ANKLE GLIDE PATH BLOCK


The first thing I look for in a ski boot I am considering is a shaft with sufficient stiffness to create a defined oval shape that will accommodate 14-15 degrees of lead segment low resistance ankle flexion before firm contact of my shank with the front of the shaft occurs. Because my shank is free to move fore and aft up to 14-15 degrees within the shaft, I tend to be acutely aware of what the tongue is doing on my shin. This is much harder to sense in boots with flexible shaft overlap segments that won’t assume a defined shape and especially in a boot with the shaft buckles and power strap cinched tight. When I took my Head World Cup boots out of the box and put them on I immediately sensed the tongue pressing firmly against the base of my shank. This was before I even tried to dorsiflex my ankle (rotate my shank forward). The curve of the transition of the tongue felt like a block pushing against the base of my shank.

Most plastic tongues amount to bent half tubes. One of the stiffest shapes known is a tube. When the trailing edges of a ski boot tongue are loaded by the leading edges of the boot liner and the curved interface of the boot shaft, the shank portion of the tongue becomes substantially rigid. When the shank presses against the tongue it bends at its transition with the forefoot portion. When it bends, the curve at the transition flattens and the tongue body moves rearward towards the shank. Unless the tongue is sewn to the toe box of the liner so it is too far forward, it can press on the lower end of the shank and block the glide path of the ankle joint. Here is a simulation of what happens. The black line represents the profile of the tongue.

 

This issue was identified in my US No. 4,534,122 and in a series of X-ray video studies done by Professor M. Pfeiffer (Kinematics of the Foot in the Ski Boot – The Shoe in Sport).  In the Type C study Dr. Pfeiffer observed that, among other things, the physiologic function of the ankle is stopped prematurely (blocked) with the effect that the talus (the bone that forms the ankle joint with the tibia) is levered backward and upward within the boot shell. The previous short video clip and the clip that follows below show this effect. If you pause the videos before and after shank loading you can see the extent of the effect of the tongue bending at the transition and pressing against the base of the shank. The flattening effect at the transition is due to the manner in which the stiffness of the half tube shape of the tongue influences the deformation.

 

Note how the foot is forced backward in the boot and the entire forefoot lifts off the boot board. This effect is easy to demonstrate with foot pressure technology by having the subject apply firm pressure to the balls of the feet and then flex the boot. As boot flex progresses, the pressure seen on the monitor under the balls of the feet will progressively decrease then disappear. The reason for this is that ski boots are flexed by decreasing the contraction of the soleus muscle. This has the effect of turning off the connection of the tibia with the balls of the feet. In his article, Dr. Pfeiffer stresses the importance of the forces on the shank in the fore aft plane being the result of active muscle participation and tonic muscular tension and that if muscular function is inhibited in the ankle area, greater loads will be placed on the knee. Tonus in a muscle is a reflex state where the muscle is primed and ready to rapidly respond to a neural signal to contract.

In my next post I will discuss the modifications I make to my boot tongue to try and minimize ankle glide path block.

 

 

4 comments

  1. David – I find this dynamic x-ray video very fascinating and illuminating.

    It is too bad that there is not a comparative video of the same motion in a more traditional 2 piece overlap shell design rather than the 3 piece Flexon style boot used here.

    Given that this boot has in independant EXO-tongue, if you will, I would expect it to exhibit similar qualities, if not with more force than those exhibited by the “inner” tongue as illustrated in your line diagram video above.
    Furthermore I would expect that the two functioning in unison would amplify the post / superior glide motion of the talus as described by your findings / theory VS the more traditional two piece conventional overlap shell. ( perhaps the narrower mid section / transition of the inner tongue mitigates some of this force? Is this why you have taken to narrowing / resecting the lateral sides of your modified tongues ? )

    My thought process is that most conventional overlaps tend to bottom out causing dorsal compression and a lateral deformation at the mid foot where the anterior transition of the lower limb / foot interface or at the point where the lower part of the “cuff” meets the “shoe” as forward flexion of the boot begins to approach its end point.

    I’ve long thought of this as a weakness in conventional overlap design and thought the flexon style 3 piece design had a BIT of a leg up on its 2 piece counterpart in this regard.
    Definitely food for thought and an eye opener re: flexion characteristics of the venerable Flexon design.
    Regards,
    G!

    1. Thanks for the input Gord. One of the boots I favoured back in the Crazy Canucks days was the Raichle Flexon but not because of the qualities of the cuff. What I like about the boot was the defined shape that the Flexion cuff provided for lead segment flexion if the cuff wasn’t done up too tight and there was no significant interference at the level of the ankle which is the portal for neural flow. If the Flexon worked for a racer it really worked. I am mystified why the glide path requirements of the talopcrual joint continue to be ignored given the X-ray studies that were done decades ago.

      1. Gord asked, “Is this why you have taken to narrowing / resecting the lateral sides of your modified tongues ?”.
        Answer: Yes. That and more. The solution is not perfect. But it works well enough to enable my balance system to orchestrate neuromuscular function in response to perturbations in snow reaction force to the point that I have no conscious perception of cuff contact on any aspect of my leg while skiing. I will explain the tongue modifications in my next post. The key is to minimize any loading of the ankle complex.

  2. Nice dynamic radiograph. However, I think the toes are lifting off the bootbase by direct pressure of the lower tongue on the extensor tendons making the hard turn from the shank into the foot. These tendons are held in place (restricted from bowstringing out away from the curve) by the extensor retinaculum. If the lower tongue is applying pressure to those tendons as the ankle dorsiflexes, then that direct pressure will cause tension on those tendons, lifting the toes and reducing contact by the forefoot on the bootbase…something counterproductive to moving the CoP forward during pronation.
    Also, thinking in terms of the sagittal moment caused by the force of the shank on the tongue… if the distributed force applied to the tongue can be thought of as a single point, the more distributed that force is low down in the tongue, the lower that single point will be. The lower it is, the shorter the moment arm it exerts around the tibiotalar joint and the less effective that force will be on creating the sagittal moment resisted by the forefoot (ie metatarsal heads). In other words, the more tongue, the more force needed to accomplish the weight shift forward.

Comments are closed.