Liner Tongues

REDUCING FOREFOOT CRASH SPACE

In my post, TONGUE SURGERY, I described how the tongue in my Head World Cup boot was blocking the glide path of my ankle joint by introducing an unwanted source of resistance at the lower end of my shank. By removing all the foam in the tongue below the lower end of the force distribution zone and adding a rectangular layer of foam directly in front of my shin bone, behind the existing layer of chip foam, I increased the space between the plastic tongue body and the lower end of my shank. But I also want to reduce the rearward movement at the transition of the tongue body that occurs when the tongue is bent in dorsiflexion.  I achieve this by trimming the sides of the tongue body as shown in the photo below.

Tongue trim 2

The red dashed lines show where I trimmed the sides of the tongue body and enlarged the neck at the narrowest point. Here’s a side view.

Tongue trim 1

I leave the fabric-foam outer skin run wild instead of trimming it to the shape of the tongue. The reason I do this is to lessen the tendency of the edges of the tongue to snag on my sock when I insert my foot into the boot. I also don’t re-sew the fabric-foam to the tongue body or glue it in place. Both these can stiffen the tongue at the transition bend. Putting my boot on can be a bit tricky a first. I place the tongue on my shin with my forefoot in the shaft. Then I grasp the boot shaft and shove my foot in. Once my foot is in the boot I wiggle the tongue to make sure it is in the right place.

To reduce the crash space over my forefoot I make a new foam pad to replace the original chip foam pad. I start off making the pad bigger than it will eventually be then trim it down as necessary to enable me to get my boot on.  Here is what the foam forefoot pad for my boot looks like.

IMG_3899

I usually taper the top edges to give the tongue a shape that won’t conflict with the shape of the boot shell above. I secure the pad in place with 2-sided tape instead of gluing it. This makes it easy to reposition the pad or remove and replace it. I fold back the fabric-foam skin,  stick the foam pad to the underside of the tongue body then fold over the fabric-foam skin.

IMG_3898

I try foams of different densities and resistance to deformation to try and find the one that works best. When I did a lot of boot work I acquired such a good supply of foams that I have not bought any in years. So I can’t recall the types or sources I am using. But here’s a photo that shows half of the original chip foam tongue alongside some samples from my stock of foams.

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Here’s what the front of the tongue looks like with the foam pad in place. Note the gap behind the forefoot pad in the transition bend that allows the tongue to bend independently of the foam-fabric skin.

IMG_3901

In terms of reducing the crash space, I just want to take up any space between the top of the high point of my foot and the boot shell while leaving space at the back end for the glide path of my ankle joint. I don’t want to feel a significant force pressing down hard on top of my foot.  If the pad is not quite thick enough to fill the space, I add a thin layer (2-3 mm) of dense foam that compresses very little. The net ramp angle of 3 degrees in combination with 14 or 15 degrees of lead segment ankle flexion turns on the stretch reflex in my legs. The stretch reflex enables my balance system to maintain the position of my CoM over my feet on what Ken Chaddock (Ski Simply Well) calls the Magic Carpet. The stretch reflex also allows my muscles to absorb energy from perturbations in snow reaction force that would tend to disturb my equilibrium. This gives me the best ride for the least effort.

In my next post I will discuss joint angles of the legs and pelvis,

TONGUE SURGERY

In my post, TONGUE TIED, I described how a conventional ski boot tongue can block the glide path of the ankle during dorsiflexion, disrupting the physiology of the ankle joint. It is essential to avoid this especially in dynamic activities such as skiing because the ankle joint is a portal for the flow of neural information. Neural flow from the more than 200,000 nerve endings in each foot and mechanoreceptors in the ankle send a flow of proprioceptive, sensory information to the central nervous system where it is processed and used to generate postural responses that sustain balance. Disrupting the physiology of the ankle joint with physical structures can disrupt neural flow and introduce foreign forces into the ankle joint that contaminate neural information from mechanoreceptors. Disrupting the physiology of the ankle joint has a similar effect on the balance system as  taking a hammer to your computer or smart phone then expecting it to still work.

My US Patent No. 4,544,299, published almost 30 years ago, discloses an in-boot tongue fit system that restrains the foot without obstructing the glide path of the ankle joint. The short video clip below shows a section through the center of the tongue system superimposed over the actual patent figures to illustrate how this system works when used in a conventional ski boot shell. The shank and forefoot portions are separate components joined by a flexible link. This allows the components to maintain their respective positions on the shank and forefoot during ankle flexion.

The in-boot system allows the ankle to flex while maintaining the position of the load centre on the shank. In order to keep the load on my shin centred in my Head World Cup boot work I had to perform some tongue surgery. The photo below is of the original tongue sectioned through the centre to reveal the core.

Section R

The first thing to note is the use of chip foam for the tongue padding. Chip foam is made from foam scraps that are ground up and held together in matrix with a bonding agent. It has been my experience that chip foam has very poor energy absorbing qualities. My first procedure will be a partial chip foamectomy. Since I only want the lower distribution of force on my shin to extend a little further below the load centre at the top of the front of the shaft than it extends above the load centre I don’t need any foam below this point where it could load my shin. I am also going to surgically remove a portion of the outer padded tongue skin since it is folded over adding thickness  in the glide path of the ankle, the very place where I don’t want any foam or padding. While I am at it, I am  going to remove all the chip foam from the forefoot of the tongue since it is next to useless anyway.

Here’s what the tongue looks like after removal of the foam.

Post foam re

In addition to removing the foam, I also trimmed off the front of the plastic tongue body that would normally be used to stitch the tongue to the liner. I want my tongue to be able to ‘float’ in the forefoot area to reduce any possibility of the transition blocking the glide path of my ankle joint. As my shin approaches the front of the boot cuff in the lead segment of flexion, I want to decelerate the movement as opposed to having my shin slam into the top edge of the shaft (aka Lange Bang). I also want to create more space below the pressure distribution zone to help maintain the centre of pressure while reducing the possibility of any load at the bottom of my shin that could block the glide path of my ankle. The solution? Add a band of foam to the tongue in front of my shin bone as shown in the photo below.

Foam add

I don’t want to add foam to the entire area of the tongue. I only want to add a rectangular band of foam that is directly in front of my shin bone. The reason for this is that there is usually a gap where the sides of the liner overlap the sides of the tongue. Placing a band of foam in front of the shin bone draws the sides of the tongue inward as my shin pushes through the gap. This assists the deceleration of the forward movement of my shin during ankle dorsiflexion while helping keep the force centred. The photo below shows the gap.

Wedge

When I am skiing, the only time I would ever have any perception of contact of my shin with the front of my cuff is if I were to get momentarily pitched forward. Even then, any sensation of any contact is minimal. When I am skiing, the only sensation I am consciously aware of is the considerable tension in the soles of my feet.

In my next post I will discuss final tongue modifications including how I reduce the crash space over my forefoot.

 

TONGUE TIED ERRATA

In reference to the photo of the tongue, which I surgically removed from the liner of my Head World Cup ski boot with a tonguectomy procedure, I also performed a bilateral resection of the tongue for the purpose of exposing the core structure. The photo on the right is of the resected tongue . I will discuss this tongue in the next post. I apologize for any confusion this omission may have caused.

Tongue section