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 show 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.

2 comments

    1. There will be a series of posts on this subject after which I will continue with my analysis of Mikaela Shiffrin. I am trying to make an extremely complex subject as simple as possible. Any comments or suggestions are appreciated because I don’t know where the gaps are between what I know and what the readers of my blog are at. Anyone who has read the posts on how I went down this ‘slippery slope’ knows that I was a huge proponent of custom moulded insoles for ski boots. In fact, after much experimentation I did find a way to cast the foot in a functionally pronated position. Still, as I started to see the potential downside of insoles I went away from anything under the arch that might interfere with it’s dynamic action. New research just published is now confirming what I figured out almost 25 years ago; interfering with the dynamics of the arch is a bad idea. The problem is that ‘pronation is bad in skiing’ appears to have started with wrong assumptions that started an out of control train. It seems that people assumed that the proponents of the ‘stop pronation movement’ were right and they jumped on board the train when they saw economic opportunities. Once this happened, it became hard to stop the train or even slow it. I will try and do at least one post a day for the next while until all the pronation pieces are in place.

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