend to make the ski continue rotating about the uphill edge past ski flat and penetrate into the snow surface on its downhill aspect as shown in the graphic below.

Rotational inertia will tend to make the inside edge of the new outside ski automatically rotate into the turn except for the fact that the force FW applied by the skier is on the wrong side of the new edge.

The graphic below has a dashed red reference that is parallel with the snow surface.

If the force FW applied by the skier is still aligned on the transverse center of the ski, it act will act to oppose edge change as shown in the graphic below. When the axis of rotation of the body of the ski changes with a change in edges, the transverse aspect of the base of the ski and the platform under the skier’s foot will tend to accelerate into an eversion translation. But this can only happen if the associated biomechanics are not interfered with by the structures of the ski boot.

The graphic below shows the change in the mechanics of rotation associated with edge change.

At the start of the transition, movement of the mass of the skier’s upper body is in phase with the downhill rotation of the ski and the force FW applied to it. But when the ski changes pivots at edge change and the mass of the skier continues to move downhill, the force FW applied to the ski will tend to rotate it back to ski flat; i.e out of the turn, unless the point of application of force FW changes during ski flat as shown in the graphic below and COM of the skier is aligned with force FW.

………. the angle between the platform and force you apply to it, the platform angle, must be 90 degrees or smaller.  – page 19, The Ski’s Platform Angle, Ultimate skiing; Le Master

The shift in center of pressure from the heel to the ball of the foot in a turn sequence seen in pressure studies of expert skiers is well documented (2., 3., 4). What the studies are really confirming is the use by expert skiers of the Two Phase Second Rocker mechanism to rock (tip) the outside ski on edge and control the edge angle during the load phase of a turn sequence.

Since the limit of the position of the application of force by the foot in relation to the inside edge of the outside ski is the center of the ball of the foot the effect of ski width underfoot and stand height should be obvious. Both rotational inertia and torque will increase as the width of a ski underfoot (profile width) is reduced and stand height increased. When Ligey says he creates pressure, he is creating far more than just pressure.

While LeMaster appears to recognize the importance of a platform angle less than 90° for edge control and, to some degree, the effect of stand height, the explanation offered for superior edging is that this can be attributed to waist width and stand height making skis more like ice skates.In my next post, I will discuss the role of Turntable Rotation in setting up a platform under the body of the outside ski for a skier to stand and balance on while maintaining edge angle.

1. http://eliteskiing.com/2017/03/31/from-psia-examining-transitions/
2. WHAT THE TWO HIGH PRESSURE COPS IN THE UNIVERSITY OF OTTAWA STUDIES MEAN – https://wp.me/p3vZhu-1fV
3. IMPLICATIONS OF THE UNIVERSITY OF OTTAWA PRESSURE STUDIES –https://wp.me/p3vZhu-1e2
4. AN INDEPENDENT STUDY IN SUPPORT OF THE UNIVERSITY OF OTTAWA FINDINGS – https://wp.me/p3vZhu-1gR

# GIRARDELLI AND STENMARK DEMONSTRATE BALANCE ON THE OUTSIDE SKI

In my last post, I discussed the movements elite Ski Pros make to balance on their outside ski.  I used Big White Ski Pro, Josh Foster as an example and reproduced his key comments from his YouTube video, Strong Platform.

Since Foster was skiing on moderate terrain, his speed is the equivalent of slomotion in comparison to typical World Cup speeds. For this post I am providing a video clip of Marc Girardelli and Ingemar Stenmark from the 1987 World Championship SL in Crans Montana, Switzerland. The video will allow you to compare the movements that create balance on the outside ski at race speeds to Foster’s movements at recreational speeds. I added reduced speed clips at the end to allow the rapid extension movement to be more easily seen.

I don’t believe there is any question that Marc Girardelli and Ingemar Stenmark can actually balance on their outside ski, especially in view of Girardelli’s statement: –

Once you can balance perfectly on the outside ski, everything else follows.

Note that the movement occurs above the gate as Girardelli and Stenmark approach the rise line and it mainly involves a rapid extension of the knee. According the predominant view, as articulated in the mental model of ski teaching and coaching, a quick extension is an unweighting movement. If this were true, why would the best skiers in the world unweight their outside ski above the gate?

