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Will increasing the friction of the topcover increase the supination resistance of the foot? And if

Discussion in 'Biomechanics, Sports and Foot orthoses' started by mike weber, May 31, 2011.

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    Moved Posts from Top covers for PTTD thread

    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.
  2. Re: Top covers for PTTD

    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.
  3. Re: Top covers for PTTD

    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
  4. Re: Top covers for PTTD

    Got to love the level of precision here :D.

    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.
  5. Re: Top covers for PTTD

    Just thinking out loud(s)
    It must be of the reasons, same reason why friction is important in adding to the pronation/supination moment froma device.

    I can see that, hurts just reading the words

    I would go with depends :D
  6. Re: Top covers for PTTD

    Its rarely the wrong answer. :D:drinks

    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?

  7. Re: Top covers for PTTD

    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

    Right a question 1st = a medial Skive device will also cause a pronation moment at the subtalar joint axis - yes or no ?
  8. Re: Top covers for PTTD

    It depends.
  9. Re: Top covers for PTTD


    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
    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.
  10. Re: Top covers for PTTD

    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. :sinking: 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?
  11. Re: Top covers for PTTD

    if it read So increased friction of the top cover must increase the Supination resistance of the Foot would that make more sense.
  12. Re: Top covers for PTTD

    Now I´m confused I think a beer will help - maybe Simon, Eric, Kevin, Ian, Ferris anyone may help
  13. 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.

    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?
  14. 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.
  15. efuller

    efuller MVP

    Re: Top covers for PTTD

    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.

  16. Re: Top covers for PTTD

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

    Griff Moderator

    Re: Top covers for PTTD

  18. RobinP

    RobinP Well-Known Member

    Re: Top covers for PTTD

    Pedro the pronation pony

    Just like pronation.......HE'S BAD!!!!!!!!

    Attached Files:

  19. Re: Top covers for PTTD

    I feel a new avatar coming on...

    Liking yours by the way Robin.
  20. RobinP

    RobinP Well-Known Member

    Re: Top covers for PTTD

    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 :deadhorse: could be Pedro the pronation pony
  21. RobinP

    RobinP Well-Known Member

    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


    Apologies for, as usual, going totally off topic. No more - 1mm EVA as I said
  22. Re: Top covers for PTTD


    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.
  23. Re: Top covers for PTTD

    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.
  24. dragon_v723

    dragon_v723 Active Member

    an easy question, what material gives you the highest friction?
    also will higher friction top cover encourage more callous formation?
  25. 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 ?
  26. efuller

    efuller MVP

    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.


    Pedro rides again
    The mark of Pedro
    Hi ho Pedro cue the William Tell overture....
  27. Phil Wells

    Phil Wells Active Member


    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!

  28. I agree with Eric.
  29. http://www.kau.edu.sa/Files/320/Researches/56847_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.
  30. As a thought experiment, what might happen to the reaction forces if we constructed a device which had zero friction at the foot-orthosis interface?

    Lets simplify it and make the orthosis an inclined plane. Without any friction you'd slide down it- right? So during your slide would the net reaction force vector be more or less vertical than on an identical device with infinitely high friction on which no siding occurred?
  31. markjohconley

    markjohconley Well-Known Member

    LESS, with no friction then you have only the one component the normal force, i think....
  32. Sounds reasonable. Why the interest in friction at the moment, Mark?
  33. Mark, this is one of the short presentations I gave at the ACFAOM meeting last october. You'll see I looked at co-efficient and angle of friction of a handful of orthotic materials versus nylon and cotton socks and talked about some of the potential implications.

    Attached Files:

  34. CraigT

    CraigT Well-Known Member

    You have posted some crackers of presentations over the past week with this one combined with the presentations from Belgium.
    For what it is worth my personal feeling is that I don't want friction to be a factor in the design of the orthotic- in other word if I don't want to need to put a high friction top cover on the orthosis to resist shear forces- I would prefer to resist the forces with the geometery of the orthosis. This is why I almost always use a shell device as you have the option of an effective deep heel cup, more lateral support through the cuboid region etc etc...
    I also do not like relying on footwear to provide the lateral heel cup support- as it is separate from the orthotic shell, it will wear and distort over time no matter how strong it is.
  35. Thanks Craig. I'm not sure why the background colour of the some of slides changed when I converted this to .PDF, but there you go.

    That you don't want friction to be a factor in the design of the orthotic is neither here nor there because it always is. :D

    Maybe you'd be good enough to share your presentation from the upcoming Delcam conference after you've presented it?
  36. CraigT

    CraigT Well-Known Member

    I guess I am saying that my aim is that the orthotic shell should function well even if the friction is very low...
    If you go back to the earlier posts I think Eric summed it up well with-
    ... which is why I prefer a shell orthosis!
  37. I think you are looking at the foot as a single unit, rather than looking at each discreet point of contact between the foot and orthosis. I don't know what a shell orthosis is.
  38. CraigT

    CraigT Well-Known Member

    Generally made from Polypropylene/ carbon fibre- (traditionally vacuum formed, but now could me milled ;))

    The sum of the forces rather than the discreet points? Yes you are probably right...

    If I apply an significant enough supination force (for example via a medial skive) that the heel wants to slide down the device, then I would also want a deep heel cup and/or support through the cuboid region to try to ensure that it cannot. I think this is where the Feehery/ Fettig/ Denton modifications came from with the Blake inverted device.

    I believe the multi-directional stiffness of the device here is important due to the desire to apply forces at discreet points and directions...

    I think we are on the same page judging by your presentations, but am I making sense?
  39. markjohconley

    markjohconley Well-Known Member

    thanks Simon; friction, your presentation at Belgium got me thinking, and I had a moment of clarity, between bouts of amorous adventure, but for the life of me can't remember and daren't ask Terri if she can recall for I doubt a favourable reaction .... so some more searching and reading, and hopefully thinking.

    Goodaye Craig, has it been determined how much extra height is needed of the lateral flange for it to exert a lateral-medial force onto bone?

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