Shells

MOVEMENT OF THE SHANK IN THE SHAFT

In the article, Biomechanical Considerations of the Ski Boot, in the book, The Shoe in Sport, Dr. Stussi raises the issue of the relationship of the levers or moment arms between the rotational axis of the shaft of the ski boot and the proximate axis of the ankle joint. As far as I know, few, if any, ski boots actually have a shaft with a functional axis of rotation. Shafts are typically fixed in place by an interface detail with the lower boot shell.  What appears to be a axis pin is actually a fixation means that secures the shaft to the lower shell. While the fixation means does not create a rotational axis, it doe influence where and how the shaft will deform when the skier’s shank applies force to it. For this reason, I prefer to have the fixation means of the shaft slightly behind the proximate centre of rotation of my ankle joint.

In general, the ski industry seems to be unaware of the fact that the ankle joint is not a fixed hinge but a gliding hinge with a centre of rotation that not only varies during plantarflexion and dorsiflexion but which has a variable axis that changes continuously throughout the range of motion of the ankle joint. In addition, there can be considerable variation in the proximate centre of the axis of rotation from the ankle joint of one person to another person. In view of this, it is not practical to design a ski boot with an axis of rotation that is congruent with the axis of rotation of the ankle. The issue I focus on in a ski boot is keeping the load centre of the shank on the shaft consistent.

Although there is no flesh on the skeleton and no liner and especially a conventional boot tongue in place, the video clearly shows that the centre of force of the shank of my shin on the front of the shaft will be on the top of the shaft.

The 3 photos below show 3 different boots with all the buckles undone. If the interface of the overlap of the shaft stays together with the buckles undone I consider that the material and shape of the shaft is stiff enough to substantially maintain its shape while skiing with the buckles in the first bale catch position. The first photo is my Head World Cup boot.  The integrity of lower shell-shank interface and the overlap is good.

Good 1

The problem is that as a boot gets stiffer it becomes increasingly difficult to insert the foot into. The upside is that a stiff cuff shape provides sufficient front-to -back space for the range of low resistance ankle flexion I need with the buckles engaged in the first bale catch.

The photo below is of my spouse’s Head boot. Although not as well defined as the World Cup the shape without the buckles engaged is acceptable.

Acceptable

The photo below is of a vintage Lange XLR race boot. Even though the shell material is in a race stiffness it does not provide the defined shape of the cuff I need to provide a defined shape for the movement of my shank within the shaft in ankle flexion.

Reject

The problem with my Head World Cup and most boots is that even if the shaft stiffness and shape is good the tongue typically introduces a secondary source of resistance to the movement of the shank of a skier that is variable. I will explain why in my next post.