What Foster, Girardelli, Stenmark, Shiffrin, Hirscher and all the best skiers in the world are really doing is loading and engaging a dual rocker system by applying a high impulse load to their outside foot at ski flat between edge change. Without knowledge of the associated mechanics, biomechanics and physics, no amount of observation will provide insights as to what is really happening. This is why 30 years after the World Championships at Crans Montana, what racers like Shiffrin, Ligety, Hirscher and other World Cup greats are doing remains a deep, dark mystery.

In my next post, I will introduce you to the Rockers.

# A SKI PRO DEMONSTRATES BALANCE ON THE OUTSIDE SKI

I have long maintained that the main reason skiers and racers ascend through the ranks to the elite is because they are able to stand and balance on their outside ski using the same natural processes of balance we were born with. My theory leading up to the Birdcage studies in 1991, was that those who are able to stand and balance on their outside ski do so by creating what amounts to solid ground under their outside foot through the application of a combination of rotational forces to the ski. It is the combination of these forces that has the effect of cantilevering the ground acting along the running length of the inside edge of the outside ski, out under the base of the ski underfoot.

I have also maintained that skiers who can stand and balance on their outside ski, don’t fully understand how and why they can do this.  So they can’t explain what they do, let alone teach it. It’s also why they don’t understand why other skiers have trouble balancing on their outside ski, something they can easily do.  Thus, Ted Ligety talks about ‘creating pressure’ while Mikaela Shiffrin talks about ‘getting over it’. This may be all they need to know. But it doesn’t help those who want to know.

Yesterday, I found an excellent YouTube video demonstration of the movement and timing associated with balance on the outside ski (1) by Big White Mountain Ski Pro, Josh Foster. Foster provides a real life visual example that most skiers can relate to. His demonstration also provides a reference I can use for future posts. To date, this is the only description I have come across that accurately describes some of the main elements.

While Foster misses a key point, he gets the role of rotation of the outside leg in combination with edge angle, right.

His comments from various parts of the video appear below. The number preceding each comment is the number of seconds into the video. The link to Fosters YouTube video is at the end of the post (1).

• 0.25 – For any structure to be in balance, it starts with a really strong platform. Skiing is no different than that. I need a strong platform.
• 0.43 – So, I need a good strong platform from the snow up so that I am balanced.
• 1:04 – But here’s how I create this platform or this foundation that I want to ski on.
• 1:11 – But it comes with a turning of the lower body. Watch how I turn my leg here. That  combination of turning also puts my ski up on its edge. So when my ski is on its edge and I turn my leg, that’s what creates that solid platform or that foundation that I am looking for.
• 1:53 – I need that platform first so I can be better balanced all the way through the turn.
• 2:14 – We do it with turning the lower body and getting balanced on those edges.

The 3 frames below are from Fosters’ video.

In the first frame below, he is approaching what I refer to as the moment of truth. This is the point where the new outside ski goes flat on the snow between edge change.

In the frame below, Foster’s new outside ski is flat on the snow. Notice the quick extension he has made in the knees since his stance in the first frame. This move is the most important part of the sequence that sets up balance on the outside ski. The move, which I will describe in the next post, is an impulse heel-rocker-forefoot loading move. This move must be made just as the outside ski is going flat on the snow. If you watch carefully, you will see all good World Cup racers make this move as they approach the rise line above a gate.

The fact that Foster does not even mention this impulse move suggests that he may not even be aware he is making it. Some ski pros and coaches confuse this move as unweighting. In fact, it is the exact opposite. It is a high impulse loading move. It tensions the forefoot and loads the inside edge under the ball of the foot. The high impulse load tips the ski on edge and causes the shovel to hook into the turn. It also starts the outside leg passively rotating internally (into the turn), from the foot up. You can see the rotation starting in the Fosters left leg.

