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. Here’s a short video clip that shows how the movement of the shank of my leg would look within the shaft of my boot if we could watch with an imaging device.

 

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.

 

 

 

 

2 comments

  1. Most boot ‘shafts’ are adjustable only in one dimension…to account for varying degrees of bowleggedness or knockkneedness…if those are words… In other words, varus and valgus knees. You’re right in that the centre of rotation of the tibio-talar joint is not constant, but most studies seem to have found three separate axes, depending on where the joint is in its dorsi/plantarflexion excursion. From fully plantarflexed, the first axis is enforced by the tension in the anterior bands of the collateral ligament sets. Then, as the ankle dorsiflexes and the tibia slides forward over the talar body, the tension in these bands reduces, and the ankle finds its second axis. Then, as the tibio-talar joint becomes more dorsiflexed, the posterior collateral bands come under tension, and the centre of rotation changes slightly again. The tibio-talar transverse axis is tilted a few degrees, running almost from the tip of the medial malleolus to the tip of the lateral malleolus, which is a little further down. It’s also inclined forward on the lateral side, which accounts for the internal rotation of the tibia as it dorsiflexes over the talus.

    So, it would be pretty tough to find a boot shaft and hinge that would accommodate all of that. Fortunately, when you’re in the boot, you’re probably only using the mid- to early-anterior range of that tibio-talar motion anyway, so that takes away some of the constraint caused by the boot shaft. However, in addition to allowing you to adjust the boot shaft position in the frontal plane, they should allow you to slide one side (or both) horizontally so the boot shaft will follow your natural tibial motion as it moves out of the sagittal plane.

    How resisted is your shaft hinge, and how many degrees does it move with low resistance when you’re skiing?

    1. Very well articulated Bob. I am guessing that the sophisticated mechanism that varies the axis is like variable valve timing in a car engine in that it changes the mechanics as dorsiflexion progresses probably to manage joint acceleration. I am grinning to myself because some coaches keep arguing that I make something that is very simple more complicated than it really is. LOL. The lower limbs are simple? I’d hate to see something that is complicated! I am trying to get a handle on how many degrees total of first phase and second phase dorsiflexion is needed. In his article, Kinematics of the Foot in the Ski Boot, Dr Pfeiffer says, “The shaft of the boot should provide the leg with good support, but not with great resistance for about two-thirds 0f the possible arc, i.e. 20-22 degrees. I think he meant 2/3 x 20 or 22 degrees which would be about 14 degrees of low resistance dorsiflexion before shaft resistance increases. One cue I have is that in walking in my ski boots from the point when my stance foot goes flat on the ground until heel lift occurs, the joint angles and sequence feel very similar to walking in my street shoes. So, I agree that it is probably mid-to-early-anterior range. What I do know is that I seldom have any sensation of loading the front of the shaft. Antero-postero perturbations seem to be managed strictly by postural responses. My first phase is no load with perhaps some very slight dampening from the sides of the liner. All dorsiflexion occurs in the antero-posterior free space in the shaft. I tension my boot buckles as loose as possible to preserve the free space and top load my forefoot to maintain plantar tension. In a few posts I will get to the accommodation of varus-valgus tibial motion which should occur within the fixed cuff configuration but for all but a small number of skiers is intentionally blocked by boot design and boot-fit measures.

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