HEEL/FIRST METATARSAL PRESSURE DIFFERENTIAL


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:

  1. The uphill edge underfoot is on the lateral (small toe) side of the foot.
  2. 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.

COP 2

(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).

Ideal

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).

IMG_3669

IMG_3668

IMG_3667

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.

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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.

 

Figure 3

In my next post, I will explain how the pressure differential drives rotation and how this is the engine of the outside ski.

4 comments

  1. You do understand that CoP (center of pressure) is the average point of force? The average point of force cannot produce a force vector.
    “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).”

    You are dancing around the inter relationship of balance center of pressure and muscle activation. Leaning forward doesn’t necessarily put more weight on the forefoot. Leaning forward and increased muscle activation in the gastroc soleus will shift the center of pressure forward with balanced maintained. Contraction of the gastroc soleus without leaning foward will shift the center of pressure forward and the center of mass will stay were it was creating a tendency to fall back wards.

    1. “You do understand that CoP (center of pressure) is the average point of force?”
      Absolutely. I went into a fair amount of detail on the issue of point center and how only one COP can exist in the foot. COP is the weighted point center of GRF. From a theoretical perspective, COP is a point no physical area.

      “You are dancing around the inter relationship of balance center of pressure and muscle activation.”
      Not really. In the stance phase of gait following weight acceptance, dorsiflexion accommodates the anterior advance of COM. The anterior advance of COM drives dorsiflexion to a point where the proximate center of the proximal tibia is aligned vertically over the proximate center of the head of the first metatarsal. At this point, the soleus has been lengthened in EC to the point where it becomes resistive and arrests dorsiflexion.

      “Leaning forward doesn’t necessarily put more weight on the forefoot.”
      I concur. This why the correct stance in skiing is critical.

      I think we are in agreement. Correct?

      1. There are some things that are the same in gait and in skiing and there are some things are different. In both tension in the Achilles will shift the center of pressure under the foot anteriorly. This is what i was referring to when talking about the inter relationship between muscle activation and center of pressure. Skiing is differnt than gait in that skiing is essentially static squating. While squatting you still have to keep your center of mass under your center of pressure or you will fall over. (In turning you have to lean into the turn because of centrifugal force, but it’s still balancing the center of mass and center of pressure.)
        So, if you lean forward while skiing, you have to activate the gastroc soleus to keep you from falling forward. If you lean backward you have to activate the anterior tibial muscle to keep from falling backward. This is independent of the rigidty of the foot.

        “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).”

        The above i disagree with. When you describe a force vector you give its magnitude direction and its point of application. The arrow you drew in the diagram does not describe a force that is happening in reallity.

      2. You are using sophistry to misrepresent the issues. Please read all of my posts on the issue of COM and COP. And also please wait until I have finished my discussion of the issue of the effect of the structural beam before you start jumping to conclusions

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