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COP and orthotic prescription

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Asher, Feb 1, 2010.

  1. Asher

    Asher Well-Known Member


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    Hi all,

    I had a colleague ask me recently about the COP line and how that could be used to decide on orthotic prescription variables. He discussed something similar to the STJ axis whereby, for example, to provide a STJ supination moment to a foot with a medially deviated STJ axis, you need to exert your force medial to that axis line.

    I'm pretty sure the same can't be used for the COP line but I don't really know why and couldn't answer the question. I do realise from Eric's previous posts that the COP line is merely the average of the ground reaction force at any one time, but that didn't help me.



    Your help would be appreciated.

    Rebecca
     
  2. Because CoP is the point of application of the force, it does not describe the vector position. Thus we may have a CoP which is medial to the STJ axis, but a vector which passes laterally to it.
     
  3. Here Eric paper on COP. Hope it helps
     

    Attached Files:

  4. Admin2

    Admin2 Administrator Staff Member

  5. efuller

    efuller MVP

    I don't really use the center of pressure path (line found with pressure analysis software.) in my prescription writing. What I do use is the concept of shifting the center of pressure more medially with varus wedging. As you stand on a varus wedge you will tend to shift the center of pressure under the foot more medially which will increase supination moment (or decrease pronation moment) from the ground. Whether or not the center of pressure actually moves in response to this is dependent on many variables including muscle activation.

    For example, placing a varus heel wedge under a foot with a laterally deviated STJ axis you will increase the supination moment from the ground. In response to this the brain may increase peroneal activity creating a pronation moment. The muscular pronation moment from the muscle may be greater than the supination moment from the ground and with this you would see STJ pronation in the presence of a varus wedge. Additionally you would see a medial shift in the center of pressure from the muscle moving the joint.

    The shift of the center of pressure in response to muscular activity is what makes using Center of Pressure line (measured with a force platform) difficult to use in prescription writing. There are two different concepts here. The moment from ground reaction force is caused by the location of the center of pressure relative to the position of the STJ axis. The other concept is using the measured line to alter your prescription. The former can be used conceptually, the latter requires a lot more thought and motion analysis to do.

    Hope this helps.

    Eric
     
  6. Eric, I have to disagree on the usefulness of CoP in prescription writing, at least when used in isolation. We could have three different net GRF vectors sharing the same CoP - one producing supination moment about the STJ axis, one producing no moment about the STJ axis and one producing pronation moment about the STJ axis. In isolation CoP only tells us the point of application of the vector, not the line of action, nor magnitude. We could see a medial shift in the CoP position in association with a foot orthosis, which results in an increase of STJ supination moment, no net change of moment, or an increase in pronation moment- but we don't know which unless we know the other components of the force vector. When used in isolation, CoP is a rather blunt instrument.
     
  7. efuller

    efuller MVP

    Simon,
    I think we are in agreement that the measured CoP is not very useful for prescription writing. Not just for the reason that you mentioned, but also the measured CoP may or may not be influenced by a change in the relative activity of peroneal and posterior tibial muscles.

    On the other hand would you agree that conceptually, when there is a medial shift of the center of pressure (and the angle of the vector is the same) then there is going to be an increase in the supination moment from the ground. The success of the medial heel skive is explained by this.

    In thinking about what would change the angle of the GRF (ground reaction force) vector you would probably see some visual change in gait. For example, if someone walked with a very wide base of gait there will be an increased lateral to medial component of the GRF vector. It's pretty hard to decrease the base of gait. What other things would change the frontal plane angle of the GRF vector?

    Cheers,

    Eric
     
  8. Bruce Williams

    Bruce Williams Well-Known Member

    Using the CoP in an isolated format is often not much help. Looking at the trajectory of the CoP can indeed be of great help.

    The CoP trajectory in the F-Scan software can be a great indicator of 1st ray function, whether there is FnHL, etc. If the trajectory is medial immediately after heel contact they often the patient has issues with their Posterior Tibial musculature and / or they abduct severely and therefore role over the medial aspect of their feet instead of thru the 1st mpj and hallux.

    While I have seen significant improvement in the CoP due to the use of a medial heel skive, or lateral heel skive, a significant change in the diretion of the CoP trajectory does not necessarily come of that.

    You must understand that utilizing postings, varus or valgus, along with digital extension and cutouts, etc can and will have a large effect on the CoP trajectory depending on what you want to happen within the prescription.

    For example, I utilize a lot of FF valgus posting wiht my medial heel skives to drive the CoP medially in late midstance / early propulsion towards the 1st mpj to overcome stiffness problems within the 1st mpj, ie FnHL.

    I personally think the CoP trajectory is very useful in orthotic prescription writing, but it is only one aspect of in-shoe pressure data that must be examined along with video analysis when determining the proper orthotic prescription for your patient's.

    Cheers
    Bruce
     
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