THE MECHANICS OF BALANCE ON THE OUTSIDE SKI


In the next series of posts, I am going to focus on the single most important, but least understood, aspect of skiing; skier balance, in particular, the ability to balance perfectly on the outside ski. Given its univerally recognized importance in the ski culture, it is both perplexing and disconcerting that little attention appears to be given to the study and analysis of the mechanics associated with balance on the outside ski.

For decades, the worlds greatest skiers, including Patrick Russell and Marc Giardelli, have stressed the importance of standing on the downhill (outside) ski. Giardelli said that once you can balance perfectly on the outside ski, everything else follows. The ability to stand on the outside ski and balance perfectly on it, implies the same mechanics of balance we engage in when we balance perfectly on one leg when we take a step to move forward in locomotion. Balancing perfectly on one leg requires a stable surface under the entire plantar aspect of the foot to provide a source of GRF. The reason why the ankle-foot complex has a triplanar joint system is so the tripod-like structure of the foot can seek stable ground. This is the classic text book definition of one-footed or monopedal balance and the standard for studies on balance performed on one foot.

The problem is that there is no ground or any form of stable GRF under the outside foot of a turn when the ski is on its inside edge other than the GRF acting along the portion of the edge in contact with the snow surface and a small portion of the base of the ski adjacent the edge. If elite skiers such as Russell and Giardelli really can stand on their outside ski and balance perfectly on it the question is where is the source of GRF coming from that acts to support weight of the body expressed on the plantar foot?

By 1990, I had an explanation in a hypothesis I had articulated. According to my hypothesis, elite skiers extend GRF acting along the portion of the inside edge of their outside ski from the snow to the base of the ski by rotating their outside leg and foot into the turn. This action causes the base of the ski on the outboard side of the inside edge to pivot upward about the portion of inside edge underfoot with sufficient force to support the weight of the body. The Birdcage studies done in 1991 were designed to find out if my hypothesis were right.

Balance on the outside ski is a Two-Step process

Having seen great skiers like Nancy Greene Raine and Toni Sailor ski with ease on pistes that would be difficult, if not impossible, for most skiers to hold an edge on, I was convinced that some skiers really could balance perfectly on their outside ski when it was on its inside edge, the same way that every skier could easily balance on one ski when the base of the ski was fully supported on a firm, stable surface.

I set out to try and figure out how this was possible. It took me about 10 years between 1980 and 1990, to formulate a hypothesis that explained the mechanics. Once I had an explanation, I understood why no one else had been able to figure it out.

Balancing on the outside ski does not adhere to the text book descriptions of single leg balance where a stable source of GRF under the plantar foot is assumed. The ability to stand on the outside ski when on its inside edge and balance perfectly on it, is a Two-Step Process. The key is that the Second Step is dependent on the First Step.  The First Step makes the Second Step possible. Without getting the First Step right within a very short window of opportunity, the Second Step is not possible.

Since my hypothesis predicted that sequence and timing is the critical, it was quite simple to prove my hypothesis with strategically placed strain gauges mounted in the Birdcage on discrete force plates positioned opposite the predicted force transfer points of the foot. The critical nature of the sequence was easily confirmed by preventing the First Step from occurring.

In my next post, I will discuss the Two Steps of the balance process and provide examples using screen shots and video clips from recent World Cup races showing the sequence in a turn where racers such as Mikaela Shiffrin make the two steps to balance on the outside ski.

 

 

10 comments

  1. It’s better when things are made simple. Try it. As far as balance in walking; it is not analogous to skiing. In walking the foot is planted on stable ground and the CoM is moving over and past the foot. In a ski turn, the support foot is stable under the CoM (core engagement) while the base of support is in motion on a sliding ski. Very different. Stabilizing the foot under the CoM does not require twisting of the foot or leg. Having a stable core to work from does facilitate twisting actions of the foot and leg though. Is it not possible that the ability to hold on a hard pistse has much to do with the amount of twisting force applied as well as the balance on that tiny bit of snow penetrated by the edge? Maybe pressure excesses can cause that tiny ledge of snow to chip off creating skid. Balance is not the only factor, just the fundamental one. It seems that you might have imagined a conclusion and designed an experiment to support it. Many variables are present, but not all are accounted for.

    1. Rather than respond to your comments, it is more productive to address the issues in posts, although most issues have already been thoroughly covered in existing posts. For now I will respond to your comment:

      “In a ski turn, the support foot is stable under the CoM (core engagement) while the base of support is in motion on a sliding ski.”

      > If by ‘the support foot’ you mean the foot that supports the weight of the superincumbent body, the fact that there is a substantially rigid element in the form of a ski bootbase/ski binding/ski system under the plantar foot is inconsequential. Whether one is standing on the top floor of a 100 storey skyscraper or the surface of hard snow, ground reaction force still originates in the ground. Even the best designed foundations will be subject to failure if the load they impose on the ground they are founded on is greater than the reaction force potential of the ground itself.

      When a ski is on one edge there is no ground on the outboard side of the edge. Therefor, there can be no Ground Reaction Force to act in opposition to the force applied to the base by the foot. With the foot supported on either its medial or lateral margin, an offset will exist between the ski edge and the applied force at the lower end of the load transfer axis. This will create a moment arm and a substantial moment of force even at G force loads of 1.5 Gs which most recreational skiers experience. To claim that the foot is supported with GRF on one on it’s margin is physically impossible.

  2. Ack! You keep me waiting. I have an idea where you’re going, given your past posts and clips of Shiffrin, and some on snow experimenting. Even though I cannot ski this year (both hips & R knee replaced this past summer) I can feel (imagine) it in my feet.

    1. GRF or grf is ground reaction force. One of the challenges in writing blog posts on skiing is that the ski culture has invented terms for things they don’t understand or don’t clearly understand. Given a lack initial scrutiny critical review, after sufficient repetition and time, these invented terms become accepted as fact and become entrenched. This is not unique to skiing. Trying to find a middle ground between correct technical terms for a discipline typically ends up causing confusion.

      A technical term like GRF assumes an applied force even when it is not mentioned because forces always occur in pairs. For the same reason Center of Pressure assumes an applied force even thought centre of pressure is the point center of (ground) reaction force which implies a corresponding center of applied force.

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