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Will increasing the friction of the topcover increase the supination resistance of the foot? And if so, how and why?

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  #1  
Old 31st May 2011, 02:25 AM
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Default Will increasing the friction of the topcover increase the supination resistance of the foot? And if so, how and why?

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Quote:
Originally Posted by Robertisaacs View Post
Before Robin P gets in and steals my thunder again, It depends (HA! In your face Robin).

Personnally, I prefer 1mm low density EVA to leather for a topcover in patients who will wear their devices very heavily. Its robust and does not go hard when it is sweated into. However it depends on many other variables. What is the cover covering? Does it have to conform into a heel cup? Is it to be worn in a heavy boot indoors or a light shoe outdoors? What is the intention of the device (the topcover may change the force vector)? Is cushioning required? Whats the climate? What is the foot like? etc etc etc.

It depends. There will never be a single right answer, just a list of factors to consider when reaching it for yourself.
Speaking form a purely mechanical stand point you could have a top cover with increased Friction medial to the STJ axis when the Tib Post is ´working´and a top cover with reduced friction lateral to the STJ at that point.
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  #2  
Old 31st May 2011, 02:48 AM
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Default Re: Top covers for PTTD

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Originally Posted by mike weber View Post
Speaking form a purely mechanical stand point you could have a top cover with increased Friction medial to the STJ axis when the Tib Post is ´working´and a top cover with reduced friction lateral to the STJ at that point.
Interesting concept. Few challenges. Murley et al Found two distinct spikes in Tib post activity. One at initial contact and the other at mid stance. Correlate that to a Sub talar kinematics graph and we have the tib post firing with the foot 1. At its most supinated (ish) and 2. at its most pronated (ish). So you have the two extremities of the weight bearing sub talar axial bundle at which the tib post fires.

Why not just give a high friction cover all over?




Murley GS, Buldt AK, Trump PJ, Wickham JB.
Tibialis posterior EMG activity during barefoot walking in people with neutral foot posture.
J Electromyogr Kinesiol. 2009 Apr;19(2):e69-77. Epub 2007 Nov 28.
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Old 31st May 2011, 03:03 AM
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Default Re: Top covers for PTTD

Quote:
Originally Posted by Robertisaacs View Post
Interesting concept. Few challenges. Murley et al Found two distinct spikes in Tib post activity. One at initial contact and the other at mid stance. Correlate that to a Sub talar kinematics graph and we have the tib post firing with the foot 1. At its most supinated (ish) and 2. at its most pronated (ish). So you have the two extremities of the weight bearing sub talar axial bundle at which the tib post fires.

Why not just give a high friction cover all over?




Murley GS, Buldt AK, Trump PJ, Wickham JB.
Tibialis posterior EMG activity during barefoot walking in people with neutral foot posture.
J Electromyogr Kinesiol. 2009 Apr;19(2):e69-77. Epub 2007 Nov 28.
Why not just give a high friction cover all over? I was going to say that but an increase in friction from a top cover should increase Supination Resistance hence the two friction level top cover idea.

I guess you would go with the increased friction medial to the STJ axis at the most pronatedish position
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Old 31st May 2011, 04:01 AM
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Default Re: Top covers for PTTD

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Originally Posted by mike weber View Post
Why not just give a high friction cover all over? I was going to say that but an increase in friction from a top cover should increase Supination Resistance hence the two friction level top cover idea.

I guess you would go with the increased friction medial to the STJ axis at the most pronatedish position
Got to love the level of precision here .

Why would an increase in friction increase supination resistance? Would it not depend on other variables? And indeed whether we are looking at the rotation of the calc around the sub talar axis or rotation of the foot around the interface axis?

