LIFT PLATES – NEUTRAL VS. PRONATED


If there two issuse the ski industry appears to be in agreement on it is that the foot functions best when it is immobilized in neutral and that pronation is bad.

Pronation is a combination of  eversion (the sole turns away from the left-right centre of the body) and vertical axial rotation of the leg towards the left-right centre of the body. In the context of skiing, if the outside foot of a turn were to pronate, the foot and leg would rotate into the turn. But this should not be allowed to happen because pronation is bad. Let’s find out if this is true.

First, let’s revisit the stick men figures in FIG. 23 A and FIG. 23 B from my US Patent No. 5,265,350. The feet in FIG. 23 A are in neutral whereas the foot in FIG. 23 B is pronated. Note the angular relationship of the leg with the foot in FIG. 23B. It  has enabled the alignment of the force W from the Centre of Mass and W of ground reaction force in opposition with each other. The force vectors are at 90 degrees to plane of the supporting surface. Note that the forces pass through innermost aspect of the pronated foot in FIG. 23B whereas in FIG. 23 A the forces are acting at the proximate transverse centre of each foot.

FIG 23A - 23BThe sketch below is from THE EFFECT OF LIFT PLATES ON SKI MANEUVERS. When the foot is in a neutral it cannot pronate and the foot cannot evert. So it is impossible to align resultant and ground reaction forces so they are perpendicular to the transverse aspect of the base of the ski. In the sketch below the base of the ski is at an angle that is greater than 90 degrees to the vector of the resultant force R and ground reaction force GRF. In this relationship of forces a shear component of force will exist that will tend to make the ski slip out of the turn.

NeutralThe sketch below shows the same mechanical constraints as the foot in neutral above except that the foot has pronated. The foot has everted sufficiently for the resultant force R and ground reaction force GRF to align in opposition to each other at right angles to the transverse aspect of the base of the ski. The force vector from the mechanical line is exiting the lift plate above the intersection of R and GRF.

Pronated

The sketch below compares the effect of lift plates on pronated foot (black) and neutral feet (light grey).

Pronated overlay

FIS  regulations permit a maximum combined height of the sole of the foot above the base of the ski of 100 mm. This is about the average width of a US men’s size 9 foot. The sketch below shows a pronated foot with an approximated maximum allowable lift height.

100 mm

Things get even more interesting when sidecut and vertical axial rotation of the leg (femur) into the turn are added to the discussion. When the outside foot of a turn can pronate, enabling the the use of internally generated vertical axial rotation of the leg (femur) into the turn to counter externally generated torque, skiing becomes ridiculously easy.

4 comments

  1. Dave, I think you’re on the right track with this, but your Fa force is difficult to interpret. You have it, by default, in line with the long axis of the leg, but from a mechanical standpoint, it could be angled in a variety of ways, depending on the angle of the ground reaction force. I’ll send you a drawing by email showing my take on it.
    In human gait, the frontal plane horizontal (transverse) joint reaction force (JRFx) at the ankle is going to be roughly the same (but opposite) as the horizontal ground reaction force (GRFx) because the mass of the foot is negligible compared to the effect the mass of the body has on the ground reaction force. The sum of all the horizontal forces in the transverse direction = mass x horizontal acceleration of the ‘segment’ COM. There are only two horizontal forces…the horizontal GRF and the horizontal joint reaction force at the first joint (subtalar? or the ‘ankle’ joint?). So… JRFx + GRFx = mass of foot times its x-acceleration. Because mass is tiny, and because in gait the x-acceleration is small, the JRFx and GRFx forces are almost equal and opposite. In skiing, though, this force would be more difficult to model, because the mass of the ski/boot/foot would be substantial. IF THE FOOT IS IMMOBILE IN THE BOOT in the frontal plane (ie. prevented from everting/inverting) then you could consider, for modelling purposes, that it was one solid unit below the knee with a stationary Centre of Mass (ski/boot/foot/shank CoM) that would be located somewhere between the knee and ski base. In this case, the mass of the shank is only about 5% body mass, and the foot/boot/ski mass would bias the ‘segment’ CoM possibly below the foot. So if you then consider the shank/foot/boot/ski as a single unit, the moment arms for those horizontal forces would be small between the snow surface and the ‘segment’ CoM, but very large in comparison from the knee. Putting on a 5cm lift could stretch this snow surface-CoM distance by about 5cm, depending on the mass of the lift material and how that affected the CoM location. Then, the GRFx force would work around a much longer moment arm, and that moment would have an effect at the knee, since the same mechanical effect exists there…the sum of the forces at the knee = mass times x-acceleration of the ski/boot/foot/shank ‘segment’.
    I would think the skier would compensate for this by angling their body more into the turn to move the Resultant GRF closer to the shank/foot/boot/ski CoM to reduce the moments. I can’t see what a lift is beneficial. Who uses them?

    1. “Who uses lifts?” Virtually very skier, including yourself. The ski, binding, boot combination has always amounted to a stack of equipment between the base of the ski and the sole of the skiers foot that elevates the foot above the snow surface. Decades ago a device called the DerbyFlex was introduced that was supposed to allow the ski flex better underfoot. The DerbyFlex dramatically improved ski performance. Soon every racer was using the device. When racers started putting blocks of solid material between the ski top plate and the binding it became apparent that the performance increase was associated with height. Soon the race was on and the sky was the limit. Racers were reaching great heights above the base of the ski by adding lifts to both bindings and boot soles. My skis are 15 mm thick underfoot. The binding lift plate adds another 15 mm for a total lift of 30 mm in the ski system. My ski boot system adds another 37 mm between the boot sole and the top of the boot board for a total lift of 67 mm. Current FIS regulation allow 50 mm + 50 mm for a total lift of 100 mm. These regulations were introduced to REDUCE lift heights, not permit greater heights. My ski system stack height is 33 mm less than the total lift height permitted by FIS regulations. Insofar as immobilizing the foot, my position is that it is relatively easy to prevent the foot from pronating by limiting the ability of the arch to compress and the foot to elongate and the metatarsals to spread. In addition, the foot needs compressive forces across the 3 points of the tripod like base of the arches to functionally pronate, something that is only possible when the ski is flat on the snow. Even then, a skier has to create compressive force by aggressively extending and moving CoM forward. And this has to happen in a very brief interval. In my US Patent 5,265,350 I used the phrase ‘without delay’ with regards to pronation. While it is relatively easy to substantially prevent pronation, it is exceedingly difficult to prevent supination. In fact, when force is applied to the outside foot in STJ neutral the neutral position will facilitate inversion. The preceding issues aside, as you point out, trying to use a simplistic explanation of complex events can leave out many relevant issues. Once I have seen your sketch I will know better what the issues are that you are raising.

  2. I could be wrong. But I suspect that in Ligety, Shiffrin and in most elite skiers who use the principles I am describing it is a combination intuitive changes made to their boots and luck. Having the right foot and leg shape is also critical. One of the few racers who I suspect knew what he was doing with equipment was Stenmark. I can’t recall where I read, it but the story was that Warren Witherall noticed that Stenmark had cants either on his boot soles or bindings that were high side out and he tried to tell Stenmark he had it wrong. Wrong? I don’t think so. This was at the point where Stenmark could and often did win runs by seconds. Stenmark knew exactly what he was doing. Once a skier gets the feel for the mechanics I am describing they will know right away if a piece of equipment interferes. The problem is that most skiers have never felt the mechanics. So they think that fighting the forces is what it is all about. In my next post I will talk about the role of leg and foot shape and characteristics. After that I will start to post on boot modifications. There is no point in trying to ski like Ligety until you have boots that let you ski that way.

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