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.







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.