Been trying to get my head around dynamic vectors since the weekend. Giving me all kinds of trauma. For eg. Assume that the vector of force from the foot is straight down. Now assume we put a foot with lots of navicualar drift into an insole with a high medial flange, like a UCBL. Now when the foot loads onto the ground it will be trying to pronate, viz, get wider. The medial part of the foot will be pushing lateral -> medial and the lateral side medial -> lateral. The medial flange of the orthotic is therefor only capable of applying medial -> lateral force as much as the lateral side / flange of the orthotic / shoe is capable of braking it. Intuitively, the higher the friction on the lateral side of the orthotic, the more lateral the force will be....


So the ORF vector starts with that. Then factor the actual vector of force from the way the foot hits the ground, the angulation of the orthotic surface and the level to which this is modified by the friction co-efficient. At which point my brain explodes.

Returning to the OP, Eva. because it comes in cool patterns.
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Old 31st May 2011, 04:11 AM
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Default Re: Top covers for PTTD

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Originally Posted by Robertisaacs View Post
Got to love the level of precision here .
Just thinking out loud(s)
Quote:
Originally Posted by Robertisaacs View Post
Why would an increase in friction increase supination resistance? Would it not depend on other variables? And indeed whether we are looking at the rotation of the calc around the sub talar axis or rotation of the foot around the interface axis?
It must be of the reasons, same reason why friction is important in adding to the pronation/supination moment froma device.

Quote:
Originally Posted by Robertisaacs View Post
Been trying to get my head around dynamic vectors since the weekend. Giving me all kinds of trauma. For eg. Assume that the vector of force from the foot is straight down. Now assume we put a foot with lots of navicualar drift into an insole with a high medial flange, like a UCBL. Now when the foot loads onto the ground it will be trying to pronate, viz, get wider. The medial part of the foot will be pushing lateral -> medial and the lateral side medial -> lateral. The medial flange of the orthotic is therefor only capable of applying medial -> lateral force as much as the lateral side / flange of the orthotic / shoe is capable of braking it. Intuitively, the higher the friction on the lateral side of the orthotic, the more lateral the force will be....


So the ORF vector starts with that. Then factor the actual vector of force from the way the foot hits the ground, the angulation of the orthotic surface and the level to which this is modified by the friction co-efficient. At which point my brain explodes.
I can see that, hurts just reading the words


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Originally Posted by Robertisaacs View Post
Returning to the OP, Eva. because it comes in cool patterns.
I would go with depends
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Old 31st May 2011, 04:43 AM
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Default Re: Top covers for PTTD

Quote:
I would go with depends
Its rarely the wrong answer.

I teach my minions that any time I, or anyone, asks them a question in biomechanics that they have no idea how to answer, they use "it depends on the morphology and functional characteristics of the individual in question."

Good one for all you undergraduates...

I completely failed to understand this though. Could you elucidate please?

Quote:
It must be of the reasons, same reason why friction is important in adding to the pronation/supination moment froma device.
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Old 31st May 2011, 05:07 AM
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Default Re: Top covers for PTTD

Quote:
Originally Posted by Robertisaacs View Post
Its rarely the wrong answer.

I teach my minions that any time I, or anyone, asks them a question in biomechanics that they have no idea how to answer, they use "it depends on the morphology and functional characteristics of the individual in question."

Good one for all you undergraduates...
Wise words just don´t write it as your total answer in your exams. ie describe the orthotic device you would use in a PTTD patient.

Answer - Depends

marker = Fail




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Originally Posted by Robertisaacs View Post
I completely failed to understand this though. Could you elucidate please?
Quote:
It must be of the reasons, same reason why friction is important in adding to the pronation/supination moment froma device.
Right a question 1st = a medial Skive device will also cause a pronation moment at the subtalar joint axis - yes or no ?
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Old 31st May 2011, 05:09 AM
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Default Re: Top covers for PTTD

It depends.
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Old 31st May 2011, 05:19 AM
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Default Re: Top covers for PTTD

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Originally Posted by Robertisaacs View Post
It depends.


Ok if we say yes for arguments sake -

the net moments acting on the STJ have increased ie both medial and lateral to the STJ axis, but the medial have increase to a greater extent and the CoP is medial, we will have an external Supination moment.

