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.

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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.

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.