In the frame below, Foster’s leg has switched gears and is actively rotating the outside leg from the pelvis down. This is the action that cantilevers the GRF acting along the running surface of the inside edge out under the base of the ski. This is possible because the internal rotator muscles of the pelvis have different origins of insertions on the pelvis than the hamstrings. The two muscle groups are complimentary while having a synergistic effect on balance and edge control.

1. Ski Tips: Josh Foster – Strong Platform   https://www.youtube.com/shared?ci=a8b5HRupcoA

You can reduce the speed on YouTube videos to 0.5 or 0.25 from Normal using the Speed menu item shown below. Slower speeds will allow you to see the timing of Fosters extension impulse loading move.

# LIGETY ANALYZED STEP-BY-STEP

In future posts, I am going to analyze video of Morgan Petitniot of France and Whistler-Blackcomb Ski Pro, Matt Williams, and identify the issues that are preventing them from reaching the level of competence on skis that they are both capable of. I am going to do this in parallel with a step-by-step analysis of Ted Ligety; one that will include analyses of Mikaela Shiffrin and Federica Brignone. If my theory of what Ligety, Shiffrin and Brignone are doing is correct, all their techniques should ultimately all have one thing in common; walking.

Today, Ligety talks about “stepping on the inside ski while it is still on its current (uphill) edge” and the need to “create pressure”. What does Ligety mean when he says he needs to create pressure? For what purpose? Create pressure how and on what? Is he referring to creating competitive pressure on himself to win? Or is he taking about putting pressure on a physical object? If the latter, is he talking about putting pressure on the shaft of his ski boot? What does creating pressure have to do with starting the next turn? If that is what he wants to do, why doesn’t Ligety just stop resisting with his outside leg at the end of a turn and allow his torso to cross over and fall into the new turn as many advocate? Meantime, Shiffrin talks about “getting over it”. Getting over what? A cold? A failed relationship? If she is talking about getting over her foot, what part of her foot does she want to get over?

These are all good questions that need good answers. But more than simply asking Ligety and Shiffrin why they are saying these things, we need to know what they are doing and for what purpose.  While the actual process behind what Ligety and Shiffrin are doing will probably turn out to be relatively simple, understanding their actions requires at least a basic knowledge of the associated principles of functional anatomy, human movement and physics. Until such time as someone analyzes what Ligety and Shiffrin are doing, applies their findings to skiers who are not skiing the same way and produces the same result, no one has any way of conclusively proving what Ligety and Shiffrin are doing. A good starting point for the analytical process is to ask what activity involves taking a step, getting over it and creating pressure? The answer? Walking and running.

Standing upright and balancing on two feet (bipedalism) is an extremely complex activity. Standing on one foot is infinitely more complex. The ease with which we are able to perform these activities, with no conscious effort, belies the complexity……. until something affects us that causes  difficulty standing upright and especially walking. The problem is that for all but a small group of skiers, ski equipment, especially the ski boot, can present problems for the hard-wired mechanisms of walking and upright balance unless steps are taken to ensure this does not happen.

What I came to appreciate, after years of searching for the answer, is that the best skiers stand on the outside ski by creating a platform that is an extension of the snow. How does a ski on its edge, with air under the remainder of the ski, support the weight of a skier? I got an early clue when I took a lesson from a ski pro the same winter I was struggling with my skiing after switching to new plastic ski boots. After watching me ski, she told me to put pressure on the ball of my outside foot. I didn’t understand what she was trying to get me to do. Until taught to do otherwise, the human tendency is to stand upright on both skis with the weight of the body felt under the heels. Because I was doing exactly that, I wasn’t able to put pressure on the ball of my outside foot. Eventually, I came to understand that putting pressure on the ball of the foot, as part of the sequence of events at ski flat, aligns the anatomical balance/force axis of the foot so that it crosses the inside edge of the outside ski in front of the foot and crosses the outside edge of the ski behind the foot. In this configuration, the anatomical balance/force axis runs diagonally across the ski. Aligning the balance/force axis diagonally across the ski is what Ligety and Shiffrin are doing at ski flat at the end of the transition.