Whats acts as resistance to the external supination moment ?

Muscle - PL, PB, PT
ligaments
Bone compression forces
and the device anywhere lateral to the Subtalat Joint axis, right ?

So increased friction of the top cover must increase the Supination resistance of the device.
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Old 31st May 2011, 05:43 AM
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Default Re: Top covers for PTTD

Quote:
Just playing

Quote:
Ok if we say yes for arguments sake -

the net moments acting on the STJ have increased ie both medial and lateral to the STJ axis, but the medial have increase to a greater extent and the CoP is medial, we will have an external Supination moment.
Sorry, still sruggling. If the ORF moments increase both medial AND lateral, would that not mean the patient has gotten heavier? If the moment is force * lever and the total lever stays the same (cos if it increases one side it must decrease the other) the only way I can see for both supination AND pronation moments arising from ORF to increase is Pie.

Quote:
Whats acts as resistance to the external supination moment ?

Muscle - PL, PB, PT
ligaments
Bone compression forces
and the device anywhere lateral to the Subtalat Joint axis, right ?
Anything which causes pronation moment. Which is going to primarily gravity causing pronation moment by the ORF lateral to the axis as you said. Not sure how bone compression causes pronation moment. Supination moment in the sinus tarsi, yes. Pronation moment, no.

Quote:
So increased friction of the top cover must increase the Supination resistance of the device.
Whoa there, where did friction come into the equation? To be honest I'm not exactly sure what you mean by supination resistance of the device. I know what supination resistance in feet means, but not "supination resistance of the device".

Sorry, I think I'm having a slow day. I'm struggling to follow you here Mike. Perhaps if you used shorter words? Or pictures? Or some kind of cartoon animals? Pedro the pronation moment pony or something?
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Old 31st May 2011, 06:07 AM
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Default Re: Top covers for PTTD

Quote:
So increased friction of the top cover must increase the Supination resistance of the device.
if it read So increased friction of the top cover must increase the Supination resistance of the Foot would that make more sense.
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Old 31st May 2011, 06:36 AM
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Default Re: Top covers for PTTD

Quote:
Originally Posted by Robertisaacs View Post
Just playing



Sorry, still sruggling. If the ORF moments increase both medial AND lateral, would that not mean the patient has gotten heavier? If the moment is force * lever and the total lever stays the same (cos if it increases one side it must decrease the other) the only way I can see for both supination AND pronation moments arising from ORF to increase is Pie.



Anything which causes pronation moment. Which is going to primarily gravity causing pronation moment by the ORF lateral to the axis as you said. Not sure how bone compression causes pronation moment. Supination moment in the sinus tarsi, yes. Pronation moment, no.



Whoa there, where did friction come into the equation? To be honest I'm not exactly sure what you mean by supination resistance of the device. I know what supination resistance in feet means, but not "supination resistance of the device".

Sorry, I think I'm having a slow day. I'm struggling to follow you here Mike. Perhaps if you used shorter words? Or pictures? Or some kind of cartoon animals? Pedro the pronation moment pony or something?
Now I´m confused I think a beer will help - maybe Simon, Eric, Kevin, Ian, Ferris anyone may help
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Old 31st May 2011, 07:14 AM
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Default Re: Top covers for PTTD

Right. One or both of us is having a brain fart. I suggest we both drink beer, then go back at it and see if it swims into focus. Or as you say perhaps someone else can help.

Quote:
if it read So increased friction of the top cover must increase the Supination resistance of the Foot would that make more sense.
That does help a bit. Makes a little more sense now, I just don't get where you get from more friction to more supination resistance. How does More friction lateral to the axis increase pronation moment?
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Old 31st May 2011, 08:42 AM
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Default Re: Top covers for PTTD

Thought I'd posted something from my i-phone earlier. Obviously not.

