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friction revisited

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Simon Spooner, Feb 9, 2011.

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    All, I was thinking about friction again this morning. When I was at school we did experiments in which an object was placed on a surface and the angulation of the surface was increased until the point when sliding of the object against the surface occurred. From this we calculated the co-efficient of friction between the surface and the object.

    Now, lets forget about socks and multiple interfaces for a moment and assume that the co-efficient of friction between a foot and an orthosis material is a constant. If we were to divide the orthosis surface into a fine grid, we would see variations in the angulation of each discreet area of the orthotic grid. So it occurs to me that in some areas of the foot-orthosis interface sliding will occur due to the angulation exceeding that required to overcome the static friction; while in other areas of the foot-orthosis interface the orthosis angulation will be below the kinetic threshold and no sliding will occur. Given that two such areas might be adjacent to each other, what are the implication in terms of the skin in these areas? Some areas might exhibit high tension, while others might be compressed/ "bunched" together- right?

    What impact might this have on joint kinetics, if any?

    Your thoughts, please.
  2. maybe not what your looking for but potential for irritation/pain avoidance. so if the device causes an increase in "bunched" areas under the medial heel. Then with pain/irritation in this area the CNS might stimulate the supination muscles to work harder which all things being equal reduce the irritation/pain under the medial heel but supinate the foot - which may or maynot be bad.
  3. RobinP

    RobinP Well-Known Member

    I'm not sure about the specific example you have given of areas of tension and compression being adjacent.

    However, is it not theoretically possible that the angle of inclination of a given point on the device my be too great to allow the intended orthotic reaction force (created by the shaping and the stiffness of the device) to be effective because there is an insufficient degree of friction at the interphase?

    So, in terms of joint kinetics, let's say that the theoretical point mentioned above was at the proximal end of the arch, toward the medial aspect. The moment being applied to the joint would be determined by the distance from the joint to the point at which there was sufficient friction to transfer the ORF - for example more into the middle of the arch (sagitally and transversely). If this were the case, then it may make the point at which the ORF can be applied closer to the sub talar joint axis, thus making the ORF "less effective"

    So, taking it a step further, might it be beneficial to have a less sloping surface with greater friction as a way of applying the ORF further from the joint axis.

    Should we have different covers for different area of the orthosis?

    I'm sure I have made many biomechanical faux pas' in my description of my rambling thoughts for which I apologise in advance
  4. That's not what I said. I said we may have two adjacent sections in which the limits of static friction are exceeded in one and not the other.
  5. drsarbes

    drsarbes Well-Known Member

  6. RobinP

    RobinP Well-Known Member

    Does it create a lot of miniature pivotting points. ie you have a fixed point and a moving segment, essentially an axis?
  7. Perhaps the best way of describing this is with a little practical demonstration: take a sheet of A4 paper, lay it on a table, place your right hand at the top edge of the paper and apply firm pressure to secure this end, at the the other end apply light pressure with a couple of fingers from your left hand and slide it toward the right hand, taking the free edge of the paper with you. Note how the paper bends up as you drag the lightly secured (left hand) end toward the hand with firm pressure = skin "bunching"; now slide your left hand away from your right hand maintaining firm pressure with your right hand, note the tension in the paper (skin). This is what is happening on a much smaller scale across the orthosis interface if we have areas of surface angulation which result in the limits of static friction being exceeded in association with other areas of the foot-orthosis interface in which static friction holds.

    So, in this example there is a translational axis- right?

    Does that make sense? I'm sure someone will give a better analogy.
  8. As I recall Craig did a study which was designed to test "proprioceptive" effects of foot orthoses in which they used identical devices with different top-covers (one was sand paper if memory serves). Now those different top-covers must have had different co-efficients of friction with the subjects socks/ feet. Just remind me of the main findings of the study please Craig..... better still, if you could post the study up here.
  9. Craig Payne

    Craig Payne Moderator


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