I have been hinting at the mechanics of heel/first metatarsal pressure differential in posts for some time. But the lack of agreement within the different camps on ski technique on how and where forces should be applied to a ski has presented a challenge in terms of the ability to establish what amounts to a beach head from which to initiate a discussion of the mechanism that has enabled the world’s best skiers to literally balance on their outside ski and dominate ski racing while appearing to be super human. Even after the two University of Ottawa pressure studies established that every elite instructor, without exception, applied high pressures to the heel and head of the first metatarsal, I still have critics say, “Sure, that was what skiers did in 1998. But technique has changed a lot since then. Skiers like Hirscher probably put different forces on their skis”. Given the structure of the foot and the physiologic function of the lower limb, this argument is without legs. The great racers of the 1930s are remarkably similar to the best racers today. There is a reason why. They had a secret. But the secret is about to be revealed.
Setting up The Force Transfer Beam
During a turn and at the end of a turn, COP will always be under the heel of the inside foot for two reasons:
- The uphill edge underfoot is on the lateral (small toe) side of the foot.
- COM is behind the inside foot.
For reasons I will discuss later, the world’s best skiers move to the inside ski while it is still on its current edge and they extend to create pressure. Why do they need to ‘create pressure? They need to establish COP under the head of the first metatarsal by positioning COM over the head of the first metatarsal so they can load the LA (longitudinal arch) and stiffen it. Shiffrin refers to this as ‘Getting Over It”.
The graphic below shows the approximate trajectory of COP as pressure moves to the head of the first metatarsal in a sweeping arc. Technically, when the pressure under the head of the first metatarsal exceeds the pressure under the heel, COP is now under the head of the first metatarsal and no longer under the heel. But a perspective of 2 COPs is required to understand how the arched beam with discrete load points at each end, transmits force to the ski. In order to show two COPs I changed the colour of the COP under the heel to blue in the graphic below. The basic underlying principle I am leading to is based on lever mechanics. But it is not obvious yet how this works.
(click on graphics to enlarge them)
Once the structural beam has been ‘stiffened’ as shown in the grey portion of the foot in the graphic below, a force vector is established between the COPs under the heel (black) and the head of the first metatarsal (red). This is where things start to get interesting. The force vector is aligned diagonally across the width of the ski, passing through the head of the first metatarsal close to or directly above the inside edge (blue line).
Here are a series of photos that show what this vector looks like on a ski. The stainless steel rule indicates the alignment of the vector. Note that the vector angles downward about 3 degrees (2.7 degrees is optimal).
As the pressure under the head of the first metatarsal increases, the load on the portion of the ski becomes biased or unbalanced in relation to the loads on the tail and shovel. As I explained in a previous post, the sidecut geometry of a ski creates what amounts to an offset lever. The pressure differential in favour of the head of the first metatarsal is pushing the portion of the ski edge under the head of the first metatarsal down. This create a tendency for the limits of the edge along the sidecut to displace away from the load under the pivot shown at Pivot P. At this point, it is probably still not obvious how and why this works.
The graphic below doesn’t show the arched beam in the LA with the 2 discrete load points. But it shows the key mechanism – forward rotation of COM about the ankle that is arrested at the Shank Restive Angle. This is the same mechanism that enables humans to walk. And it is the same mechanism that enables racers to walk to the podium.
In my next post, I will explain how the pressure differential drives rotation and how this is the engine of the outside ski.