I suggest you go back a step. Take an incline plane with block on it, draw in the forces acting on the block with the block in static equilibrium. Then draw in the forces for the block with it sliding down the incline; then draw in the forces with the block being drawn up the incline. What difference do these situation make to the direction of the net force vector?

You'll probably find a java app to do this for you.
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Old 31st May 2011, 11:31 AM
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Default Re: Top covers for PTTD

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Now I´m confused I think a beer will help - maybe Simon, Eric, Kevin, Ian, Ferris anyone may help
What was the question? I liked Simon's response. I also liked Pedro the pronation pony. Should Pedro look like eeyore from Winnie the Pooh? Should he have big muscles and an "s" on his chest. Should he carry a big lever?

Endless possibilities. I'm going to have to work on the avatar.

Eric
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Old 31st May 2011, 12:08 PM
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Default Re: Top covers for PTTD

Quote:
What was the question?
Will increasing the friction of the topcover increase the supination resistance of the foot? And if so, how and why?
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Old 31st May 2011, 12:30 PM
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Default Re: Top covers for PTTD

Quote:
Originally Posted by Simon Spooner View Post
Thought I'd posted something from my i-phone earlier. Obviously not.

I suggest you go back a step. Take an incline plane with block on it, draw in the forces acting on the block with the block in static equilibrium. Then draw in the forces for the block with it sliding down the incline; then draw in the forces with the block being drawn up the incline. What difference do these situation make to the direction of the net force vector?

You'll probably find a java app to do this for you.
http://zebu.uoregon.edu/1998/ph101/ex4.html
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Old 31st May 2011, 12:44 PM
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Default Re: Top covers for PTTD

Pedro the pronation pony

Just like pronation.......HE'S BAD!!!!!!!!
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Old 31st May 2011, 12:59 PM
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I feel a new avatar coming on...

Liking yours by the way Robin.
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Old 31st May 2011, 01:06 PM
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Default Re: Top covers for PTTD

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I feel a new avatar coming on...

Liking yours by the way Robin.
Having a Flight of the Conchords binge at the moment.

Go for it Robert, the Arena needs a bit of spicing up

Just a thought, but could be Pedro the pronation pony
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Old 31st May 2011, 01:24 PM
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Default Re: Top covers for PTTD

As in smack the pony?

http://www.youtube.com/watch?v=x-6HqMHpkFo

Poor pedro.
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Old 31st May 2011, 01:28 PM
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Default Re: Top covers for PTTD

Only a second before the "punchline" did I realise I had seen it before - didn't make it any less funny.

thanks for that robert, brightened up my evening

robin

Apologies for, as usual, going totally off topic. No more - 1mm EVA as I said
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Old 31st May 2011, 11:33 PM
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Default Re: Top covers for PTTD

Yeah.

A warning though. I downloaded the conchords album and had it on my ipod yesterday when in the lab. Turns out that the inevitable singing under the breath that headphones cause is not ideal with that particular album. The poor girl sharing the lab with me was, I'm told, initially quite terrified then disturbed. She apparently has no interest in my sugarlumps, nor that I consider myself freaky. She was mildly interested in the distribution of mail genitalia on the dance floor though. Also, I have an excellant falsetto "gay voice" for the appropriate moments.
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Old 31st May 2011, 11:40 PM
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Default Re: Top covers for PTTD

Quote:
Originally Posted by Simon Spooner View Post
Thought I'd posted something from my i-phone earlier. Obviously not.

I suggest you go back a step. Take an incline plane with block on it, draw in the forces acting on the block with the block in static equilibrium. Then draw in the forces for the block with it sliding down the incline; then draw in the forces with the block being drawn up the incline. What difference do these situation make to the direction of the net force vector?

You'll probably find a java app to do this for you.
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Originally Posted by Ian G View Post
Thanks for the link Ian.

Had a few beers and it doesn´t seemed to have helped.

I think I´m in way over my head Physics wise.

