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ORF normal to the orthosis surface, right?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by markjohconley, Nov 25, 2009.

  1. markjohconley

    markjohconley Well-Known Member

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    Couldn't remember, and couldn't think of a search term to use, so, just to clarify the ORF is directed normally? to the orthosis inclination angle (at each point on the orthosis surface)? thanks, mark
  2. Hi Mark,
    ORF or orthoses reaction force. Will be directed at the angle that device comes incontact with the foot.

    Is is important to consider that this angle will change constantly because of this like GRF, compression of device material, flex of material, angle of foot strike, muscle activity etc.

    So even if a medial skive is used at 15 degrees the ORF on the medial heel may be at 15 degree or more or less depending on many many variables
  3. markjohconley

    markjohconley Well-Known Member

    Thanks Michael,
    so if
    then a more efficacious orthoses to move the CoP would be a flat (parallel to insole) addition added to the orthosis. I have tried this on a couple of pts (a 4mm EVA rearfoot medial addition with front and one side, away from upper, steeply bevelled).
    Michael as for tips I do recall two statistics, from analysing those 500 thorougbred races, 30 odd years ago, was that the outside barrier in a large field (13 and over runners) had an amazingly good return %, and never back short priced favourites drawn the inside barrier (no matter how many runners) ..... this is weird!,
    ... why don't we throw in some 'current affairs", with the upcoming 9/11 trials ...
    ...I remember that immediately after the 9/11 catastrophe an ad-hoc? association of USA commercial jet pilots announced that the alleged 'terrorist' pilots could not have flown the planes, including a statement that "it would be like my mother winning Daytona three times in a row" (not necessarily correct quote but gist is right) ... this webpage like many, including all the tapes of the incoming planes from a distance, were 'removed' from the webworld,
    all the best, mark
  4. Mark, If we assume that each area on the orthosis is an inclined plane then a simple 2-dimensional analysis can be performed.
    See here: http://en.wikipedia.org/wiki/Inclined_plane scroll to calculation of forces on an inclined plane.

    However, in truth the surface is made up of 3-dimensional inclined planes, so we would need to perform the analysis in two directions.

    In terms of moving the Centre of Pressure (COP), it is important to remember that in isolation the COP tells us little, that is, it does not define the magnitude nor line of action of the net reaction force vector. We could have two net vectors which share the same COP but of very different magnitudes and directions. Similarly, we could have two net vectors that have different COP's but due to their magnitudes and lines of action, create the same moment about a joint.
  5. efuller

    efuller MVP

    It is normal at each point of the orthosis. So, a varus wedge under the heel will tend to shift the center of pressure more medially and then have a large vertical component and a smaller medial to lateral component vector. In order to maintain equilibrium, or to prevent medial to lateral acceleration of the foot relative to the wedge there has to be a lateral to medial force from some other location. This can be from the shoe, or from the lateral heel cup of an orthosis or friction.

  6. Only in the absence of any shear components.
  7. markjohconley

    markjohconley Well-Known Member

    Simon, keep forgetting the friction shear force and had completely forgotten it had a max value
    Eric, methinks i'm giving too much emphasis to the medial to lateral vector component.
    Thanks gents!
  8. efuller

    efuller MVP

    The vertical component has to equal body weight +/- accelerations. The horizontal component will equal the vertical x sin(angle of wedge). I'm too lazy to pull out the calculator right now, but you can figure it out for your self. At some point it will matter.

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