A letter I wrote on December 31, 2000 in response to an article in a ski racing magazine.

Dear Editor,

Knowles is correct in stressing the importance of movement in skiing. But he failed to identify the key factor critical to effective movement. Ground.

Action [movement] is dependent on reaction [resistance to movement]. In accordance with Newton’s Third Law, every action must have an equal and opposite reaction. It follows that action potential must equal reaction potential. Reaction potential is limited by the quality of the ground or supporting surface that provides a source of ground reaction force. Therefore movement potential will be dependent on, and thus limited to, the reactive potential of the supporting surface [action = reaction]. No amount of conditioning or strength will enable an athlete to exceed the reactive potential.

Why is reactive potential important? Most activities occur on continuous hard surfaces where reaction potential is far in excess of action potential [action potential < reaction potential]. Here, the foot typically has unlimited access to ground reaction force because it acts in what amounts to direct contact with the ground. In this environment, the foot can adapt to the supporting surface and acquire a stable base of support achieving 100% loading of its tripod like contact points. In this relationship, physical strength and conditioning of the athlete is the factor limiting performance.

This is not the case in skiing. Here the source of ground reaction force is the snow surface and the racer is limited in terms of their ability to access it. Snow only provides a continuous source of ground reaction force when the racer is supported on both feet with the skis flat on the snow. When a ski is on edge, ground reaction force is limited to the contact area between the ski and the snow surface. Access to the ground [snow] is dependent on the ability of the balance system to negotiate a balance solution wherein the force acting at the sole of the foot is aligned in opposition to the force acting at the snow. This relationship is called the Centre of Pressure or COP.

Center of Pressure is the point centre of ground reaction force. It is where the racer is connected with the snow [ground]. In this relationship, the ball of the foot [1st MTP] is the key mechanical point. In order to act against GRF the 1st MTP must be able to act between the edge of the ski below the 1st MTP and the area of the ski edge in contact with the snow as defined by the sidecut. Several factors affect the ability to create this alignment of forces. Among them are the functional status of the foot and lower limb, the structures of the ski boot, the sidecut geometry of the ski and the elevation of the foot above the snow surface. All are important but the width of the ski at its waist is critical. It must be compatible with the transverse position of the 1st MTP in relation to the centreline of the foot. This dimension decreases in proportion to a decrease in foot size.

Consider this. Ski boots have a 70 mm wide sole plate standardized for bindings. Boot makers shrink the width of their foot lasts proportionally towards to the centre of the sole plate as the size of the foot decreases. This moves the position of the 1st MTP farther away from the edge of the ski. This has enormous implications on the ability of the lower limbs to access ground reaction force.

Here’s why. Assume 2 racers are on the same ski. One racer is a male with size 10 feet. The other a female with size 4 feet. For the male skier a waist width in the area of 62-63 mm will allow access to ground reaction force assuming foot function is not compromised [elevation of the foot also comes into play]. On the other hand, the female skier requires a waist width < 58 mm to access ground reaction force. If the ski does not meet these requirements CoP cannot exist by definition because an applied force cannot be aligned against a ground reaction force. Instead, a transverse moment arm will be set up between the 2 opposing forces. Any attempt to apply an active force against the distant ground reaction force will tend to rotate the ski about its point of contact with the snow. In this situation, reactive potential < than active potential. Muscle activity will be limited by this factor. Coordinated muscle activity cannot occur because the unbalanced moment of force acting at the ski [and by implication in the foot] will result in unbalanced moments of force in the joints of the lower limb system. Body postures must be generated through top down strategies originating in the torso that use concentric and isometric muscle activity. These tension driven strategies produce static postures that render the racer stiff and unable to absorb perturbations in GRF. Thus it matters not how well the athlete is conditioned or how effective their balance skills the factor limiting performance in this situation is reactive potential.

Skating is similar to skiing in that access to ground reaction force at the ice surface must occur through the skate blade. But there is a fundamental difference. In skating the blade is placed relative to the key mechanical points of the foot [i.e. the heel and 1st MTP] whereas in skiing the position of the ski edge is fixed in relation to the centre of the DIN standard sole plate interface with the centre of the foot referenced to the centre of the sole plate. The design of ski boots does not respect the same mechanical relationship that the design of skates does. It should.

The FIS should produce scientific research to support the current limits they have placed on waist width of skis. Such restrictions can severely impact the ability of skiers with small feet to perform while favouring skiers with larger feet. This could be considered a form of discrimination. The relationship of sidecut geometry to 1ST MTP positioning could also go a long way towards explaining why females have twice as many knee injuries as male skiers. At the very least this factor deserves a thorough investigation.

David MacPhail







  1. Not far off, considering this was 14 years ago. The only problem is in your paragraph 7, talking about the ski’s waist dimension and CoP. The CoP will always be at or near the edge of the ski, when the ski is on edge, especially on a harder snow base, which is where the waist dimension is especially important. I think what you’re talking about is the discrepancy between the skier’s Centre of Mass, in relation to the ski edge and CoP. From the CoP standpoint, the boot and ski simply become part of the distal limb anatomy. A skier with a smaller foot, with the 1st MTP located slightly more laterally relative to the medial ski edge, will still apply the force through the edge of the ski, which is where the CoP is. By definition, the GRF acts through the CoP, although it may be distributed over a larger plantar surface (like on a softer surface). The skier with the smaller foot, with it’s anatomical centre slightly more lateral, will need to lean sideways more to adjust their body CoM in relation to the CoP, for balance.
    I’m heading off to Wanaka, on the South Island, to ski next week. The season has just opened…!! Hoping for decent snow. Going to try Cardrona, and Treble Cone.

    1. Leaning or inclining will not address the issue. It is tilting as I will explain in a future post. In his book, Ultimate Skiing, the author Ron LeMaster, is literally standing on the answer when he says, “that platform (cut into the snow by the edge) has to be at an angle that is 90 degrees or less to a line from it to your centre of gravity”. I agree to a point. The platform has to be less than 90 degrees to the resultant force from CoM and the vector of CoP. The only way the platform cut by the (inside) edge (of the outside ski) and base of the ski could be less than 90 degrees is if the platform of the ski-boot base is tilted in relation to the sole of the foot. This is so obvious I find it hard to believe no one has figured it out.

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