THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: THE TURNTABLE EFFECT


Neither the Two Phase Second Rocker (heel to ball of foot rocker) described in THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: HEEL/FOREFOOT ROCKER (1.) or the Rotating Turntable Effect described in THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: THE ROCKER/TURNTABLE EFFECT (2.) are new. They have been the trademark technique of the world’s best skiers for decades. But the ability to engage the associated mechanics and biomechanics requires what amounts to a perfect storm that typically occurs early in the development of a young skier. More than raw athletic talent, discipline and dedication, the ski boot appears to be the critical factor that determines who acquires the ability to engage these effects.

In working with skiers and racers who are gifted natural athletes, it has been my consistent finding that a change in ski boots that compromises neuromuscular function will result in the body adopting compensatory mechanisms that can reduce competence on skis to survival reactions. Given sufficient time, the survival mechanism will become imprinted until a point is reached where it is accepted as normal by the body. Even after the cause is corrected, it can take years of retraining to erase and replace survival motor patterns. A good example of this is what happened to Mikaela Shiffrin at the start of the 2014-2015 World Cup after changes were made to her boots in the fall of 2014. Fortunately, she was able to revert to her previous boots over Christmas and quickly restore her former competitive competence.

Four synergistic mechanisms associated with the mechanics of edge change result in the creation of a platform under the outside ski that a skier can stand and balance on. These are:

  1. The Two Phase Second Rocker (heel to ball of foot rocker) Mechanism
  2. Impulse rocker loading that occurs at edge change
  3. The Over-Center mechanism, and
  4. Open and Closed Chain Whole Leg Rotation; The Rotating Turntable Effect.

The most critical and seemingly least appreciated and understood mechanism in skiing is the mechanics and biomechanics of whole leg rotation.

LeMaster recognized the role of whole leg rotation in skiing in his book Ultimate Skiing when he stated under Twisting Actions (p 13) that torques play important roles in turning skis and holding them on edge. In Chapter 7, Turning the Skis (p 107), LeMaster states, Rotating the leg inward generally rolls the ski on its edge, too, combining the increase in the edge and platform angles—often a desirable combination while acknowledging that leg rotation is powerful and can produce large torques through the whole turn. But LeMaster does not describe the mechanics associated with whole leg rotation in this context.

The Center of Rotation

Whole leg rotational force is applied to the femur primarily by the gluteus medius.

The most important source of rotational power with which to apply torque to the footwear (ski boot) is the adductor/rotator muscle groups of the hip joint. – US Patent 5,265,350 MacPhail

Rotation of the femur is transferred through the tibia where it is applied through its lower or distal aspect to the talus that forms the ankle joint with the tibia.

The graphic below shows a skeleton of the foot aligned on a fixed reference axis (dashed line).The graphic below shows the same skeleton of the foot rotated 15° medially (towards the center of the body) against the fixed reference axis (dashed line).

The graphic below shows the relative displacement of the heel and forefoot in relation to the fixed reference axis (dashed line).

The graphic below compares the displacements of the heel and limit of the forefoot at the end of the second toe with horizontal lines in the center of the graphic. The lines show that the end of the second toe displaces almost 4 times as much as the rearmost end of the center of the heel during whole leg rotation of the foot. Hence the advice in my post, THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: PRESS AND POINT THE BIG TOE (3.), to point the big toe in the direction you want to go.

Open Chain Rotation vs Closed Chain Rotation

  • Open Chain Rotation – occurs when the foot can rotate in the horizontal plane in conjunction with the rotation of the whole leg from pelvis. In ski technique, this is referred to as steering.
  • Closed Chain Rotation – occurs when the foot is fixed on its long axis and whole leg rotational force is applied to the foot from pelvis.

Open Chain whole leg rotation acting about the axis of the ankle joint in combination with a Two Phase Second Rocker induced Over Centre mechanism are prerequisites to the application of Closed Chain Rotation. The emerging profile created by the steering angle of the outside ski as it crosses the fall line below a gate yields important clues as to the technique of a racer.

In my next post, I will discuss Closed Chain Rotation applied to the outside ski in a turn and the transfer path of torques applied to the foot by the leg through the boot-binding interface to the ski.


  1. http://wp.me/p3vZhu-2at
  2. http://wp.me/p3vZhu-2bb
  3. http://wp.me/p3vZhu-25W

3 comments

  1. Simply said, coupling of the shank and talus in foot pronation provides the skeletal framework for the closed chain rotation of the leg and foot. Reflex synergistic neuromuscular responses add power in the turn initiation to the soon to be turning leg in an ascending postural response.

    Kim Hewson, MD

  2. Loved the article. Really provided me with a much more detailed understanding of a technical foundation of CSIA technique summarized as “The turning effort is led by the lower body”. Thanks again for all your passion and for sharing your insights and experience.

    1. Thanks for the input.

      In reading through volumes of material, I see signs that some were seeing some of the issues I am raising. But for reasons that puzzle me, no one seemed to take the initiative to pursue them to a conclusion and connect the dots. A good example is LeMaster’s comment under Forces, Pressure, and Momentum on page 4 of Ultimate Skiing; “A quick extension to unweight the skis uses internal forces to push the skis downward against the snow”. A quick extension (of the knees) unweights the skis while simultaneously pushing the skis downward against the snow? This doesn’t make sense. And what pushing the skis downward against the snow achieve? LeMaster appears to be providing his interpretation of the impulse loading mechanism elite skiers use to generate the two phase second rocker action. But for some reason, he didn’t take steps to investigate.

      It is my hope that by providing explanations of issues like open and closed chain whole leg rotation, those such as yourself will take up the cause and advance the technical and analytical aspects of skiing.

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