THE EFFECT OF LIFT PLATES


By their own (FIS) admission, boots are too complex, and plates are, too. – Black Diamond: The Deaf Ears of the FIS | Ski Racing 11/18/11.

Assuming the preceding statement is accurate, it raises more questions than answers. What does the FIS mean by too complicated? Too complicated in what respect? Is the FIS saying that boots and plates are too complicated to understand their effects?  The esoteric aspects of both issues are indeed complicated. I have already addressed the complex issues pertaining to the design and fitting of ski boots in US Patent No. 5,265,350.

So I will attempt to address some aspects associated with the introduction of lift plates between the  sole of the user’s foot and the base of the ski. The last time I checked (2013-14), FIS regulations limit the maximum stack heights from the base of the ski to the highest point on the binding interface to 50 mm (later reduced to 43 mm)  and from the sole of a ski boot to the sole of the foot to 50 mm for an aggregate maximum stack height of 100 mm (later reduced to 93 mm). I assume the FIS does not count the thickness of any socks worn by a racer. But who knows for sure?

NOTE: Check FIS regulations for current stack heights.

 

A common explanation for the noticeable effects of lift plates is that they increase the pressure that can be applied to a ski. In order to understand the effect of lift plates or any means that elevates the foot above the base of a ski one needs to understand how force acting on the CoM of a skier is transferred from the pelvis to the base of a ski.

The initial force path is by what is called the mechanical line. Since force travels in a straight line the mechanical line runs from the proximate centre of the trochanter (the ball joint of the femur with the pelvis) to the distal (lower end) tibia. This is the simple force path. Transferring force from the mechanical line to the soles of the feet gets a lot more complicated depending on the configuration of the triplanar joint system of the ankle complex and the intrinsic tension in the arches of the foot.

The sketch below shows the mechanical lines in the lower limbs. The mechanical line in each leg actually extends down as far as the talus, the bone that forms what is commonly called the ankle joint. In quiet erect standing, the force of gravity G pulls CoM down towards the center on the earth. The ball joints of the pelvis apply force to the mechanical line of each leg which extends to the distal tibia. Depending on the physiological state of the foot, force will be applied to the ground or supporting surface with a Centre of Pressure somewhere under the sole of the foot. In this graphic, the feet are in a neutral position and lie directly under the centres of the ball joints of the pelvis. Centre of Pressure will reside on a line running through the proximate centre of the heel and the centre of the head of the 2nd metatarsal (aka – ball of the 2nd toe).

ML1In the graphic below, lift plates have been inserted under each foot. According to the position of some on this issue, lift plates increase the pressure that can be applied to a ski. Seriously? How could this work? It couldn’t. Where forces are linear with no components it would make no difference whether lift plates were 1 cm high or 1 metre high. They would have no impact on pressure aside from any increase in pressure resulting from the added mass of the lift plates. Do people just make this sort of stuff up?

ML 2

In the graphic below, the feet are wider apart than the centre-to-centre dimension between the ball joints of the pelvis. The mechanical lines still run from the centre of the ball joints in the pelvis to the distal tibia. But there are now vertical and horizontal components of force Fh and Fv with a resultant force R aligned with the mechanical line. The horizontal component Fh of the resultant force R of the mechanical line is tending to rotate the feet about their outside or lateral borders. In other words, the angular relation of the mechanical line to vertical has created a moment of force or torque  that is tending to rotate the foot. What would happen if lift plates were introduced between the soles of the feet and the ground?

ML 3…………… to be continued