EDGE CHANGE INERTIA: WHY THE TRANSITION PHASE MATTERS


One of the most important events in the turn sequence is edge change. Yet, it is rarely mentioned in technical discussions. One of the few references I was able to find on edge change is in the CSIA Technical Reference which states:

Edge Change = Balance Change: Changing edges requires a change of balance.

Edge change occurs during an unbalanced, controlled fall in the transition phase that leads to the development of a balanced position on the outside ski as it crosses the fall line in the bottom of a turn. Properly executed, edge change leads to the development of a platform under the outside ski for the skier to stand and balance on.

The edge change sequence starts in the transition phase when a skier begins to transfer weight from the outside (downhill) ski to the inside (uphill ski). At the start of the transition, the edges of the inside ski are uphill and on the lateral (little toe) side of the foot. From a perspective of the gait cycle, the base of the ski is inverted (turned inward towards the center of the body). This is the normal configuration when the foot is unweighted in the gait cycle. The foot strikes the ground on the lateral (little toe) side and rotates about it’s long axis in the direction of eversion to bring the three points of the tripod of the foot into contact with the ground. As the foot everts, the leg rotates internally through torque coupling in the subtalar joint. The normal kinetic flow from foot strike to the support phase in mid to late stance is one of inversion of the foot/external rotation of the leg to eversion of the foot/internal rotation of the leg. Put another way, the human lower limbs will naturally rotate into a turn so long as the biomechanics are not interfered with.

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.

The Eversion/Internal Rotation Cascade

Transferring the weight from the outside foot and ski to the inside foot and ski in the transition phase sets in motion what I call the  Eversion/Internal Rotation Cascade. When the cascade starts, the force F W applied to the ski by the foot  by the weight of the body will impart rotational inertia as the ski rotates about the pivot point formed by its inside edge.

For the sake of simplicity, the stack of equipment between the sole of the skier’s foot and the snow is represented by a rectangle in a 3:2 ratio where the stand height is 50% higher than the width (FIS maximum stand height = 93 mm – maximum profile width = 63 mm). Sidecut is also not shown.

The following graphics show the sequence of the Eversion Cascade. Note: Internal rotation of the leg is not shown in this sequence.

The first graphic below shows the moment or torque arm ma that is set up by the offset that exists between GRF from the firm piste acting at the inside edge and the point where the center of pressure of the weight of the body acts in the plane of the base of the ski. The large red arc shows the radius of rotation. The small red arc shows the radius of the moment of force. In this sequence, the ski is rotating downhill away from the pivot at the uphill edge.

When the base of the ski comes into full contact with the surface of the snow, rotational inertia, will make it want to continue rotating about the uphill edge and penetrate into the snow surface on the downhill aspect. If the force FW applied by the weight of the body is still aligned on the transverse center of the ski, it will oppose edge change.

In my next post I will discuss how the Second Rocker affects the mechanics of edge change at ski flat.

 

4 comments

  1. Dear David,

    This is brilliant. To highlight it may be even more important in skiing than in gait cycle to strike with a slightly inverted foot. This is the little toe edge and as with gait the slightly inverted foot gives the body proper chaining reaction to establish postural stability on the outside ski as the weight is transferred from old outside ski to new outside ski. (Emily Splichal, “Walking Gait Assessment The most functional movement assessment?” min 10:00+-) https://www.youtube.com/watch?v=8kNo-cJcacU

    A skier who is does not first establish postural stability on the new stance foot, read new outside ski will find it neurologically difficult if not impossible to go forward with the parallel turn. Stop frame video will show how the body will compensate in a variety of inefficient or impractical ways, many of which are taught. Perhaps readers know the teaching phrase “to fall” or “throw ones self” into the turn. The old Mambo was likely another variation – it looks cool.

    Only after neurological and physical homeostasis are established with postural stability centered on the new outside ski (little toe edge) can the neurological system engage the body in an orderly transition to bearing pressure on the inside edge of the new out side ski as your diagrams and words so accurately describe. If I am not mistaken this is one of the things you like about Lito Tejada-Flores’s approach. Harald Harb expands on this concept as well.

    The next problem for skiers is how to align their bodies so they can get the new outside ski leg to support both their weight and angle of the foot in the last diagram above. Looking forward to continued discussion, thomas

    1. The next problem for skiers is how to align their bodies so they can get the new outside ski leg to support both their wait and angle of the foot in the last diagram above. Looking forward to continued discussion, thomas

      > It’s coming.

  2. Excellent transition description for the feet. Ski flat is a transient moment for recentering. I am interested in the resultant postural response as associated with lower body leg rotation and upper body stability in a countered position.

    1. “Ski flat is a transient moment for recentering”
      I would amend that to state, “Ski flat is a transient opportunity for recentering; an opportunity that is usually missed”.

      Although I did not possess my current level of knowledge in 1990 when I formulated my hypothetical model of the mechanics, biomechanics and physics associated with edge change, I understood the basic mechanism. The primary purpose of the 1991 Birdcage studies was to test this hypothesis. Once I saw the mechanism, it became obvious and should been obvious to anyone studying and analyzing ski technique with a critical eye. However, almost 30 years later, it still seems to eluded everyone.

      My last post came after a lot of thought as to how to best approach what is a very complex issue. I am going to proceed in small steps. The input and guidance of those such as yourself will be very helpful and much appreciated.

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