# 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

# 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.

In my next post, I will discuss impulse heel-rocker-forefoot loading.

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’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.

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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.

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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.

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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.

# THE IDEAL SKIER’S FOOT AND LEG

Do you have the ideal shape foot and leg for ski boots? Let’s find out.

Not long after I starting working on ski boots, I began to notice that the best skiers, those who made high performance skiing look easy, seemed to be able to ski in boots right out of the box that usually required major modifications for most skiers just to get their feet into. This small group of elite skiers consistently had very specific foot characteristics, especially the shape of their feet and legs. Although there were some exceptions, the feet of elite male skiers tended to be US size 8 or 9. Podborksi’s foot was a US men’s size 6. The feet of elite female skiers tended to be the equivalent of US men’s size 5 or 6 with some feet as small as size 4. As I worked on the boots of more National Ski Team racers I soon developed a reputation for being able to describe how a racer skied by studying their feet. On one occasion I was with a group of racers in the waiting area of a steak house. Unseen by me, Dave Murray came into the room. He snuck up on me while I was on my knees on the floor studying a racer’s foot. Dave put his bare foot in my face so to speak. Without looking up and without missing a beat I said, “What are you doing here Dave?”

The image below shows the characteristics I have identified as common to racers like Ligety, Shiffrin, Vonn and other World Cup racers. The characteristics of their feet and legs enables them to create the mechanics and biomechanics within the ski boot necessary for them to engage the external forces to drive their outside ski into a turn. Depending on a number of factors, including luck, racer’s with these foot characteristics can often ski in a boot right out of the box with minimal or no modifications. Modifications take their skiing to another whole level.  I believe that young racers such as Shiffrin make the connection with the right feel early in their career. Once a racer, or any sensitive skier, connects with the right feel, especially at an early age, they know as soon as they take a run in a new ski boot whether it will work for them. This group of skiers has the ‘magic touch’.

Here’s what the right shape of leg looks like in the cuff of a ski boot. I align the cuff so the leg is centred in the cuff when the skier is standing on two feet in the boot shells with the feet hip width under the pelvis. Note the amount of space on either side of the shin. This is critical for reasons I will explain in a future post.

In my next post I will show what problematic foot and leg shapes look like and the challenges presented in creating a functional environment in a ski boot.