Centre of Pressure

PRONATION: FORCES IN THE FEET

In order to appreciate why pronation is crucial to skiing you have to know something of the forces at play between the foot and the snow.

Hidden inside ski boot, out of sight, is where the real action is going down in the world’s best skiers. But it is also inside the ski boot where the attempts of most skiers to ski like the world’s best fall short.

In order to understand what is playing out in the background, you have to know how balance works. In balance, everything centres around the relationship of Centre of Mass to Ground or Snow Reaction Force and Centre of Pressure.  Excluding Centre of Pressure from the balance equation is like giving a lesson in geography and saying New York is in North America. It doesn’t really give specifics of where New York is. Similarly, any discussion of balance or edging forces in skiing without including Centre of Pressure (C0P) renders the discussion meaningless.

Conclusions about what is happening in skiing based on what is visible; edge angle, body angulation etc., tend to  invariably be wrong because everything happens from the feet up in snow/feet interactions. Top up or top down interactions above the level of the feet in the form of such things as knee movements cannot produce balance synergies.

Despite the admonishment to know all the forces at work in skiing, little attention appears to have been directed to the place where the rubber meets the road; the feet. In fact, I have yet to come across a discussion of ski technique and/or the biomechanics of skiing wherein the forces in the feet get more than a passing mention. Like Rodney Dangerfield, the feet “don’t get no respect. You know what I’m talkin’ about?” And despite the rare mention of ‘pressures in the feet’, I have yet to find a specific reference to the ground zero centre of force in the feet; Centre of Pressure. It is not clear to me why this force is not mentioned. Centre of Mass, Centre of Pressure and Ground Reaction Force are an integral part of every discussion on studies pertaining to balance. Without including CoP, any discussion of balance is irrelevant. This is especially true in skiing where the path to Ground (Snow) Reaction Force, the foundation of postural or balance responses, is defined by the narrow band of the inside ski edges.

Let’s start by looking at Centre of Mass and Centre of Pressure in quiet bipedal, (two-footed) erect standing. The graphic below shows a typical pressure pattern produced in quiet bipedal standing by what is called foot pressure mapping. The term pressure is used instead of force because the force applied by the boney structures of the foot that support the weight of the body is distributed or spread through the soft tissue between the bones and the supporting surface. Hence the applied force manifests itself as pressure.

Bipedal Standing

In quiet bipedal standing, approximately 50% of the total applied pressure is under the heel of each foot. The remaining 50% of the pressure is applied under the five balls of the feet and toes. Centre of Pressure is shown on the centre axis of each foot where it would normally be in quiet bipedal standing. Centre of Mass is just in front of the base of the shin but behind the Centre of Pressure. Forward movement or excursion of the Centre of Mass will provoke a balance response that will push the balls of the feet towards the floor by extending the ankle joint. This action will push the Centre of Mass back towards the heel. Should Centre of Mass pass Centre of Pressure, a forward fall will result.

In the next post we will look at how the relationship of Centre of Pressure, Centre of Mass, Snow Reaction Force and ski width underfoot affect the ability to assume a dynamically tensioned base of support in skiing.

BALANCE ON THE OUTSIDE SKI: IT’S ABOUT BALANCING TORQUES

Showing balance in skiing as opposing vertical forces that meet at the inside edge of the outside ski might look good on paper. But, there’s one small problem (actually a big problem) it could not possibly work this way.

For starters, the edge of a ski with sidecut isn’t a straight line. It’s a curve. The narrowest part of the sidecut (the waist) must lie under or close to the ball of the foot. But the actual force applied to the edge by an world class skier is not applied at the narrowest part of the sidecut. The force is applied across 2 points of the curve of the sidecut to the inside of the turn. In addition, the force applied to the ski must be a torque or twisting force and not just in one plane. The  torques must be applied simultaneously in multiple planes. Ron LeMaster is right when he says the angle of the transverse aspect of the ski must be 90 degrees or less with the resultant force. The reality is that the angle is tending to be less than 90 degrees with the resultant force because of the torque that is tending to rotating the base of the inside ski about what……………..? About the inside edge at the waist.

One more thing. And this is the key to skiing. The mechanisms that create a dynamically balanced base of support and edging and steering forces on the outside ski of a turn are the same. There is no way this could be otherwise. I will expand on this later. For now let’s look at some basic sketches that show the mechanics of edging.

Edge Mechanics

SKIER BALANCE: EDGE ANGLE

In his book, Ultimate Skiing, Ron LeMaster says  that once the forces of skiing are understood, everything else makes sense. I would qualify this statement and say that until the forces of skiing are understood, nothing makes sense and any assumptions are nothing less than wild guesses.

In his book, Le Master says 2 things that I believe  are fundamental to skiing:

1. Most of the expert skiers’ weight is balanced over the outside ski and,

2. The edge angle of the ski with the applied force must be 90 degrees or smaller.

In point 2, it is my position that the angle must be less than 90 degrees. Although he doesn’t appear to say so, I am assuming that LeMaster is referring to the edge angle of the outside ski of a turn.

On page 20 of his book, FIGURE 2.3, a and b show two different alignments of opposing forces, R and S. In Figure a the angle of the transverse aspect of the base of the ski with the snow reaction force S is 90 degrees. In Figure b the angle of the transverse aspect of the base of the ski with the snow reaction force S is greater than 90 degrees.  LeMaster doesn’t provide details on what force R represents. As explained my my last post, in order to assess balance in skiing as it pertains to a dynamically balance base of support one needs to know the relationship of Centre of Mass (CoM) and Centre of Pressure (CoP) with the Snow Reaction Force. One also needs to know where the ball of the foot and centre axis of the foot are positioned in relation to the inside edge of the outside ski.

While all of the above are relevant, the key issue, which neither LeMaster’s book or any other literature I have read on ski technique doesn’t touch on, is the nature of the forces associated with a dynamically balanced base of support that would act to cause the transverse aspect of the ski base to assume  an angle with the applied force that is less than 90 degrees. Here LeMaster provides all kinds of clues that he is standing on the answer, but not seeing it. For example, on page 44 (Janica Kostelic) he describes how one of Kostelic’s strengths was her ability to maintain contact with the snow through the transition (from one turn to another) with perfect flexion moves so she could get pressure on the edge early in the turn. He goes on to say that Kostelic also knew when and how to use her inside ski to advantage. LeMaster is so close, yet so far.

As I will explain in future posts, Kostelic and all the other great skiers including Mikaela Shiffrin and Ted Ligety, more than simply getting pressure on the new outside ski early in the turn, apply forces to the new outside ski (current inside ski) that sets up and over-centre mechanism that allows them to use external forces acting on them to drive 3-dimensional forces into the new turn. They set this mechanism up in the transition from the current outside ski to the new outside ski, before they apply steering forces to the ski .


References: Ultimate Skiing – Ron LeMaster