THE EFFECT OF LIFT PLATES – CONTINUED


In skiing,  a myriad of complex issues are associated with riser plates that elevate the foot above the base of s ski. FIS regulations for 2013-2014 permit a maximum stack height of 100 mm total between the base of a ski and the sole of a racer’s foot.  Because of the complex nature of the issues, I am going to use a simplistic model to explain the primary effects of riser plates.

NOTE: Check FIS regulations for current stack heights.

Platform shoes, high heels and similar footwear that elevate the sole of the foot above the supporting surface, tend to be make the wearer susceptible to ankle sprains when a lateral thrust or cutting move is made off the stance foot. The high centre of the ankle joint in relation to the supporting surface makes the ankle susceptible to lateral rotation and twisting above the contact surface of the sole when angular forces are applied. As the ankle progressively rolls over, the twisting force dramatically increases due to the resulting over-centre mechanism. This is called an inversion sprain because the sole of the foot turns inward towards the centre of the body. In the days when skiers used low-cut leather boots, the experts, who could make their edges hold on hard pistes, would call it ‘falling off the edge’ when their outside foot and ski rolled downhill, away from an edge set.

The model I am going use for my explanation assumes that the feet and legs have been aligned and fixed in the magical neutral position. Technically, a neutral position means that the joint that underlies the subtalar joint (what most people think of as the ankle joint) has been effectively rendered non-functional.

In a neutral position it doesn’t really matter whether the foot is a block of wood or a marvel of anatomy, the force applied by the weight of the body resulting from the force of gravity applied on the mechanical line will impress a portion of the weight to the proximate transverse centre line of the foot. For the following explanation the foot will be considered as solid entity like a block of wood. In a neutral position, both feet would be under the femoral heads. For the sake of simplicity I am showing one foot with the mechanical line vertical to gravity. Static references are shown with dashed red lines.  In skiing the force are more complicated.

The sketch below shows a schematic model of a right leg. As shown in my previous post on this subject, the mechanical line has a ball joint at the top (pelvis) with the subtalar joint below the ankle at the bottom. Both joints allow for rotation in the plane facing the reader. The subtalar joint acts in two coupled planes. For the following explanation only the effects associated with rotation of the foot about an axis below the edge of its inner aspect are considered. Fa is the force applied to the base of foot from the force of gravity acting on the mechanical line. Ma is the length of the moment or torque arm resulting from a line from the pivot axis that is perpendicular to the force vector of Fa.

1In the sketch below, the foot model has rotated counterclockwise 10 degrees about the pivot from the original configuration. The original configuration prior to rotation is superimposed in light grey over the new rotated configuration. Note that the mechanical line as a whole and the centres of the ball and subtalar joints have dropped in relation to the pivot point. The vector of force Fa has shifted to the left and is now angled towards the left hand side of the base of the foot model. The moment arm, Ma, that drives the rotation, has grown longer.  This is called an over-centre mechanism because reversing or unwinding the rotated configuration requires that the load that created it be overcome. As the rotation progresses the mechanics associated with reversing direction become increasingly unfavourable.

2In the sketch below a lift plate has been added to the bottom of the foot model. The previous rotated configuration is superimposed in light grey over the new configuration. Note that the mechanical line as a whole and the centre of the subtalar joint have not changed significantly in relation to the position in the previous sketch. But the vector of force Fa has shifted significantly further to the left and is now almost at the left hand edge of the base of the foot model. In addition moment arm Ma, that drives the rotation, has grown significantly longer.

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The sketch below compares the original unrotated configuration (light grey)  to the rotated configuration with the lift.

4Preventing the outside foot of a turn from pronating by fixing the foot in neutral or otherwise obstructing pronation with arch supporting insoles and/or injected and heat formable liners ensures that any force applied by the balls of the feet will be on the outside turn aspect of the inside edge of the outside ski and that the tibia cannot rotate into the turn. The mechanics described above would be similar to that of a situation where a skier gets inside, ends up losing contact with the snow of their outside ski then re-establishes contact. Although the constraints imposed on the foot and leg by the structures of a rigid ski boot would probably make 10 degrees of rotation unlikely, having the applied force on the outside turn aspect of the inside edge of the outside ski will almost guarantee mechanics similar to those described above.  Clearly lift plates can have a positive effect but only if the moments forces acting on the ski are going into the turn and can be dynamically balanced by muscular effort mediated by the skier.