From a common sense side Pulling the block up the incline should have greater friction Vector what this does to the net force vector I really have no idea.
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Old 1st June 2011, 08:10 PM
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Default Re: Will increasing the friction of the topcover increase the supination resistance of the foot? And if so, how and why?

an easy question, what material gives you the highest friction?
also will higher friction top cover encourage more callous formation?
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Old 2nd June 2011, 12:28 PM
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Default Re: Will increasing the friction of the topcover increase the supination resistance of the foot? And if so, how and why?

No one ?

If we add a cuboid lift to a device with felt, we will have changed the make up of the external moments acting on the subtalar joint right ?

Friction is an important factor in how a device works

So will friction lateral to the axis effect the effectiveness of the device ? In reducing pronation moment ?

Anyone ?
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Old 2nd June 2011, 10:13 PM
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Default Re: Will increasing the friction of the topcover increase the supination resistance of the foot? And if so, how and why?

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Originally Posted by mike weber View Post
No one ?

If we add a cuboid lift to a device with felt, we will have changed the make up of the external moments acting on the subtalar joint right ?

Friction is an important factor in how a device works

So will friction lateral to the axis effect the effectiveness of the device ? In reducing pronation moment ?

Anyone ?
Take a medial heel skive, the orthosis on the medial side of the heel cup will apply forces normal to the heel cup. Assume a varus wedge. So the force from the wedge will have a vertical component and a medial to lateral component. The medial to lateral component would tend to accelerate the heel laterally. However, if there is friction this force will be added to the medial to lateral component to get a net medial to lateral force of zero. You would know the net force was zero if there was no medial to lateral acceleration.

Thinking back to a slippery heel cup in a medial heel skive device... The medial to lateral force from the surface will cause the foot to slide down the hill until something stops it. What will stop the slide is a high lateral heel cup that will apply a lateral to medial force to bring the net medial lateral force to zero. So, a surface with high friction would prevent the initial slide, but probably not gain you anything in the end.

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Old 2nd June 2011, 11:46 PM
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Default Re: Will increasing the friction of the topcover increase the supination resistance of the foot? And if so, how and why?

Mike

The issue I have with friction is whether the coefficient of friction will ever be powerful enough to effect CoP progression enough to be worth worrying about.
As friction from the top cover will have to act through the soft tissue and therefore be 'dampened', will any effect be seen on force? (Need a mathematician quickly)
I do think that friction is important to consider when using a sticky material that will potentially decelerate epidermal translation over the dermis = interstitial friction.
You see this with silicone top covers causing arch blistering when a higher, more slippy insole doesn't.
Anecdotaly we have found that polyprop insoles without top covers have better compliance than covered ones!

Phil
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Old 3rd June 2011, 04:56 AM
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Default Re: Will increasing the friction of the topcover increase the supination resistance of the foot? And if so, how and why?

Quote:
Originally Posted by efuller View Post
Thinking back to a slippery heel cup in a medial heel skive device... The medial to lateral force from the surface will cause the foot to slide down the hill until something stops it. What will stop the slide is a high lateral heel cup that will apply a lateral to medial force to bring the net medial lateral force to zero. So, a surface with high friction would prevent the initial slide, but probably not gain you anything in the end.

Eric
I agree with Eric.
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Old 3rd June 2011, 08:19 AM
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Default Re: Will increasing the friction of the topcover increase the supination resistance of the foot? And if so, how and why?

http://www.kau.edu.sa/Files/320/Rese...6847_27169.pdf

A nice little study would be to look at the coefficient of friction between sock and top-cover materials using the inclined plane method.

The thing about foot orthoses is that the superior surface is a complex series of inclined planes, if the angulation between two points on the surface exceeds the angle of friction for sock versus top-cover material then there will be slippage of the foot in the area between these two points. So we might have areas of the foot-orthosis interface were static friction is not overcome, and areas of the foot-orthosis interface where static friction is overcome. This will likely create "hot-spots". I talked about this before, but I don't know which thread.
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