The diagonal axis is the equivalent of a gas pedal for a turn. The actual pedal is like a button about an inch in diameter under the ball of the outside foot. The harder a skier presses on the gas pedal button, while rotating the leg into the turn, the greater will be the turning force applied to the  ski. I designed the Roll Over Trainer to serve as a feedback device to train skiers how to find the gas pedal at ski flat and press down on it in the turn.

In the next post I will talk about the events that need to happen at ski flat so we can find the gas pedal and step on the gas.

# CORRECTION OF ERROR IN MATTS’ NOTES

Matts’ Notes

Feelings in my body when the trainer is used correctly:

Activation of the glute muscles and a fairly relaxed quad. Further feelings of the glutes (not quads) being pulled on by the upper body is also in the correct place with a flexed hip and shoulders rounded forward. I’ve also noticed the importance of the controlling of the other leg, using it to practise releasing what would be the old outside ski as I move over onto the new turning foot.

COMMENT: Monopedal stance, that results from Roll Over, extends the fascial tension in the foot from the balls of the foot to the pelvis through foot to core extension, where it is felt in the glutes. Pulling the shoulders outward and forward, while rounding the back, extends the tension from the ball of the foot to the shoulders resulting in tension above and below the pelvis. It is this configuration that produces the quiet upper body.

# LIGETY’S ROLL OVER EXPLAINED: THE SIMPLE VERSION – PART 3, THE REVOLUTION BEGINS

This post marks the World debut of the revolutionary Roll Over Trainer. I designed the Roll Over Trainer to enable skiers to train the movements of Roll Over in a controlled indoor environment. Roll Over initiates rotation of the ski into the turn about its horizontal and long axes in a manner that extends snow reaction force under the transverse axis of the outside ski to create a base of support for the foot while aligning resultant and snow reaction forces in opposition to each other. This creates true skier balance in accordance with Newton’s Third Law; for every action, there is an equal and opposite reaction. The translation of the plane that the foot is supported on provides the impetus for the foot to initiate multi-plane rotation of forces into the turn.

The first two users talk about their experience with the Roll Over Trainer in their own words.

__________________________________________________________

Morgan from FONT-ROMEU Ski resort in the south of France. Morgan is the first skier in world to build and test a version of the Roll Over Trainer.

Morgan’s words

FOR ME, IT IS THE PERFECT DEVICE TO TRAIN THE MOVEMENT SEQUENCES WE ALL NEED TO OPERATE THE SKI AND SKIERS. IN OTHER WORDS, IT IS REVOLUTIONARY!

Often, I was wondering how to train without snow and indoor : and like dreams come true David made my year.

It was in April 2015. And I remember That I said to me:”Now I can work on technique and improve my skiing”

So I built the David’s invention, the Roll Over Device. I started training with it and I am totally impressed by how it run.

You can train on the good sequence of movements needed to engage the physics and biomechanics and forces acting with us and not against us.

That is very interesting is you can not cheat or be helped by the centripetal force.

That I can feel :

– When I lose balance and why (What I missed in the sequence of movements)

– How and how much I need to move my joints to reach the monopedal-pronated-balanced position.

– I can feel the need about other movements to improve balance as counter rotation of the pelvis, and feel the consequences of the right movement of my upper body

– How exactly I have to swing my free leg to assist and help my balance

– Also, I notice that a small amount of move can make me lose balance

– And above all : As I can feel several time the good sequence of movement on the device, I can control it in skiboot, without the liner. And have a look to check if anything prevents the needs of my feet and leg in multiple planes.

__________________________________________________________

Matt Williams, Ski Pro, Whistler-Blackcomb Ski School: Matt is the second skier in the world and the first in North America to test and train on the revolutionary Roll Over Trainer.

Matt’s words

Some notes on the new trainer:

Firstly about the trainer itself. It feels much more like a ski with less rolling of the top plate. The contact plates on top are key in relating the feeling to the movements when wearing boots and skis. Using the trainer with a balance aid on the inside or outside are helpful. Also using a balance aid on the inside can promote loss of remaining in the targeted reference point on the front of the foot (for me personally).

