Roll Over


The Two Phase Second Rocker (Heel to Ball of Foot) described in the previous post is dependent on inertia impulse loading. A good discussion of the basics of inertia and momentum is found in Inertia, Momentum, Impulse and Kinetic Energy (1.)

Limitations of Pressure Insoles used in Skiing

A paper published on May 4, 2017 called Pressure Influence of slope steepness, foot position and turn phase on plantar pressure distribution during giant slalom alpine ski racing by Falda-Buscaiot T, Hintzy F, Rougier P, Lacouture P, Coulmy N. while noting that:

Pressure insoles are a useful measurement system to assess kinetic parameters during posture, gait or dynamic activities in field situations, since they have a minimal influence on the subject’s skill.

acknowledge limitations in pressure insoles:

However, several limitations should be pointed out. The compressive force is underestimated from 21% to 54% compared to a force platform, and this underestimation varies depending on the phase of the turn, the skier’s skill level, the pitch of the slope and the skiing mode.

It has been stated this underestimation originates from a significant part of the force actually being transferred through the ski boot’s cuff. As a result, the CoP trajectory also tends to be underestimated along both the anterior-posterior (A-P) and medial-lateral (M-L) axes compared to force platforms.

Forces transferred through the cuff of a ski boot to the ski can limit or even prevent the inertia impulse loading associated with the Two Phase Second Rocker/Turntable Effect. In addition, forces transferred through the cuff of a ski boot to the ski intercept forces that would otherwise be transferred to a supportive footbed or orthotic.

Rocker Roll Over

In his comment to my post, OUTSIDE SKI BALANCE BASICS: STEP-BY-STEP, Robert Colborne said:

In the absence of this internal rotation movement, the center of pressure remains somewhere in the middle of the forefoot, which is some distance from the medial edge of the ski, where it is needed.

Rock n’ Roll

To show how the Two Phase Second Rocker rocks and then rolls the inside ski onto its inside edge at ski flat during edge change, I constructed a simple simulator. The simulator is hinged so as to tip inward when the Two Phase Second Rocker shifts the center of pressure (COP) from under the heel, on the proximate center of a ski, diagonally, to the ball of the foot.

The red ball in the photo below indicates the center of gravity (COG) of the subject. When COP shifts from the proximate center to the inside edge aspect, the platform will tilt and the point of COP will drop with the COG in an over-center mechanism.

A sideways (medial) translation of the structures of the foot away from the COG will also occur as shown in the graphic below. The black lines indicate the COP center configuration of the foot. The medial translation of the foot imparts rotational inertia on the platform under the foot.

Two Phase Second Rocker: The Movie

The video below shows the Two Phase Second Rocker.

Click on the X on the right side of the lower menu bar of the video to enter full screen.

The graphic below shows to Dual Plane Turntable Effect that initiates whole leg rotation from the pelvis applying multi-plane torque to the ski platform cantilevering reaction force acting along the running edge of the outside ski out under the body of the ski. A combination of over-center mechanics and internal (medial or into the turn) application of rotation of the leg from the pelvis, counters torques resulting from external forces.







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.


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


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.

Screen Shot 2015-11-24 at 11.16.14 AM



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.

Matt 1

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.


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.

Fenniger Abduct

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

Rotational Momentum


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.



Ski Flat SRF

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.



“Yet a further problem relates to the efficient transfer of torque from the lower leg and foot to the footwear. When the leg is rotated inwardly relative to the foot by muscular effort, a torsional load is applied to the foot. Present footwear does not adequately provide support or surfaces on and against which the wearer can transfer biomechanically generated forces such as torque to the footwear. Alternatively, the footwear presents sources of resistance which interfere with the movements necessary to initiate such transfer. It is desirable to provide for appropriate movement and such sources of resistance in order to increase the efficiency of this torque transfer and, in so doing, enhance the turning response of the ski.

“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.” – MacPhail, US Patent No 525,265, 350 November 30, 1993 in reference to conventional ski boots and ski technique

COMMENT: The application of torque to the outside ski of a turn resulting from balancing on the outside leg (monopedal stance) and applying rotational force to the leg from the pelvis engages and loads the shovel of the ski while enabling edge hold at angles of the base with the snow in excess of 45 degrees.

As more racers like Federica Brignone adopt a technique based on the application of torque to the outside ski, they will suddenly begin to appear on the podium.

Here is a clip of Brignone in very slow frame-by-frame. Watch carefully as she Rolls Over and applies rotation to her outside ski as evidenced by movement of her knee into the new turn. As she crosses the fall line, her outside ski will be seen to converge at the shovel with her inside ski then lock up with the snow, closing the kinetic chain and translating rotation of her leg into rotation of her ski about its length into the the turn. Brignone is using the powerful rotators in her pelvis to wind her outside ski into the turn like a corkscrew against external torque that is unwind her outside ski out of the turn resulting in a loss of edge grip.

Molto Bene Federica!



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.

Fenniger Outside

(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.

Fenniger inside 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.

Fenniger Abduct

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.




Roll Over 1


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.