Other notes:

Training the movement of moving forward and up on to the foot in the trainer to create a tipping on the trainer creates immediate positive feelings on skis. When performed correctly, it feels like the trainer tipping inside allows for continuation of the leg turning which has already started from loading the head of the first metatarsal. Tipping the trainer inside with a lateral movement, having not loaded on the front of the foot (ball of the foot) creates feelings of weakness and quad muscles engage to stabilise.

Feelings in my body when the trainer is used correctly:

Activation of the glute muscles and a fairly relaxed quad. Further feelings of the quads being pulled on by the upper body is also in the correct place with a flexed hip and shoulders Rounded forward. I’ve also noticed the importance of the controlling of the other leg, using it to practise releasing what would be the old outside ski as I move over onto the new turning foot.

__________________________________________________________

The principles behind the Roll Over Trainer

The normal kinetic flow in gait is from supination of the foot and leg (inversion of the foot, external rotation of the leg and plantarflexion of the ankle) in the unweighted state to pronation of the foot and leg (eversion, internal rotation of the leg and dorsiflexion of the ankle) in the fully weighted state.  Either the outer aspect of the heel or forefoot, makes contact with the ground first. Then the foot rotates on the multi-axial joint of the ankle about the initial point of contact until the load transfer points under the heel and heads of the five metatarsal are in contact with the ground.

When Ligety steps on his inside ski while it is still on its current (uphill edge), he sets the normal kinetic flow pattern in motion. At initial contact, the structure of the stance or support foot is loose. After the foot has achieved full contact, it becomes progressively tensioned by physiology tighteners as the ankle dorsiflexes and COM moves towards the balls of the feet. As the ankle dorsiflexes an intrinsic mechanism in the foot and leg causes the foot to progressively pronate; the sole of the foot everts  (turns outward, away from the center of the body) and the leg rotates internally (towards the center of the body). Contrary to the position of some experts in skiing, it is not normal for the knee to track straight ahead. This has been known and proved since at least 1950.

In order to more easily appreciate how translation of the ski width profile affects foot function, I  designed the Roll Over Trainer to train skiers in the correct movement pattern at ski flat at the end of the transition phase. In the spirit of the mission that I started 40 years ago to bring principles of science to ski technique, I have no intention to patent the Roll Over Trainer or derive any commercial or financial benefit from the Roll Over Trainer. I am making the principles and design freely available to the world for anyone to make, use and enjoy for the benefit of the sport of skiing.

Based on feedback from Morgan and Matt, I have just completed a 2nd generation Roll Over Trainer that has the capability for users to tune it to replicate the feel of Roll Over on different pistes.

# LIGETY’S ROLL OVER EXPLAINED: THE SIMPLE VERSION – PART 2

Part 1 left off with the inside ski flat on the surface of the snow after it had completed its rotation about its current (uphill) edge when pressure was applied to the ski by stepping on it. The current or uphill edge was the point where snow reaction force S was acting. The pressure W, applied under the heel of the inside leg and foot, on the proximate center of the ski, was offset from S resulting in a moment arm that tended to rotate the ski downhill. This rotation was opposed by a force exerted against the inner aspect of outside of the boot shaft  by the inside leg being abducted (moved outward) by the thigh as shown in the insert in the graphic of Ana Fenninger below.

When the ski rotates into contact with the snow surface, rotational momentum wants to maintain the rotation.

If the piste is firm or icy, there will be little or no penetration into the surface as the ski moves beyond full contact with the snow surface as it changes edges.

One way or another, there will be a translation of the plane of the base  of the ski into a different plane as it changes edges and begins to rotate about the inside edge of what will become the outside ski of the new turn. Translation is part of the event that I call Roll Over.

If the pressure stays in the center of the ski as it changes edges and translation starts, there will be a problem. Even though COM will eventually pass the axis of rotation of the new edge, translation will be resisted by the Pressure applied to the center of the ski. This is the literal Moment of Truth. If the Pressure stays on the center of the ski, force exerted on the inside of the inner side of the boot shaft will cause translation to occur against the Pressure that continues to rotate the ski out of the turn. What has to happen for Pressure and Translation to be in phase, so Roll Over can occur, will be the subject of Part 3.